go/types, types2: instantiate generic alias types

For #46477.

Change-Id: Ifa47d3ff87f67c60fa25654e54194ca8b31ea5a2
Reviewed-on: https://go-review.googlesource.com/c/go/+/567617
Auto-Submit: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>

src/cmd/compile/internal/types2/alias.go

@@ -4,7 +4,10 @@
 
 package types2
 
-import "fmt"
+import (
+	"cmd/compile/internal/syntax"
+	"fmt"
+)
 
 // An Alias represents an alias type.
 // Whether or not Alias types are created is controlled by the
@@ -30,7 +33,10 @@ func NewAlias(obj *TypeName, rhs Type) *Alias {
 	return alias
 }
 
-func (a *Alias) Obj() *TypeName { return a.obj }
+// Obj returns the type name for the declaration defining the alias type a.
+// For instantiated types, this is same as the type name of the origin type.
+func (a *Alias) Obj() *TypeName { return a.orig.obj }
+
 func (a *Alias) String() string { return TypeString(a, nil) }
 
 // Underlying returns the [underlying type] of the alias type a, which is the
@@ -125,6 +131,20 @@ func (check *Checker) newAlias(obj *TypeName, rhs Type) *Alias {
 	return a
 }
 
+// newAliasInstance creates a new alias instance for the given origin and type
+// arguments, recording pos as the position of its synthetic object (for error
+// reporting).
+func (check *Checker) newAliasInstance(pos syntax.Pos, orig *Alias, targs []Type, ctxt *Context) *Alias {
+	assert(len(targs) > 0)
+	obj := NewTypeName(pos, orig.obj.pkg, orig.obj.name, nil)
+	rhs := check.subst(pos, orig.fromRHS, makeSubstMap(orig.TypeParams().list(), targs), nil, ctxt)
+	res := check.newAlias(obj, rhs)
+	res.orig = orig
+	res.tparams = orig.tparams
+	res.targs = newTypeList(targs)
+	return res
+}
+
 func (a *Alias) cleanup() {
 	// Ensure a.actual is set before types are published,
 	// so Unalias is a pure "getter", not a "setter".

src/cmd/compile/internal/types2/instantiate.go

@@ -21,11 +21,13 @@ type genericType interface {
 }
 
 // Instantiate instantiates the type orig with the given type arguments targs.
-// orig must be a *Named or a *Signature type. If there is no error, the
-// resulting Type is an instantiated type of the same kind (either a *Named or
-// a *Signature). Methods attached to a *Named type are also instantiated, and
-// associated with a new *Func that has the same position as the original
-// method, but nil function scope.
+// orig must be an *Alias, *Named, or *Signature type. If there is no error,
+// the resulting Type is an instantiated type of the same kind (*Alias, *Named
+// or *Signature, respectively).
+//
+// Methods attached to a *Named type are also instantiated, and associated with
+// a new *Func that has the same position as the original method, but nil function
+// scope.
 //
 // If ctxt is non-nil, it may be used to de-duplicate the instance against
 // previous instances with the same identity. As a special case, generic
@@ -35,10 +37,10 @@ type genericType interface {
 // not guarantee that identical instances are deduplicated in all cases.
 //
 // If validate is set, Instantiate verifies that the number of type arguments
-// and parameters match, and that the type arguments satisfy their
-// corresponding type constraints. If verification fails, the resulting error
-// may wrap an *ArgumentError indicating which type argument did not satisfy
-// its corresponding type parameter constraint, and why.
+// and parameters match, and that the type arguments satisfy their respective
+// type constraints. If verification fails, the resulting error may wrap an
+// *ArgumentError indicating which type argument did not satisfy its type parameter
+// constraint, and why.
 //
 // If validate is not set, Instantiate does not verify the type argument count
 // or whether the type arguments satisfy their constraints. Instantiate is
@@ -101,8 +103,9 @@ func (check *Checker) instance(pos syntax.Pos, orig genericType, targs []Type, e
 		hashes[i] = ctxt.instanceHash(orig, targs)
 	}
 
-	// If local is non-nil, updateContexts return the type recorded in
-	// local.
+	// Record the result in all contexts.
+	// Prefer to re-use existing types from expanding context, if it exists, to reduce
+	// the memory pinned by the Named type.
 	updateContexts := func(res Type) Type {
 		for i := len(ctxts) - 1; i >= 0; i-- {
 			res = ctxts[i].update(hashes[i], orig, targs, res)
@@ -122,6 +125,21 @@ func (check *Checker) instance(pos syntax.Pos, orig genericType, targs []Type, e
 	case *Named:
 		res = check.newNamedInstance(pos, orig, targs, expanding) // substituted lazily
 
