go/types: mark implicit interfaces as such

This is a straightforward port of CL 353396 to go/types.

For #48424

Change-Id: I3040c2ad3a8c9573026277de01deb9c47953cec8
Reviewed-on: https://go-review.googlesource.com/c/go/+/354991
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/decl.go

@@ -715,11 +715,13 @@ func (check *Checker) bound(x ast.Expr) Type {
 		wrap = op.Op == token.OR
 	}
 	if wrap {
-		// TODO(gri) Should mark this interface as "implicit" somehow
-		//           (and propagate the info to types2.Interface) so
-		//           that we can elide the interface again in error
-		//           messages. Could use a sentinel name for the field.
 		x = &ast.InterfaceType{Methods: &ast.FieldList{List: []*ast.Field{{Type: x}}}}
+		t := check.typ(x)
+		// mark t as implicit interface if all went well
+		if t, _ := t.(*Interface); t != nil {
+			t.implicit = true
+		}
+		return t
 	}
 	return check.typ(x)
 }

src/go/types/interface.go

@@ -19,6 +19,7 @@ type Interface struct {
 	methods   []*Func      // ordered list of explicitly declared methods
 	embeddeds []Type       // ordered list of explicitly embedded elements
 	embedPos  *[]token.Pos // positions of embedded elements; or nil (for error messages) - use pointer to save space
+	implicit  bool         // interface is wrapper for type set literal (non-interface T, ~T, or A|B)
 	complete  bool         // indicates that obj, methods, and embeddeds are set and type set can be computed
 
 	tset *_TypeSet // type set described by this interface, computed lazily
@@ -108,6 +109,9 @@ func (t *Interface) IsComparable() bool { return t.typeSet().IsComparable() }
 // set.
 func (t *Interface) IsMethodSet() bool { return t.typeSet().IsMethodSet() }
 
+// IsImplicit reports whether the interface t is a wrapper for a type set literal.
+func (t *Interface) IsImplicit() bool { return t.implicit }
+
 // Complete computes the interface's type set. It must be called by users of
 // NewInterfaceType and NewInterface after the interface's embedded types are
 // fully defined and before using the interface type in any way other than to
@@ -143,7 +147,6 @@ func (check *Checker) interfaceType(ityp *Interface, iface *ast.InterfaceType, d
 
 	for _, f := range iface.Methods.List {
 		if len(f.Names) == 0 {
-			// We have an embedded type; possibly a union of types.
 			addEmbedded(f.Type.Pos(), parseUnion(check, flattenUnion(nil, f.Type)))
 			continue
 		}

src/go/types/testdata/examples/typesets.go2

@@ -46,3 +46,13 @@ func _() *int {
 // A type parameter may not be embedded in an interface;
 // so it can also not be used as a constraint.
 func _[A any, B A /* ERROR cannot use a type parameter as constraint */ ]() {}
+
+// Error messages refer to the type constraint as it appears in the source.
+// (No implicit interface should be exposed.)
+func _[T string](x T) T {
+	return x /* ERROR constrained by string */ * x
+}
+
+func _[T int|string](x T) T {
+	return x /* ERROR constrained by int|string */ * x
+}

src/go/types/typeparam.go

@@ -108,6 +108,7 @@ func (t *TypeParam) iface() *Interface {
 	// TODO(gri) mark it as implicit - see comment in Checker.bound
 	if ityp == nil {
 		ityp = NewInterfaceType(nil, []Type{bound})
+		ityp.implicit = true
 		t.bound = ityp // update t.bound for next time (optimization)
 	}
 

src/go/types/typestring.go

@@ -191,6 +191,15 @@ func (w *typeWriter) typ(typ Type) {
 		}
 
 	case *Interface:
+		if t.implicit {
+			if len(t.methods) == 0 && len(t.embeddeds) == 1 {
+				w.typ(t.embeddeds[0])
+				break
+			}
+			// Something's wrong with the implicit interface.
+			// Print it as such and continue.
+			w.string("/* implicit */ ")
+		}
 		w.string("interface{")
 		first := true
 		for _, m := range t.methods {

src/go/types/universe.go

@@ -89,7 +89,7 @@ func defPredeclaredTypes() {
 		res := NewVar(token.NoPos, nil, "", Typ[String])
 		sig := NewSignatureType(nil, nil, nil, nil, NewTuple(res), false)
 		err := NewFunc(token.NoPos, nil, "Error", sig)
-		ityp := &Interface{nil, obj, []*Func{err}, nil, nil, true, nil}
+		ityp := &Interface{nil, obj, []*Func{err}, nil, nil, false, true, nil}
 		computeInterfaceTypeSet(nil, token.NoPos, ityp) // prevent races due to lazy computation of tset
 		typ := NewNamed(obj, ityp, nil)
 		sig.recv = NewVar(token.NoPos, nil, "", typ)
@@ -100,7 +100,7 @@ func defPredeclaredTypes() {
 	{
 		obj := NewTypeName(token.NoPos, nil, "comparable", nil)
 		obj.setColor(black)
-		ityp := &Interface{nil, obj, nil, nil, nil, true, &_TypeSet{true, nil, allTermlist}}
+		ityp := &Interface{nil, obj, nil, nil, nil, false, true, &_TypeSet{true, nil, allTermlist}}
 		NewNamed(obj, ityp, nil)
 		def(obj)
 	}