diff --git a/src/go/internal/gccgoimporter/importer_test.go b/src/go/internal/gccgoimporter/importer_test.go
index e4236a5867..e42684d107 100644
--- a/src/go/internal/gccgoimporter/importer_test.go
+++ b/src/go/internal/gccgoimporter/importer_test.go
@@ -95,7 +95,7 @@ var importerTests = [...]importerTest{
 	{pkgpath: "nointerface", name: "I", want: "type I int"},
 	{pkgpath: "issue29198", name: "FooServer", gccgoVersion: 7, want: "type FooServer struct{FooServer *FooServer; user string; ctx context.Context}"},
 	{pkgpath: "issue30628", name: "Apple", want: "type Apple struct{hey sync.RWMutex; x int; RQ [517]struct{Count uintptr; NumBytes uintptr; Last uintptr}}"},
-	{pkgpath: "issue31540", name: "S", gccgoVersion: 7, want: "type S struct{b int; map[Y]Z}"},
+	{pkgpath: "issue31540", name: "S", gccgoVersion: 7, want: "type S struct{b int; A2 /* = map[Y]Z */}"},
 	{pkgpath: "issue34182", name: "T1", want: "type T1 struct{f *T2}"},
 	{pkgpath: "notinheap", name: "S", want: "type S struct{}"},
 }
diff --git a/src/go/types/check.go b/src/go/types/check.go
index 5e7bd92076..8d9e9a217e 100644
--- a/src/go/types/check.go
+++ b/src/go/types/check.go
@@ -77,6 +77,7 @@ type Checker struct {
 	fset *token.FileSet
 	pkg  *Package
 	*Info
+	nextId uint64                     // unique Id for type parameters (first valid Id is 1)
 	objMap map[Object]*declInfo       // maps package-level objects and (non-interface) methods to declaration info
 	impMap map[importKey]*Package     // maps (import path, source directory) to (complete or fake) package
 	posMap map[*Interface][]token.Pos // maps interface types to lists of embedded interface positions
diff --git a/src/go/types/decl.go b/src/go/types/decl.go
index 17b66ca387..cd610036b8 100644
--- a/src/go/types/decl.go
+++ b/src/go/types/decl.go
@@ -544,36 +544,35 @@ func (check *Checker) varDecl(obj *Var, lhs []*Var, typ, init ast.Expr) {
 	check.initVars(lhs, []ast.Expr{init}, token.NoPos)
 }
 
-// underlying returns the underlying type of typ; possibly by following
-// forward chains of named types. Such chains only exist while named types
-// are incomplete. If an underlying type is found, resolve the chain by
-// setting the underlying type for each defined type in the chain before
-// returning it.
-//
-// If no underlying type is found, a cycle error is reported and Typ[Invalid]
-// is used as underlying type for each defined type in the chain and returned
-// as result.
-func (check *Checker) underlying(typ Type) Type {
-	// If typ is not a defined type, its underlying type is itself.
-	n0, _ := typ.(*Named)
-	if n0 == nil {
-		return typ // nothing to do
+// under returns the expanded underlying type of n0; possibly by following
+// forward chains of named types. If an underlying type is found, resolve
+// the chain by setting the underlying type for each defined type in the
+// chain before returning it. If no underlying type is found or a cycle
+// is detected, the result is Typ[Invalid]. If a cycle is detected and
+// n0.check != nil, the cycle is reported.
+func (n0 *Named) under() Type {
+	u := n0.underlying
+	if u == nil {
+		return Typ[Invalid]
 	}
 
 	// If the underlying type of a defined type is not a defined
 	// type, then that is the desired underlying type.
-	typ = n0.underlying
-	n, _ := typ.(*Named)
+	n := asNamed(u)
 	if n == nil {
-		return typ // common case
+		return u // common case
 	}
 
 	// Otherwise, follow the forward chain.
 	seen := map[*Named]int{n0: 0}
 	path := []Object{n0.obj}
 	for {
-		typ = n.underlying
-		n1, _ := typ.(*Named)
+		u = n.underlying
+		if u == nil {
+			u = Typ[Invalid]
+			break
+		}
+		n1 := asNamed(u)
 		if n1 == nil {
 			break // end of chain
 		}
@@ -584,8 +583,12 @@ func (check *Checker) underlying(typ Type) Type {
 
 		if i, ok := seen[n]; ok {
 			// cycle
-			check.cycleError(path[i:])
-			typ = Typ[Invalid]
+			// TODO(rFindley) revert this to a method on Checker. Having a possibly
+			// nil Checker on Named and TypeParam is too subtle.
+			if n0.check != nil {
+				n0.check.cycleError(path[i:])
+			}
+			u = Typ[Invalid]
 			break
 		}
 	}
@@ -594,13 +597,14 @@ func (check *Checker) underlying(typ Type) Type {
 		// We should never have to update the underlying type of an imported type;
 		// those underlying types should have been resolved during the import.
 		// Also, doing so would lead to a race condition (was issue #31749).
-		if n.obj.pkg != check.pkg {
+		// Do this check always, not just in debug more (it's cheap).
+		if n0.check != nil && n.obj.pkg != n0.check.pkg {
 			panic("internal error: imported type with unresolved underlying type")
 		}
-		n.underlying = typ
+		n.underlying = u
 	}
 
-	return typ
+	return u
 }
 
 func (n *Named) setUnderlying(typ Type) {
@@ -623,7 +627,7 @@ func (check *Checker) typeDecl(obj *TypeName, typ ast.Expr, def *Named, alias bo
 
 	} else {
 
-		named := &Named{obj: obj}
+		named := &Named{check: check, obj: obj}
 		def.setUnderlying(named)
 		obj.typ = named // make sure recursive type declarations terminate
 