+	case *Alias:
+		// TODO(gri) is this correct?
+		assert(expanding == nil) // Alias instances cannot be reached from Named types
+
+		tparams := orig.TypeParams()
+		// TODO(gri) investigate if this is needed (type argument and parameter count seem to be correct here)
+		if !check.validateTArgLen(pos, orig.String(), tparams.Len(), len(targs)) {
+			return Typ[Invalid]
+		}
+		if tparams.Len() == 0 {
+			return orig // nothing to do (minor optimization)
+		}
+
+		return check.newAliasInstance(pos, orig, targs, ctxt)
+
 	case *Signature:
 		assert(expanding == nil) // function instances cannot be reached from Named types
 

src/cmd/compile/internal/types2/predicates.go

@@ -137,6 +137,9 @@ func hasEmptyTypeset(t Type) bool {
 // TODO(gri) should we include signatures or assert that they are not present?
 func isGeneric(t Type) bool {
 	// A parameterized type is only generic if it doesn't have an instantiation already.
+	if alias, _ := t.(*Alias); alias != nil && alias.tparams != nil && alias.targs == nil {
+		return true
+	}
 	named := asNamed(t)
 	return named != nil && named.obj != nil && named.inst == nil && named.TypeParams().Len() > 0
 }

src/cmd/compile/internal/types2/subst.go

@@ -96,17 +96,41 @@ func (subst *subster) typ(typ Type) Type {
 		// nothing to do
 
 	case *Alias:
-		rhs := subst.typ(t.fromRHS)
-		if rhs != t.fromRHS {
-			// This branch cannot be reached because the RHS of an alias
-			// may only contain type parameters of an enclosing function.
-			// Such function bodies are never "instantiated" and thus
-			// substitution is not called on locally declared alias types.
-			// TODO(gri) adjust once parameterized aliases are supported
-			panic("unreachable for unparameterized aliases")
-			// return subst.check.newAlias(t.obj, rhs)
+		// This code follows the code for *Named types closely.
+		// TODO(gri) try to factor better
+		orig := t.Origin()
+		n := orig.TypeParams().Len()
+		if n == 0 {
+			return t // type is not parameterized
 		}
 
+		// TODO(gri) do we need this for Alias types?
+		var newTArgs []Type
+		if t.TypeArgs().Len() != n {
+			return Typ[Invalid] // error reported elsewhere
+		}
+
+		// already instantiated
+		// For each (existing) type argument targ, determine if it needs
+		// to be substituted; i.e., if it is or contains a type parameter
+		// that has a type argument for it.
+		for i, targ := range t.TypeArgs().list() {
+			new_targ := subst.typ(targ)
+			if new_targ != targ {
+				if newTArgs == nil {
+					newTArgs = make([]Type, n)
+					copy(newTArgs, t.TypeArgs().list())
+				}
+				newTArgs[i] = new_targ
+			}
+		}
+
+		if newTArgs == nil {
+			return t // nothing to substitute
+		}
+
+		return subst.check.newAliasInstance(subst.pos, t.orig, newTArgs, subst.ctxt)
+
 	case *Array:
 		elem := subst.typOrNil(t.elem)
 		if elem != t.elem {

src/cmd/compile/internal/types2/typestring.go

@@ -335,6 +335,10 @@ func (w *typeWriter) typ(typ Type) {
 
 	case *Alias:
 		w.typeName(t.obj)
+		if list := t.targs.list(); len(list) != 0 {
+			// instantiated type
+			w.typeList(list)
+		}
 		if w.ctxt != nil {
 			// TODO(gri) do we need to print the alias type name, too?
 			w.typ(Unalias(t.obj.typ))

src/cmd/compile/internal/types2/typexpr.go

@@ -453,6 +453,10 @@ func (check *Checker) instantiatedType(x syntax.Expr, xlist []syntax.Expr, def *
 		}()
 	}
 
+	defer func() {
+		setDefType(def, res)
+	}()
+
 	var cause string
 	gtyp := check.genericType(x, &cause)
 	if cause != "" {
@@ -462,21 +466,23 @@ func (check *Checker) instantiatedType(x syntax.Expr, xlist []syntax.Expr, def *
 		return gtyp // error already reported
 	}
 
