diff --git a/src/go/types/builtins.go b/src/go/types/builtins.go
index 65d760f3b7..a81bdc70b4 100644
--- a/src/go/types/builtins.go
+++ b/src/go/types/builtins.go
@@ -265,7 +265,21 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 		}
 
 		// the argument types must be of floating-point type
-		if !isFloat(x.typ) {
+		f := func(x Type) Type {
+			if t, _ := x.Underlying().(*Basic); t != nil {
+				switch t.kind {
+				case Float32:
+					return Typ[Complex64]
+				case Float64:
+					return Typ[Complex128]
+				case UntypedFloat:
+					return Typ[UntypedComplex]
+				}
+			}
+			return nil
+		}
+		resTyp := applyTypeFunc(f, x.typ)
+		if resTyp == nil {
 			check.invalidArg(x.pos(), "arguments have type %s, expected floating-point", x.typ)
 			return
 		}
@@ -277,20 +291,6 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 			x.mode = value
 		}
 
-		// determine result type
-		var res BasicKind
-		switch x.typ.Underlying().(*Basic).kind {
-		case Float32:
-			res = Complex64
-		case Float64:
-			res = Complex128
-		case UntypedFloat:
-			res = UntypedComplex
-		default:
-			unreachable()
-		}
-		resTyp := Typ[res]
-
 		if check.Types != nil && x.mode != constant_ {
 			check.recordBuiltinType(call.Fun, makeSig(resTyp, x.typ, x.typ))
 		}
@@ -383,7 +383,21 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 		}
 
 		// the argument must be of complex type
-		if !isComplex(x.typ) {
+		f := func(x Type) Type {
+			if t, _ := x.Underlying().(*Basic); t != nil {
+				switch t.kind {
+				case Complex64:
+					return Typ[Float32]
+				case Complex128:
+					return Typ[Float64]
+				case UntypedComplex:
+					return Typ[UntypedFloat]
+				}
+			}
+			return nil
+		}
+		resTyp := applyTypeFunc(f, x.typ)
+		if resTyp == nil {
 			check.invalidArg(x.pos(), "argument has type %s, expected complex type", x.typ)
 			return
 		}
@@ -399,20 +413,6 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 			x.mode = value
 		}
 
-		// determine result type
-		var res BasicKind
-		switch x.typ.Underlying().(*Basic).kind {
-		case Complex64:
-			res = Float32
-		case Complex128:
-			res = Float64
-		case UntypedComplex:
-			res = UntypedFloat
-		default:
-			unreachable()
-		}
-		resTyp := Typ[res]
-
 		if check.Types != nil && x.mode != constant_ {
 			check.recordBuiltinType(call.Fun, makeSig(resTyp, x.typ))
 		}
@@ -647,6 +647,51 @@ func (check *Checker) builtin(x *operand, call *ast.CallExpr, id builtinId) (_ b
 	return true
 }
 
+// applyTypeFunc applies f to x. If x is a type parameter,
+// the result is an anonymous type parameter constrained by
+// an unnamed contract. The type constraints of that contract
+// are computed by applying f to each of the contract type
+// constraints of x. If any of these applications of f return
+// nil, applyTypeFunc returns nil.
+// If x is not a type parameter, the result is f(x).
+func applyTypeFunc(f func(Type) Type, x Type) Type {
+	if tp, _ := x.Underlying().(*TypeParam); tp != nil {
+		// Test if t satisfies the requirements for the argument
+		// type and collect possible result types at the same time.
+		var resTypes []Type
+		if !tp.Interface().is(func(x Type) bool {
+			if r := f(x); r != nil {
+				resTypes = append(resTypes, r)
+				return true
+			}
+			return false
+		}) {
+			return nil
+		}
+
+		// TODO(gri) Would it be ok to return just the one type
+		//           if len(resType) == 1? What about top-level
+		//           uses of real() where the result is used to
+		//           define type and initialize a variable?
+
+		// construct a suitable new type parameter
+		tpar := NewTypeName(token.NoPos, nil /* = Universe pkg */, "<type parameter literal>", nil)
+		resTyp := NewTypeParam(tpar, 0, nil) // assigns type to tpar as a side-effect
+
+		// construct a corresponding interface and contract
+		iface := &Interface{allMethods: markComplete, types: resTypes}
+		contr := &Contract{
+			TParams: []*TypeName{tpar},
+			IFaces:  map[*TypeName]*Interface{tpar: iface},
+		}
+		resTyp.contr = contr
+
+		return resTyp
+	}
+
+	return f(x)
+}
+
 // makeSig makes a signature for the given argument and result types.
 // Default types are used for untyped arguments, and res may be nil.
 func makeSig(res Type, args ...Type) *Signature {
diff --git a/src/go/types/examples/contracts.go2 b/src/go/types/examples/contracts.go2
index 009a504f6c..cd199f0c4c 100644
--- a/src/go/types/examples/contracts.go2
+++ b/src/go/types/examples/contracts.go2
@@ -3,7 +3,6 @@
 // license that can be found in the LICENSE file.
 