@@ -643,8 +647,7 @@ func (check *Checker) typeDecl(obj *TypeName, typ ast.Expr, def *Named, alias bo
 		// and which has as its underlying type the named type B.
 		// Determine the (final, unnamed) underlying type by resolving
 		// any forward chain.
-		named.underlying = check.underlying(named)
-
+		named.underlying = under(named)
 	}
 
 	check.addMethodDecls(obj)
diff --git a/src/go/types/expr.go b/src/go/types/expr.go
index eb2056125a..51714790cb 100644
--- a/src/go/types/expr.go
+++ b/src/go/types/expr.go
@@ -591,7 +591,7 @@ func (check *Checker) implicitType(x *operand, target Type) Type {
 			return Typ[UntypedNil]
 		}
 		// cannot assign untyped values to non-empty interfaces
-		check.completeInterface(t)
+		check.completeInterface(token.NoPos, t)
 		if !t.Empty() {
 			return nil
 		}
diff --git a/src/go/types/lookup.go b/src/go/types/lookup.go
index 3c9ff182ec..2497843b21 100644
--- a/src/go/types/lookup.go
+++ b/src/go/types/lookup.go
@@ -6,6 +6,8 @@
 
 package types
 
+import "go/token"
+
 // LookupFieldOrMethod looks up a field or method with given package and name
 // in T and returns the corresponding *Var or *Func, an index sequence, and a
 // bool indicating if there were any pointer indirections on the path to the
@@ -175,7 +177,7 @@ func (check *Checker) rawLookupFieldOrMethod(T Type, addressable bool, pkg *Pack
 			case *Interface:
 				// look for a matching method
 				// TODO(gri) t.allMethods is sorted - use binary search
-				check.completeInterface(t)
+				check.completeInterface(token.NoPos, t)
 				if i, m := lookupMethod(t.allMethods, pkg, name); m != nil {
 					assert(m.typ != nil)
 					index = concat(e.index, i)
@@ -276,7 +278,7 @@ func MissingMethod(V Type, T *Interface, static bool) (method *Func, wrongType b
 // To improve error messages, also report the wrong signature
 // when the method exists on *V instead of V.
 func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method, wrongType *Func) {
-	check.completeInterface(T)
+	check.completeInterface(token.NoPos, T)
 
 	// fast path for common case
 	if T.Empty() {
@@ -284,7 +286,7 @@ func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method,
 	}
 
 	if ityp, _ := V.Underlying().(*Interface); ityp != nil {
-		check.completeInterface(ityp)
+		check.completeInterface(token.NoPos, ityp)
 		// TODO(gri) allMethods is sorted - can do this more efficiently
 		for _, m := range T.allMethods {
 			_, obj := lookupMethod(ityp.allMethods, m.pkg, m.name)
diff --git a/src/go/types/predicates.go b/src/go/types/predicates.go
index 057908eacd..a139a4eb09 100644
--- a/src/go/types/predicates.go
+++ b/src/go/types/predicates.go
@@ -6,7 +6,10 @@
 
 package types
 
-import "sort"
+import (
+	"go/token"
+	"sort"
+)
 
 func isNamed(typ Type) bool {
 	if _, ok := typ.(*Basic); ok {
@@ -225,8 +228,8 @@ func (check *Checker) identical0(x, y Type, cmpTags bool, p *ifacePair) bool {
 			// that case, interfaces are expected to be complete and lazy completion
 			// here is not needed.
 			if check != nil {
-				check.completeInterface(x)
-				check.completeInterface(y)
+				check.completeInterface(token.NoPos, x)
+				check.completeInterface(token.NoPos, y)
 			}
 			a := x.allMethods
 			b := y.allMethods
diff --git a/src/go/types/type.go b/src/go/types/type.go
index 087cda429d..e0cff1b976 100644
--- a/src/go/types/type.go
+++ b/src/go/types/type.go
@@ -4,12 +4,18 @@
 
 package types
 
-import "sort"
+import (
+	"fmt"
+	"go/token"
+	"sort"
+)
 
 // A Type represents a type of Go.
 // All types implement the Type interface.
 type Type interface {
-	// Underlying returns the underlying type of a type.
+	// Underlying returns the underlying type of a type
+	// w/o following forwarding chains. Only used by
+	// client packages (here for backward-compatibility).
 	Underlying() Type
 
 	// String returns a string representation of a type.
@@ -98,7 +104,7 @@ type Array struct {
 
 // NewArray returns a new array type for the given element type and length.
 // A negative length indicates an unknown length.
-func NewArray(elem Type, len int64) *Array { return &Array{len, elem} }
+func NewArray(elem Type, len int64) *Array { return &Array{len: len, elem: elem} }
 
 // Len returns the length of array a.
 // A negative result indicates an unknown length.
@@ -113,7 +119,7 @@ type Slice struct {
 }
 
 // NewSlice returns a new slice type for the given element type.
-func NewSlice(elem Type) *Slice { return &Slice{elem} }
+func NewSlice(elem Type) *Slice { return &Slice{elem: elem} }
 
 // Elem returns the element type of slice s.
 func (s *Slice) Elem() Type { return s.elem }
@@ -176,8 +182,10 @@ type Tuple struct {
 // NewTuple returns a new tuple for the given variables.
 func NewTuple(x ...*Var) *Tuple {
 	if len(x) > 0 {
-		return &Tuple{x}
+		return &Tuple{vars: x}
 	}
+	// TODO(gri) Don't represent empty tuples with a (*Tuple)(nil) pointer;
+	//           it's too subtle and causes problems.
 	return nil
 }
 
@@ -199,11 +207,13 @@ type Signature struct {
 	// and store it in the Func Object) because when type-checking a function
 	// literal we call the general type checker which returns a general Type.
 	// We then unpack the *Signature and use the scope for the literal body.
-	scope    *Scope // function scope, present for package-local signatures
-	recv     *Var   // nil if not a method
-	params   *Tuple // (incoming) parameters from left to right; or nil
-	results  *Tuple // (outgoing) results from left to right; or nil
-	variadic bool   // true if the last parameter's type is of the form ...T (or string, for append built-in only)
+	rparams  []*TypeName // receiver type parameters from left to right, or nil
+	tparams  []*TypeName // type parameters from left to right, or nil
+	scope    *Scope      // function scope, present for package-local signatures
+	recv     *Var        // nil if not a method
+	params   *Tuple      // (incoming) parameters from left to right; or nil
+	results  *Tuple      // (outgoing) results from left to right; or nil
+	variadic bool        // true if the last parameter's type is of the form ...T (or string, for append built-in only)
 }
 