+	// evaluate arguments
+	targs := check.typeList(xlist)
+	if targs == nil {
+		return Typ[Invalid]
+	}
+
+	if orig, _ := gtyp.(*Alias); orig != nil {
+		return check.instance(x.Pos(), orig, targs, nil, check.context())
+	}
+
 	orig := asNamed(gtyp)
 	if orig == nil {
 		panic(fmt.Sprintf("%v: cannot instantiate %v", x.Pos(), gtyp))
 	}
 
-	// evaluate arguments
-	targs := check.typeList(xlist)
-	if targs == nil {
-		setDefType(def, Typ[Invalid]) // avoid errors later due to lazy instantiation
-		return Typ[Invalid]
-	}
-
 	// create the instance
 	inst := asNamed(check.instance(x.Pos(), orig, targs, nil, check.context()))
-	setDefType(def, inst)
 
 	// orig.tparams may not be set up, so we need to do expansion later.
 	check.later(func() {

src/cmd/compile/internal/types2/util.go

@@ -36,7 +36,7 @@ func dddErrPos(call *syntax.CallExpr) *syntax.CallExpr {
 	return call
 }
 
-// argErrPos returns the node (poser) for reportign an invalid argument count.
+// argErrPos returns the node (poser) for reporting an invalid argument count.
 func argErrPos(call *syntax.CallExpr) *syntax.CallExpr { return call }
 
 // ExprString returns a string representation of x.

src/go/types/alias.go

@@ -7,7 +7,10 @@
 
 package types
 
-import "fmt"
+import (
+	"fmt"
+	"go/token"
+)
 
 // An Alias represents an alias type.
 // Whether or not Alias types are created is controlled by the
@@ -33,7 +36,10 @@ func NewAlias(obj *TypeName, rhs Type) *Alias {
 	return alias
 }
 
-func (a *Alias) Obj() *TypeName { return a.obj }
+// Obj returns the type name for the declaration defining the alias type a.
+// For instantiated types, this is same as the type name of the origin type.
+func (a *Alias) Obj() *TypeName { return a.orig.obj }
+
 func (a *Alias) String() string { return TypeString(a, nil) }
 
 // Underlying returns the [underlying type] of the alias type a, which is the
@@ -128,6 +134,20 @@ func (check *Checker) newAlias(obj *TypeName, rhs Type) *Alias {
 	return a
 }
 
+// newAliasInstance creates a new alias instance for the given origin and type
+// arguments, recording pos as the position of its synthetic object (for error
+// reporting).
+func (check *Checker) newAliasInstance(pos token.Pos, orig *Alias, targs []Type, ctxt *Context) *Alias {
+	assert(len(targs) > 0)
+	obj := NewTypeName(pos, orig.obj.pkg, orig.obj.name, nil)
+	rhs := check.subst(pos, orig.fromRHS, makeSubstMap(orig.TypeParams().list(), targs), nil, ctxt)
+	res := check.newAlias(obj, rhs)
+	res.orig = orig
+	res.tparams = orig.tparams
+	res.targs = newTypeList(targs)
+	return res
+}
+
 func (a *Alias) cleanup() {
 	// Ensure a.actual is set before types are published,
 	// so Unalias is a pure "getter", not a "setter".

src/go/types/generate_test.go

@@ -100,7 +100,7 @@ func generate(t *testing.T, filename string, write bool) {
 type action func(in *ast.File)
 
 var filemap = map[string]action{
-	"alias.go": nil,
+	"alias.go": fixTokenPos,
 	"assignments.go": func(f *ast.File) {
 		renameImportPath(f, `"cmd/compile/internal/syntax"->"go/ast"`)
 		renameSelectorExprs(f, "syntax.Name->ast.Ident", "ident.Value->ident.Name", "ast.Pos->token.Pos") // must happen before renaming identifiers