 // This file shows examples of contract declarations.
-// They are not type-checked at the moment.
 
 package p
 
diff --git a/src/go/types/predicates.go b/src/go/types/predicates.go
index 5f5e14f75d..f01ea780d5 100644
--- a/src/go/types/predicates.go
+++ b/src/go/types/predicates.go
@@ -21,7 +21,7 @@ func is(typ Type, what BasicInfo) bool {
 	case *Basic:
 		return t.info&what != 0
 	case *TypeParam:
-		return t.contr.ifaceAt(t.index).is(func(typ Type) bool { return is(typ, what) })
+		return t.Interface().is(func(typ Type) bool { return is(typ, what) })
 	}
 	return false
 }
diff --git a/src/go/types/testdata/contracts.go2 b/src/go/types/testdata/contracts.go2
index ef128e1115..e83db783fe 100644
--- a/src/go/types/testdata/contracts.go2
+++ b/src/go/types/testdata/contracts.go2
@@ -143,4 +143,21 @@ func sum(type T Integer)(data []T) T {
                 s += x
         }
         return s
-}
\ No newline at end of file
+}
+
+// Contracts and built-ins whose result type depends on the argument type.
+
+contract FloatComplex(F, C) {
+        F float32, float64
+        C complex64, complex128
+}
+
+func _(type F, C FloatComplex)(c C) {
+        _ = real(c)
+        _ = imag(c)
+        re := real(c)
+        im := imag(c)
+        var _ F = re // TODO(gri) why does this work? (no type conversion needed?)
+        var _ F = im // TODO(gri) why does this work? (no type conversion needed?)
+        c = C(complex(re, im))
+}
diff --git a/src/go/types/type.go b/src/go/types/type.go
index 0b50fd6f31..6883c37b8d 100644
--- a/src/go/types/type.go
+++ b/src/go/types/type.go
@@ -521,6 +521,9 @@ type Parameterized struct {
 }
 
 // A Contract represents a contract.
+// TODO(gri) Do we need the ability to represent unnamed type parameter literals?
+//           For instance, when creating (result) type parameters out of whole cloth
+//           say for the result type of real/imag(x) where x is of a parameterized type.
 type Contract struct {
 	TParams []*TypeName
 	IFaces  map[*TypeName]*Interface
@@ -559,6 +562,7 @@ func NewTypeParam(obj *TypeName, index int, contr *Contract) *TypeParam {
 // Interface returns the type parameter's interface as
 // specified via its contract. If there is no contract,
 // the result is the empty interface.
+// TODO(gri) should this be Underlying instead?
 func (t *TypeParam) Interface() *Interface {
 	return t.contr.ifaceAt(t.index)
 }
@@ -578,7 +582,7 @@ func (c *Chan) Underlying() Type          { return c }
 func (t *Named) Underlying() Type         { return t.underlying }
 func (p *Parameterized) Underlying() Type { return p.tname.typ.Underlying() }
 func (c *Contract) Underlying() Type      { return c }
-func (t *TypeParam) Underlying() Type     { return t }
+func (t *TypeParam) Underlying() Type     { return t } // TODO(gri) should this return t.Interface() instead?
 
 func (b *Basic) String() string         { return TypeString(b, nil) }
 func (a *Array) String() string         { return TypeString(a, nil) }