 // NewSignature returns a new function type for the given receiver, parameters,
@@ -220,7 +230,7 @@ func NewSignature(recv *Var, params, results *Tuple, variadic bool) *Signature {
 			panic("types.NewSignature: variadic parameter must be of unnamed slice type")
 		}
 	}
-	return &Signature{nil, recv, params, results, variadic}
+	return &Signature{recv: recv, params: params, results: results, variadic: variadic}
 }
 
 // Recv returns the receiver of signature s (if a method), or nil if a
@@ -231,6 +241,12 @@ func NewSignature(recv *Var, params, results *Tuple, variadic bool) *Signature {
 // contain methods whose receiver type is a different interface.
 func (s *Signature) Recv() *Var { return s.recv }
 
+// TParams returns the type parameters of signature s, or nil.
+func (s *Signature) TParams() []*TypeName { return s.tparams }
+
+// SetTParams sets the type parameters of signature s.
+func (s *Signature) SetTParams(tparams []*TypeName) { s.tparams = tparams }
+
 // Params returns the parameters of signature s, or nil.
 func (s *Signature) Params() *Tuple { return s.params }
 
@@ -240,12 +256,89 @@ func (s *Signature) Results() *Tuple { return s.results }
 // Variadic reports whether the signature s is variadic.
 func (s *Signature) Variadic() bool { return s.variadic }
 
+// A Sum represents a set of possible types.
+// Sums are currently used to represent type lists of interfaces
+// and thus the underlying types of type parameters; they are not
+// first class types of Go.
+type Sum struct {
+	types []Type // types are unique
+}
+
+// NewSum returns a new Sum type consisting of the provided
+// types if there are more than one. If there is exactly one
+// type, it returns that type. If the list of types is empty
+// the result is nil.
+func NewSum(types []Type) Type {
+	if len(types) == 0 {
+		return nil
+	}
+
+	// What should happen if types contains a sum type?
+	// Do we flatten the types list? For now we check
+	// and panic. This should not be possible for the
+	// current use case of type lists.
+	// TODO(gri) Come up with the rules for sum types.
+	for _, t := range types {
+		if _, ok := t.(*Sum); ok {
+			panic("sum type contains sum type - unimplemented")
+		}
+	}
+
+	if len(types) == 1 {
+		return types[0]
+	}
+	return &Sum{types: types}
+}
+
+// is reports whether all types in t satisfy pred.
+func (s *Sum) is(pred func(Type) bool) bool {
+	if s == nil {
+		return false
+	}
+	for _, t := range s.types {
+		if !pred(t) {
+			return false
+		}
+	}
+	return true
+}
+
 // An Interface represents an interface type.
 type Interface struct {
 	methods   []*Func // ordered list of explicitly declared methods
+	types     Type    // (possibly a Sum) type declared with a type list (TODO(gri) need better field name)
 	embeddeds []Type  // ordered list of explicitly embedded types
 
 	allMethods []*Func // ordered list of methods declared with or embedded in this interface (TODO(gri): replace with mset)
+	allTypes   Type    // intersection of all embedded and locally declared types  (TODO(gri) need better field name)
+
+	obj Object // type declaration defining this interface; or nil (for better error messages)
+
+}
+
+// unpack unpacks a type into a list of types.
+// TODO(gri) Try to eliminate the need for this function.
+func unpackType(typ Type) []Type {
+	if typ == nil {
+		return nil
+	}
+	if sum := asSum(typ); sum != nil {
+		return sum.types
+	}
+	return []Type{typ}
+}
+
+// is reports whether interface t represents types that all satisfy pred.
+func (t *Interface) is(pred func(Type) bool) bool {
+	if t.allTypes == nil {
+		return false // we must have at least one type! (was bug)
+	}
+	for _, t := range unpackType(t.allTypes) {
+		if !pred(t) {
+			return false
+		}
+	}
+	return true
 }
 
 // emptyInterface represents the empty (completed) interface
@@ -344,8 +437,101 @@ func (t *Interface) assertCompleteness() {
 func (t *Interface) Method(i int) *Func { t.assertCompleteness(); return t.allMethods[i] }
 
 // Empty reports whether t is the empty interface.
-// The interface must have been completed.
-func (t *Interface) Empty() bool { t.assertCompleteness(); return len(t.allMethods) == 0 }
+func (t *Interface) Empty() bool {
+	if t.allMethods != nil {
+		// interface is complete - quick test
+		// A non-nil allTypes may still be empty and represents the bottom type.
+		return len(t.allMethods) == 0 && t.allTypes == nil
+	}
+	return !t.iterate(func(t *Interface) bool {
+		return len(t.methods) > 0 || t.types != nil
+	}, nil)
+}
+
+// HasTypeList reports whether interface t has a type list, possibly from an embedded type.
+func (t *Interface) HasTypeList() bool {
+	if t.allMethods != nil {
+		// interface is complete - quick test
+		return t.allTypes != nil
+	}
+
+	return t.iterate(func(t *Interface) bool {
+		return t.types != nil
+	}, nil)
+}
+
+// IsComparable reports whether interface t is or embeds the predeclared interface "comparable".
+func (t *Interface) IsComparable() bool {
+	if t.allMethods != nil {
+		// interface is complete - quick test
+		_, m := lookupMethod(t.allMethods, nil, "==")
+		return m != nil
+	}
+
+	return t.iterate(func(t *Interface) bool {
+		_, m := lookupMethod(t.methods, nil, "==")
+		return m != nil
+	}, nil)
+}
+
+// IsConstraint reports t.HasTypeList() || t.IsComparable().
+func (t *Interface) IsConstraint() bool {
+	if t.allMethods != nil {
+		// interface is complete - quick test
+		if t.allTypes != nil {
+			return true
+		}
+		_, m := lookupMethod(t.allMethods, nil, "==")
+		return m != nil
+	}
+
+	return t.iterate(func(t *Interface) bool {
+		if t.types != nil {
+			return true
+		}
+		_, m := lookupMethod(t.methods, nil, "==")
+		return m != nil
+	}, nil)
+}
+
+// iterate calls f with t and then with any embedded interface of t, recursively, until f returns true.
+// iterate reports whether any call to f returned true.
+func (t *Interface) iterate(f func(*Interface) bool, seen map[*Interface]bool) bool {
+	if f(t) {
+		return true
+	}
+	for _, e := range t.embeddeds {
+		// e should be an interface but be careful (it may be invalid)
+		if e := asInterface(e); e != nil {
+			// Cyclic interfaces such as "type E interface { E }" are not permitted
+			// but they are still constructed and we need to detect such cycles.
+			if seen[e] {
+				continue
+			}
+			if seen == nil {
+				seen = make(map[*Interface]bool)
+			}
+			seen[e] = true
+			if e.iterate(f, seen) {
+				return true
+			}
+		}
+	}
+	return false
+}
+
+// isSatisfiedBy reports whether interface t's type list is satisfied by the type typ.
+// If the the type list is empty (absent), typ trivially satisfies the interface.
+// TODO(gri) This is not a great name. Eventually, we should have a more comprehensive
+//           "implements" predicate.
+func (t *Interface) isSatisfiedBy(typ Type) bool {
+	t.Complete()
+	if t.allTypes == nil {
+		return true
+	}
+	types := unpackType(t.allTypes)
+	return includes(types, typ) || includes(types, under(typ))
+}
 