src/go/types/instantiate.go

@@ -24,11 +24,13 @@ type genericType interface {
 }
 
 // Instantiate instantiates the type orig with the given type arguments targs.
-// orig must be a *Named or a *Signature type. If there is no error, the
-// resulting Type is an instantiated type of the same kind (either a *Named or
-// a *Signature). Methods attached to a *Named type are also instantiated, and
-// associated with a new *Func that has the same position as the original
-// method, but nil function scope.
+// orig must be an *Alias, *Named, or *Signature type. If there is no error,
+// the resulting Type is an instantiated type of the same kind (*Alias, *Named
+// or *Signature, respectively).
+//
+// Methods attached to a *Named type are also instantiated, and associated with
+// a new *Func that has the same position as the original method, but nil function
+// scope.
 //
 // If ctxt is non-nil, it may be used to de-duplicate the instance against
 // previous instances with the same identity. As a special case, generic
@@ -38,10 +40,10 @@ type genericType interface {
 // not guarantee that identical instances are deduplicated in all cases.
 //
 // If validate is set, Instantiate verifies that the number of type arguments
-// and parameters match, and that the type arguments satisfy their
-// corresponding type constraints. If verification fails, the resulting error
-// may wrap an *ArgumentError indicating which type argument did not satisfy
-// its corresponding type parameter constraint, and why.
+// and parameters match, and that the type arguments satisfy their respective
+// type constraints. If verification fails, the resulting error may wrap an
+// *ArgumentError indicating which type argument did not satisfy its type parameter
+// constraint, and why.
 //
 // If validate is not set, Instantiate does not verify the type argument count
 // or whether the type arguments satisfy their constraints. Instantiate is
@@ -104,8 +106,9 @@ func (check *Checker) instance(pos token.Pos, orig genericType, targs []Type, ex
 		hashes[i] = ctxt.instanceHash(orig, targs)
 	}
 
-	// If local is non-nil, updateContexts return the type recorded in
-	// local.
+	// Record the result in all contexts.
+	// Prefer to re-use existing types from expanding context, if it exists, to reduce
+	// the memory pinned by the Named type.
 	updateContexts := func(res Type) Type {
 		for i := len(ctxts) - 1; i >= 0; i-- {
 			res = ctxts[i].update(hashes[i], orig, targs, res)
@@ -125,6 +128,21 @@ func (check *Checker) instance(pos token.Pos, orig genericType, targs []Type, ex
 	case *Named:
 		res = check.newNamedInstance(pos, orig, targs, expanding) // substituted lazily
 
+	case *Alias:
+		// TODO(gri) is this correct?
+		assert(expanding == nil) // Alias instances cannot be reached from Named types
+
+		tparams := orig.TypeParams()
+		// TODO(gri) investigate if this is needed (type argument and parameter count seem to be correct here)
+		if !check.validateTArgLen(pos, orig.String(), tparams.Len(), len(targs)) {
+			return Typ[Invalid]
+		}
+		if tparams.Len() == 0 {
+			return orig // nothing to do (minor optimization)
+		}
+
+		return check.newAliasInstance(pos, orig, targs, ctxt)
+
 	case *Signature:
 		assert(expanding == nil) // function instances cannot be reached from Named types
 

src/go/types/predicates.go

@@ -140,6 +140,9 @@ func hasEmptyTypeset(t Type) bool {
 // TODO(gri) should we include signatures or assert that they are not present?
 func isGeneric(t Type) bool {
 	// A parameterized type is only generic if it doesn't have an instantiation already.
+	if alias, _ := t.(*Alias); alias != nil && alias.tparams != nil && alias.targs == nil {
+		return true
+	}
 	named := asNamed(t)
 	return named != nil && named.obj != nil && named.inst == nil && named.TypeParams().Len() > 0
 }

src/go/types/subst.go

@@ -99,17 +99,41 @@ func (subst *subster) typ(typ Type) Type {
 		// nothing to do
 
 	case *Alias:
-		rhs := subst.typ(t.fromRHS)
-		if rhs != t.fromRHS {
-			// This branch cannot be reached because the RHS of an alias
-			// may only contain type parameters of an enclosing function.
-			// Such function bodies are never "instantiated" and thus
-			// substitution is not called on locally declared alias types.
-			// TODO(gri) adjust once parameterized aliases are supported
-			panic("unreachable for unparameterized aliases")
-			// return subst.check.newAlias(t.obj, rhs)
+		// This code follows the code for *Named types closely.
+		// TODO(gri) try to factor better
+		orig := t.Origin()
+		n := orig.TypeParams().Len()
+		if n == 0 {
+			return t // type is not parameterized
 		}
 