 // Complete computes the interface's method set. It must be called by users of
 // NewInterfaceType and NewInterface after the interface's embedded types are
@@ -379,12 +565,22 @@ func (t *Interface) Complete() *Interface {
 		addMethod(m, true)
 	}
 
+	allTypes := t.types
+
 	for _, typ := range t.embeddeds {
-		typ := typ.Underlying().(*Interface)
-		typ.Complete()
-		for _, m := range typ.allMethods {
+		utyp := under(typ)
+		etyp := asInterface(utyp)
+		if etyp == nil {
+			if utyp != Typ[Invalid] {
+				panic(fmt.Sprintf("%s is not an interface", typ))
+			}
+			continue
+		}
+		etyp.Complete()
+		for _, m := range etyp.allMethods {
 			addMethod(m, false)
 		}
+		allTypes = intersect(allTypes, etyp.allTypes)
 	}
 
 	for i := 0; i < len(todo); i += 2 {
@@ -399,6 +595,7 @@ func (t *Interface) Complete() *Interface {
 		sort.Sort(byUniqueMethodName(methods))
 		t.allMethods = methods
 	}
+	t.allTypes = allTypes
 
 	return t
 }
@@ -410,7 +607,7 @@ type Map struct {
 
 // NewMap returns a new map for the given key and element types.
 func NewMap(key, elem Type) *Map {
-	return &Map{key, elem}
+	return &Map{key: key, elem: elem}
 }
 
 // Key returns the key type of map m.
@@ -437,7 +634,7 @@ const (
 
 // NewChan returns a new channel type for the given direction and element type.
 func NewChan(dir ChanDir, elem Type) *Chan {
-	return &Chan{dir, elem}
+	return &Chan{dir: dir, elem: elem}
 }
 
 // Dir returns the direction of channel c.
@@ -446,13 +643,16 @@ func (c *Chan) Dir() ChanDir { return c.dir }
 // Elem returns the element type of channel c.
 func (c *Chan) Elem() Type { return c.elem }
 
-// A Named represents a named type.
+// A Named represents a named (defined) type.
 type Named struct {
-	info       typeInfo  // for cycle detection
-	obj        *TypeName // corresponding declared object
-	orig       Type      // type (on RHS of declaration) this *Named type is derived of (for cycle reporting)
-	underlying Type      // possibly a *Named during setup; never a *Named once set up completely
-	methods    []*Func   // methods declared for this type (not the method set of this type); signatures are type-checked lazily
+	check      *Checker    // for Named.under implementation
+	info       typeInfo    // for cycle detection
+	obj        *TypeName   // corresponding declared object
+	orig       Type        // type (on RHS of declaration) this *Named type is derived of (for cycle reporting)
+	underlying Type        // possibly a *Named during setup; never a *Named once set up completely
+	tparams    []*TypeName // type parameters, or nil
+	targs      []Type      // type arguments (after instantiation), or nil
+	methods    []*Func     // methods declared for this type (not the method set of this type); signatures are type-checked lazily
 }
 
 // NewNamed returns a new named type for the given type name, underlying type, and associated methods.
@@ -472,6 +672,19 @@ func NewNamed(obj *TypeName, underlying Type, methods []*Func) *Named {
 // Obj returns the type name for the named type t.
 func (t *Named) Obj() *TypeName { return t.obj }
 
+// TODO(gri) Come up with a better representation and API to distinguish
+//           between parameterized instantiated and non-instantiated types.
+
+// TParams returns the type parameters of the named type t, or nil.
+// The result is non-nil for an (originally) parameterized type even if it is instantiated.
+func (t *Named) TParams() []*TypeName { return t.tparams }
+
+// TArgs returns the type arguments after instantiation of the named type t, or nil if not instantiated.
+func (t *Named) TArgs() []Type { return t.targs }
+
+// SetTArgs sets the type arguments of Named.
+func (t *Named) SetTArgs(args []Type) { t.targs = args }
+
 // NumMethods returns the number of explicit methods whose receiver is named type t.
 func (t *Named) NumMethods() int { return len(t.methods) }
 
@@ -496,28 +709,251 @@ func (t *Named) AddMethod(m *Func) {
 	}
 }
 
-// Implementations for Type methods.
+// A TypeParam represents a type parameter type.
+type TypeParam struct {
+	check *Checker  // for lazy type bound completion
+	id    uint64    // unique id
+	ptr   bool      // pointer designation
+	obj   *TypeName // corresponding type name
+	index int       // parameter index
+	bound Type      // *Named or *Interface; underlying type is always *Interface
+}
 
-func (b *Basic) Underlying() Type     { return b }
-func (a *Array) Underlying() Type     { return a }
-func (s *Slice) Underlying() Type     { return s }
-func (s *Struct) Underlying() Type    { return s }
-func (p *Pointer) Underlying() Type   { return p }
+// NewTypeParam returns a new TypeParam.
+func (check *Checker) NewTypeParam(ptr bool, obj *TypeName, index int, bound Type) *TypeParam {
+	assert(bound != nil)
+	typ := &TypeParam{check: check, id: check.nextId, ptr: ptr, obj: obj, index: index, bound: bound}
+	check.nextId++
+	if obj.typ == nil {
+		obj.typ = typ
+	}
+	return typ
+}
+
+func (t *TypeParam) Bound() *Interface {
+	iface := asInterface(t.bound)
+	// use the type bound position if we have one
+	pos := token.NoPos
+	if n, _ := t.bound.(*Named); n != nil {
+		pos = n.obj.pos
+	}
+	// TODO(rFindley) switch this to an unexported method on Checker.
+	t.check.completeInterface(pos, iface)
+	return iface
+}
+
+// optype returns a type's operational type. Except for
+// type parameters, the operational type is the same
+// as the underlying type (as returned by under). For
+// Type parameters, the operational type is determined
+// by the corresponding type bound's type list. The
+// result may be the bottom or top type, but it is never
+// the incoming type parameter.
+func optype(typ Type) Type {
+	if t := asTypeParam(typ); t != nil {
+		// If the optype is typ, return the top type as we have
+		// no information. It also prevents infinite recursion
+		// via the asTypeParam converter function. This can happen
+		// for a type parameter list of the form:
+		// (type T interface { type T }).
+		// See also issue #39680.
+		if u := t.Bound().allTypes; u != nil && u != typ {
+			// u != typ and u is a type parameter => under(u) != typ, so this is ok
+			return under(u)
+		}
+		return theTop
+	}
+	return under(typ)
+}
+
+// An instance represents an instantiated generic type syntactically
+// (without expanding the instantiation). Type instances appear only
+// during type-checking and are replaced by their fully instantiated
+// (expanded) types before the end of type-checking.
+type instance struct {
+	check   *Checker    // for lazy instantiation
+	pos     token.Pos   // position of type instantiation; for error reporting only
+	base    *Named      // parameterized type to be instantiated
+	targs   []Type      // type arguments
+	poslist []token.Pos // position of each targ; for error reporting only
+	value   Type        // base(targs...) after instantiation or Typ[Invalid]; nil if not yet set
+}
+
+// expand returns the instantiated (= expanded) type of t.
+// The result is either an instantiated *Named type, or
+// Typ[Invalid] if there was an error.
+func (t *instance) expand() Type {
+	// TODO(rFindley) add this in a follow-up CL.
+	panic("not implemented")
+}
+
+// expand expands a type instance into its instantiated
+// type and leaves all other types alone. expand does
+// not recurse.
+func expand(typ Type) Type {
+	if t, _ := typ.(*instance); t != nil {
+		return t.expand()
+	}
+	return typ
+}
+
+// expandf is set to expand.
+// Call expandf when calling expand causes compile-time cycle error.
+var expandf func(Type) Type
+
+func init() { expandf = expand }
+
+// bottom represents the bottom of the type lattice.
+// It is the underlying type of a type parameter that
+// cannot be satisfied by any type, usually because
+// the intersection of type constraints left nothing).
+type bottom struct{}
+
+// theBottom is the singleton bottom type.
+var theBottom = &bottom{}
+
+// top represents the top of the type lattice.
+// It is the underlying type of a type parameter that
+// can be satisfied by any type (ignoring methods),
+// usually because the type constraint has no type
+// list.
+type top struct{}
+
+// theTop is the singleton top type.
+var theTop = &top{}
+
+// Type-specific implementations of Underlying.
+func (t *Basic) Underlying() Type     { return t }
+func (t *Array) Underlying() Type     { return t }
+func (t *Slice) Underlying() Type     { return t }
+func (t *Struct) Underlying() Type    { return t }
+func (t *Pointer) Underlying() Type   { return t }
 func (t *Tuple) Underlying() Type     { return t }
-func (s *Signature) Underlying() Type { return s }
+func (t *Signature) Underlying() Type { return t }
+func (t *Sum) Underlying() Type       { return t }
 func (t *Interface) Underlying() Type { return t }
-func (m *Map) Underlying() Type       { return m }
-func (c *Chan) Underlying() Type      { return c }
+func (t *Map) Underlying() Type       { return t }
+func (t *Chan) Underlying() Type      { return t }
 func (t *Named) Underlying() Type     { return t.underlying }
+func (t *TypeParam) Underlying() Type { return t }
+func (t *instance) Underlying() Type  { return t }
+func (t *bottom) Underlying() Type    { return t }
+func (t *top) Underlying() Type       { return t }
 