+		// TODO(gri) do we need this for Alias types?
+		var newTArgs []Type
+		if t.TypeArgs().Len() != n {
+			return Typ[Invalid] // error reported elsewhere
+		}
+
+		// already instantiated
+		// For each (existing) type argument targ, determine if it needs
+		// to be substituted; i.e., if it is or contains a type parameter
+		// that has a type argument for it.
+		for i, targ := range t.TypeArgs().list() {
+			new_targ := subst.typ(targ)
+			if new_targ != targ {
+				if newTArgs == nil {
+					newTArgs = make([]Type, n)
+					copy(newTArgs, t.TypeArgs().list())
+				}
+				newTArgs[i] = new_targ
+			}
+		}
+
+		if newTArgs == nil {
+			return t // nothing to substitute
+		}
+
+		return subst.check.newAliasInstance(subst.pos, t.orig, newTArgs, subst.ctxt)
+
 	case *Array:
 		elem := subst.typOrNil(t.elem)
 		if elem != t.elem {

src/go/types/typestring.go

@@ -338,6 +338,10 @@ func (w *typeWriter) typ(typ Type) {
 
 	case *Alias:
 		w.typeName(t.obj)
+		if list := t.targs.list(); len(list) != 0 {
+			// instantiated type
+			w.typeList(list)
+		}
 		if w.ctxt != nil {
 			// TODO(gri) do we need to print the alias type name, too?
 			w.typ(Unalias(t.obj.typ))

src/go/types/typexpr.go

@@ -443,6 +443,10 @@ func (check *Checker) instantiatedType(ix *typeparams.IndexExpr, def *TypeName)
 		}()
 	}
 
+	defer func() {
+		setDefType(def, res)
+	}()
+
 	var cause string
 	gtyp := check.genericType(ix.X, &cause)
 	if cause != "" {
@@ -452,21 +456,23 @@ func (check *Checker) instantiatedType(ix *typeparams.IndexExpr, def *TypeName)
 		return gtyp // error already reported
 	}
 
+	// evaluate arguments
+	targs := check.typeList(ix.Indices)
+	if targs == nil {
+		return Typ[Invalid]
+	}
+
+	if orig, _ := gtyp.(*Alias); orig != nil {
+		return check.instance(ix.Pos(), orig, targs, nil, check.context())
+	}
+
 	orig := asNamed(gtyp)
 	if orig == nil {
 		panic(fmt.Sprintf("%v: cannot instantiate %v", ix.Pos(), gtyp))
 	}
 
-	// evaluate arguments
-	targs := check.typeList(ix.Indices)
-	if targs == nil {
-		setDefType(def, Typ[Invalid]) // avoid errors later due to lazy instantiation
-		return Typ[Invalid]
-	}
-
 	// create the instance
 	inst := asNamed(check.instance(ix.Pos(), orig, targs, nil, check.context()))
-	setDefType(def, inst)
 
 	// orig.tparams may not be set up, so we need to do expansion later.
 	check.later(func() {

src/go/types/util.go

@@ -33,7 +33,7 @@ func hasDots(call *ast.CallExpr) bool { return call.Ellipsis.IsValid() }
 // dddErrPos returns the positioner for reporting an invalid ... use in a call.
 func dddErrPos(call *ast.CallExpr) positioner { return atPos(call.Ellipsis) }
 
-// argErrPos returns positioner for reportign an invalid argument count.
+// argErrPos returns positioner for reporting an invalid argument count.
 func argErrPos(call *ast.CallExpr) positioner { return inNode(call, call.Rparen) }
 
 // startPos returns the start position of node n.

src/internal/types/testdata/spec/typeAliases1.23b.go

@@ -28,14 +28,20 @@ type _[P any, Q int] = RHS[P, Q]
 type _[P int | float64] = RHS[P, int]
 type _[P, Q any] = RHS[P, Q /* ERROR "Q does not satisfy ~int" */]
 
-// ----------------------------------------------------------------------------
-// NOTE: The code below does now work yet.
-// TODO: Implement this.
-
 // A generic type alias may be used like any other generic type.
 type A[P any] = RHS[P, int]
 
-func _(a A /* ERROR "not a generic type" */ [string]) {
+func _(a A[string]) {
 	a.p = "foo"
 	a.q = 42
 }
+
+// A generic alias may refer to another generic alias.
+type B[P any] = A[P]
+
+func _(a B[string]) {
+	a.p = "foo"
+	a.q = 42
+	// error messages print the instantiated alias type
+	a.r /* ERROR "a.r undefined (type B[string] has no field or method r)" */ = 0
+}