-func (b *Basic) String() string     { return TypeString(b, nil) }
-func (a *Array) String() string     { return TypeString(a, nil) }
-func (s *Slice) String() string     { return TypeString(s, nil) }
-func (s *Struct) String() string    { return TypeString(s, nil) }
-func (p *Pointer) String() string   { return TypeString(p, nil) }
+// Type-specific implementations of String.
+func (t *Basic) String() string     { return TypeString(t, nil) }
+func (t *Array) String() string     { return TypeString(t, nil) }
+func (t *Slice) String() string     { return TypeString(t, nil) }
+func (t *Struct) String() string    { return TypeString(t, nil) }
+func (t *Pointer) String() string   { return TypeString(t, nil) }
 func (t *Tuple) String() string     { return TypeString(t, nil) }
-func (s *Signature) String() string { return TypeString(s, nil) }
+func (t *Signature) String() string { return TypeString(t, nil) }
+func (t *Sum) String() string       { return TypeString(t, nil) }
 func (t *Interface) String() string { return TypeString(t, nil) }
-func (m *Map) String() string       { return TypeString(m, nil) }
-func (c *Chan) String() string      { return TypeString(c, nil) }
+func (t *Map) String() string       { return TypeString(t, nil) }
+func (t *Chan) String() string      { return TypeString(t, nil) }
 func (t *Named) String() string     { return TypeString(t, nil) }
+func (t *TypeParam) String() string { return TypeString(t, nil) }
+func (t *instance) String() string  { return TypeString(t, nil) }
+func (t *bottom) String() string    { return TypeString(t, nil) }
+func (t *top) String() string       { return TypeString(t, nil) }
+
+// under returns the true expanded underlying type.
+// If it doesn't exist, the result is Typ[Invalid].
+// under must only be called when a type is known
+// to be fully set up.
+func under(t Type) Type {
+	// TODO(gri) is this correct for *Sum?
+	if n := asNamed(t); n != nil {
+		return n.under()
+	}
+	return t
+}
+
+// Converters
+//
+// A converter must only be called when a type is
+// known to be fully set up. A converter returns
+// a type's operational type (see comment for optype)
+// or nil if the type argument is not of the
+// respective type.
+
+func asBasic(t Type) *Basic {
+	op, _ := optype(t).(*Basic)
+	return op
+}
+
+func asArray(t Type) *Array {
+	op, _ := optype(t).(*Array)
+	return op
+}
+
+func asSlice(t Type) *Slice {
+	op, _ := optype(t).(*Slice)
+	return op
+}
+
+// TODO (rFindley) delete this on the dev.typeparams branch. This is only
+// exported in the prototype for legacy compatibility.
+func AsStruct(t Type) *Struct {
+	return asStruct(t)
+}
+
+func asStruct(t Type) *Struct {
+	op, _ := optype(t).(*Struct)
+	return op
+}
+
+// TODO(rFindley) delete this on the dev.typeparams branch (see ToStruct).
+func AsPointer(t Type) *Pointer {
+	return asPointer(t)
+}
+
+func asPointer(t Type) *Pointer {
+	op, _ := optype(t).(*Pointer)
+	return op
+}
+
+func asTuple(t Type) *Tuple {
+	op, _ := optype(t).(*Tuple)
+	return op
+}
+
+func asSignature(t Type) *Signature {
+	op, _ := optype(t).(*Signature)
+	return op
+}
+
+func asSum(t Type) *Sum {
+	op, _ := optype(t).(*Sum)
+	return op
+}
+
+func asInterface(t Type) *Interface {
+	op, _ := optype(t).(*Interface)
+	return op
+}
+
+func asMap(t Type) *Map {
+	op, _ := optype(t).(*Map)
+	return op
+}
+
+func asChan(t Type) *Chan {
+	op, _ := optype(t).(*Chan)
+	return op
+}
+
+// If the argument to asNamed and asTypeParam is of the respective types
+// (possibly after expanding an instance type), these methods return that type.
+// Otherwise the result is nil.
+
+func asNamed(t Type) *Named {
+	e, _ := expand(t).(*Named)
+	return e
+}
+
+func asTypeParam(t Type) *TypeParam {
+	u, _ := under(t).(*TypeParam)
+	return u
+}
diff --git a/src/go/types/typestring.go b/src/go/types/typestring.go
index 31c572f83b..c82fa3395b 100644
--- a/src/go/types/typestring.go
+++ b/src/go/types/typestring.go
@@ -9,6 +9,7 @@ package types
 import (
 	"bytes"
 	"fmt"
+	"unicode/utf8"
 )
 
 // A Qualifier controls how named package-level objects are printed in
@@ -75,6 +76,10 @@ func WriteType(buf *bytes.Buffer, typ Type, qf Qualifier) {
 	writeType(buf, typ, qf, make([]Type, 0, 8))
 }
 
+// instanceMarker is the prefix for an instantiated type
+// in "non-evaluated" instance form.
+const instanceMarker = '#'
+
 func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 	// Theoretically, this is a quadratic lookup algorithm, but in
 	// practice deeply nested composite types with unnamed component
@@ -82,7 +87,7 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 	// using a map.
 	for _, t := range visited {
 		if t == typ {
-			fmt.Fprintf(buf, "○%T", typ) // cycle to typ
+			fmt.Fprintf(buf, "○%T", goTypeName(typ)) // cycle to typ
 			return
 		}
 	}
@@ -121,11 +126,19 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 			if i > 0 {
 				buf.WriteString("; ")
 			}
-			if !f.embedded {
-				buf.WriteString(f.name)
+			buf.WriteString(f.name)
+			if f.embedded {
+				// emphasize that the embedded field's name
+				// doesn't match the field's type name
+				if f.name != embeddedFieldName(f.typ) {
+					buf.WriteString(" /* = ")
+					writeType(buf, f.typ, qf, visited)
+					buf.WriteString(" */")
+				}
+			} else {
 				buf.WriteByte(' ')
+				writeType(buf, f.typ, qf, visited)
 			}
-			writeType(buf, f.typ, qf, visited)
 			if tag := t.Tag(i); tag != "" {
 				fmt.Fprintf(buf, " %q", tag)
 			}
@@ -143,6 +156,14 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 		buf.WriteString("func")
 		writeSignature(buf, t, qf, visited)
 
+	case *Sum:
+		for i, t := range t.types {
+			if i > 0 {
+				buf.WriteString(", ")
+			}
+			writeType(buf, t, qf, visited)
+		}
+
 	case *Interface:
 		// We write the source-level methods and embedded types rather
 		// than the actual method set since resolved method signatures
@@ -168,6 +189,13 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 				writeSignature(buf, m.typ.(*Signature), qf, visited)
 				empty = false
 			}
+			if !empty && t.allTypes != nil {
+				buf.WriteString("; ")
+			}
+			if t.allTypes != nil {
+				buf.WriteString("type ")
+				writeType(buf, t.allTypes, qf, visited)
+			}
 		} else {
 			// print explicit interface methods and embedded types
 			for i, m := range t.methods {
@@ -178,8 +206,19 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 				writeSignature(buf, m.typ.(*Signature), qf, visited)
 				empty = false
 			}
+			if !empty && t.types != nil {
+				buf.WriteString("; ")
+			}
+			if t.types != nil {
+				buf.WriteString("type ")
+				writeType(buf, t.types, qf, visited)
+				empty = false
+			}
+			if !empty && len(t.embeddeds) > 0 {
+				buf.WriteString("; ")
+			}
 			for i, typ := range t.embeddeds {
-				if i > 0 || len(t.methods) > 0 {
+				if i > 0 {
 					buf.WriteString("; ")
 				}
 				writeType(buf, typ, qf, visited)
@@ -227,17 +266,36 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 		}
 
 	case *Named:
-		s := "<Named w/o object>"
-		if obj := t.obj; obj != nil {
-			if obj.pkg != nil {
-				writePackage(buf, obj.pkg, qf)
-			}
-			// TODO(gri): function-local named types should be displayed
-			// differently from named types at package level to avoid
-			// ambiguity.
-			s = obj.name
+		writeTypeName(buf, t.obj, qf)
+		if t.targs != nil {
+			// instantiated type
+			buf.WriteByte('[')
+			writeTypeList(buf, t.targs, qf, visited)
+			buf.WriteByte(']')
+		} else if t.tparams != nil {
+			// parameterized type
+			writeTParamList(buf, t.tparams, qf, visited)
 		}
-		buf.WriteString(s)
+
+	case *TypeParam:
+		s := "?"
+		if t.obj != nil {
+			s = t.obj.name
+		}
+		buf.WriteString(s + subscript(t.id))
+
+	case *instance:
+		buf.WriteByte(instanceMarker) // indicate "non-evaluated" syntactic instance
+		writeTypeName(buf, t.base.obj, qf)
+		buf.WriteByte('[')
+		writeTypeList(buf, t.targs, qf, visited)
+		buf.WriteByte(']')
+
+	case *bottom:
+		buf.WriteString("⊥")
+
+	case *top:
+		buf.WriteString("⊤")
 
 	default:
 		// For externally defined implementations of Type.
@@ -245,6 +303,64 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 	}
 }
 
+func writeTypeList(buf *bytes.Buffer, list []Type, qf Qualifier, visited []Type) {
+	for i, typ := range list {
+		if i > 0 {
+			buf.WriteString(", ")
+		}
+		writeType(buf, typ, qf, visited)
+	}
+}
+
+func writeTParamList(buf *bytes.Buffer, list []*TypeName, qf Qualifier, visited []Type) {
+	buf.WriteString("[")
+	var prev Type
+	for i, p := range list {
+		// TODO(rFindley) support 'any' sugar here.
+		var b Type = &emptyInterface
+		if t, _ := p.typ.(*TypeParam); t != nil && t.bound != nil {
+			b = t.bound
+		}
+		if i > 0 {
+			if b != prev {
+				// type bound changed - write previous one before advancing
+				buf.WriteByte(' ')
+				writeType(buf, prev, qf, visited)
+			}
+			buf.WriteString(", ")
+		}
+		prev = b
+
+		if t, _ := p.typ.(*TypeParam); t != nil {
+			if t.ptr {
+				buf.WriteByte('*')
+			}
+			writeType(buf, t, qf, visited)
+		} else {
+			buf.WriteString(p.name)
+		}
+	}
+	if prev != nil {
+		buf.WriteByte(' ')
+		writeType(buf, prev, qf, visited)
+	}
+	buf.WriteByte(']')
+}
+
+func writeTypeName(buf *bytes.Buffer, obj *TypeName, qf Qualifier) {
+	s := "<Named w/o object>"
+	if obj != nil {
+		if obj.pkg != nil {
+			writePackage(buf, obj.pkg, qf)
+		}
+		// TODO(gri): function-local named types should be displayed
+		// differently from named types at package level to avoid
+		// ambiguity.
+		s = obj.name
+	}
+	buf.WriteString(s)
+}
+
 func writeTuple(buf *bytes.Buffer, tup *Tuple, variadic bool, qf Qualifier, visited []Type) {
 	buf.WriteByte('(')
 	if tup != nil {
@@ -264,7 +380,7 @@ func writeTuple(buf *bytes.Buffer, tup *Tuple, variadic bool, qf Qualifier, visi
 				} else {
 					// special case:
 					// append(s, "foo"...) leads to signature func([]byte, string...)
-					if t, ok := typ.Underlying().(*Basic); !ok || t.kind != String {
+					if t := asBasic(typ); t == nil || t.kind != String {
 						panic("internal error: string type expected")
 					}
 					writeType(buf, typ, qf, visited)
@@ -287,6 +403,10 @@ func WriteSignature(buf *bytes.Buffer, sig *Signature, qf Qualifier) {
 }
 
 func writeSignature(buf *bytes.Buffer, sig *Signature, qf Qualifier, visited []Type) {
+	if sig.tparams != nil {
+		writeTParamList(buf, sig.tparams, qf, visited)
+	}
+
 	writeTuple(buf, sig.params, sig.variadic, qf, visited)
 
 	n := sig.results.Len()
@@ -305,3 +425,38 @@ func writeSignature(buf *bytes.Buffer, sig *Signature, qf Qualifier, visited []T
 	// multiple or named result(s)
 	writeTuple(buf, sig.results, false, qf, visited)
 }
+
+// embeddedFieldName returns an embedded field's name given its type.
+// The result is "" if the type doesn't have an embedded field name.
+func embeddedFieldName(typ Type) string {
+	switch t := typ.(type) {
+	case *Basic:
+		return t.name
+	case *Named:
+		return t.obj.name
+	case *Pointer:
+		// *T is ok, but **T is not
+		if _, ok := t.base.(*Pointer); !ok {
+			return embeddedFieldName(t.base)
+		}
+	case *instance:
+		return t.base.obj.name
+	}
+	return "" // not a (pointer to) a defined type
+}
+
+// subscript returns the decimal (utf8) representation of x using subscript digits.
+func subscript(x uint64) string {
+	const w = len("₀") // all digits 0...9 have the same utf8 width
+	var buf [32 * w]byte
+	i := len(buf)
+	for {
+		i -= w
+		utf8.EncodeRune(buf[i:], '₀'+rune(x%10)) // '₀' == U+2080
+		x /= 10
+		if x == 0 {
+			break
+		}
+	}
+	return string(buf[i:])
+}
diff --git a/src/go/types/typestring_test.go b/src/go/types/typestring_test.go
index 5d9db39bfc..b16529dc64 100644
--- a/src/go/types/typestring_test.go
+++ b/src/go/types/typestring_test.go
@@ -96,6 +96,10 @@ var independentTestTypes = []testEntry{
 	dup("interface{m()}"),
 	dup(`interface{String() string; m(int) float32}`),
 
+	// TODO(rFindley) uncomment this once this AST is accepted, and add more test
+	// cases.
+	// dup(`interface{type int, float32, complex128}`),
+
 	// maps
 	dup("map[string]int"),
 	{"map[struct{x, y int}][]byte", "map[struct{x int; y int}][]byte"},
diff --git a/src/go/types/typexpr.go b/src/go/types/typexpr.go
index 2b398010f4..41ba7e9bca 100644
--- a/src/go/types/typexpr.go
+++ b/src/go/types/typexpr.go
@@ -7,11 +7,13 @@
 package types
 
 import (
+	"fmt"
 	"go/ast"
 	"go/constant"
 	"go/token"
 	"sort"
 	"strconv"
+	"strings"
 )
 
 // ident type-checks identifier e and initializes x with the value or type of e.
@@ -207,6 +209,12 @@ func (check *Checker) funcType(sig *Signature, recvPar *ast.FieldList, ftyp *ast
 	sig.variadic = variadic
 }
 
+// goTypeName returns the Go type name for typ and
+// removes any occurences of "types." from that name.
+func goTypeName(typ Type) string {
+	return strings.ReplaceAll(fmt.Sprintf("%T", typ), "types.", "")
+}
+
 // typInternal drives type checking of types.
 // Must only be called by definedType.
 //
@@ -498,7 +506,7 @@ func (check *Checker) interfaceType(ityp *Interface, iface *ast.InterfaceType, d
 			// it if it's a valid interface.
 			typ := check.typ(f.Type)
 
-			utyp := check.underlying(typ)
+			utyp := under(typ)
 			if _, ok := utyp.(*Interface); !ok {
 				if utyp != Typ[Invalid] {
 					check.errorf(f.Type, _InvalidIfaceEmbed, "%s is not an interface", typ)
@@ -521,10 +529,10 @@ func (check *Checker) interfaceType(ityp *Interface, iface *ast.InterfaceType, d
 	sort.Sort(byUniqueMethodName(ityp.methods))
 	sort.Stable(byUniqueTypeName(ityp.embeddeds))
 
-	check.later(func() { check.completeInterface(ityp) })
+	check.later(func() { check.completeInterface(iface.Pos(), ityp) })
 }
 
-func (check *Checker) completeInterface(ityp *Interface) {
+func (check *Checker) completeInterface(pos token.Pos, ityp *Interface) {
 	if ityp.allMethods != nil {
 		return
 	}
@@ -538,11 +546,18 @@ func (check *Checker) completeInterface(ityp *Interface) {
 	}
 
 	if trace {
-		check.trace(token.NoPos, "complete %s", ityp)
+		// Types don't generally have position information.
+		// If we don't have a valid pos provided, try to use
+		// one close enough.
+		if !pos.IsValid() && len(ityp.methods) > 0 {
+			pos = ityp.methods[0].pos
+		}
+
+		check.trace(pos, "complete %s", ityp)
 		check.indent++
 		defer func() {
 			check.indent--
-			check.trace(token.NoPos, "=> %s", ityp)
+			check.trace(pos, "=> %s (methods = %v, types = %v)", ityp, ityp.allMethods, ityp.allTypes)
 		}()
 	}
 
@@ -592,25 +607,77 @@ func (check *Checker) completeInterface(ityp *Interface) {
 		addMethod(m.pos, m, true)
 	}
 
+	// collect types
+	allTypes := ityp.types
+
 	posList := check.posMap[ityp]
 	for i, typ := range ityp.embeddeds {
 		pos := posList[i] // embedding position
-		typ, ok := check.underlying(typ).(*Interface)
-		if !ok {
-			// An error was reported when collecting the embedded types.
-			// Ignore it.
+		utyp := under(typ)
+		etyp := asInterface(utyp)
+		if etyp == nil {
+			if utyp != Typ[Invalid] {
+				var format string
+				if _, ok := utyp.(*TypeParam); ok {
+					format = "%s is a type parameter, not an interface"
+				} else {
+					format = "%s is not an interface"
+				}
+				// TODO: correct error code.
+				check.errorf(atPos(pos), 0, format, typ)
+			}
 			continue
 		}
-		check.completeInterface(typ)
-		for _, m := range typ.allMethods {
+		check.completeInterface(pos, etyp)
+		for _, m := range etyp.allMethods {
 			addMethod(pos, m, false) // use embedding position pos rather than m.pos
 		}
+		allTypes = intersect(allTypes, etyp.allTypes)
 	}
 
 	if methods != nil {
 		sort.Sort(byUniqueMethodName(methods))
 		ityp.allMethods = methods
 	}
+	ityp.allTypes = allTypes
+}
+
+// intersect computes the intersection of the types x and y.
+// Note: A incomming nil type stands for the top type. A top
+// type result is returned as nil.
+func intersect(x, y Type) (r Type) {
+	defer func() {
+		if r == theTop {
+			r = nil
+		}
+	}()
+
+	switch {
+	case x == theBottom || y == theBottom:
+		return theBottom
+	case x == nil || x == theTop:
+		return y
+	case y == nil || x == theTop:
+		return x
+	}
+
+	xtypes := unpackType(x)
+	ytypes := unpackType(y)
+	// Compute the list rtypes which includes only
+	// types that are in both xtypes and ytypes.
+	// Quadratic algorithm, but good enough for now.
+	// TODO(gri) fix this
+	var rtypes []Type
+	for _, x := range xtypes {
+		if includes(ytypes, x) {
+			rtypes = append(rtypes, x)
+		}
+	}
+
+	if rtypes == nil {
+		return theBottom
+	}
+	return NewSum(rtypes)
 }
 
 // byUniqueTypeName named type lists can be sorted by their unique type names.
@@ -762,3 +829,13 @@ func embeddedFieldIdent(e ast.Expr) *ast.Ident {
 	}
 	return nil // invalid embedded field
 }
+
+// includes reports whether typ is in list.
+func includes(list []Type, typ Type) bool {
+	for _, e := range list {
+		if Identical(typ, e) {
+			return true
+		}
+	}
+	return false
+}