cmd/compile/internal/types2: mark implicit interfaces as such

Provide an accessor for clients, and don't print the interface
around implicitly wrapped embedded types.

For #48424.

Change-Id: Ib2c76315508fc749ea4337d52e13d17de80e04da
Reviewed-on: https://go-review.googlesource.com/c/go/+/353396
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -652,11 +652,12 @@ func (check *Checker) bound(x syntax.Expr) Type {
 	// embed it in an implicit interface so that only interface type-checking
 	// needs to take care of such type expressions.
 	if op, _ := x.(*syntax.Operation); op != nil && (op.Op == syntax.Tilde || op.Op == syntax.Or) {
-		// 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 = &syntax.InterfaceType{MethodList: []*syntax.Field{{Type: x}}}
+		t := check.typ(&syntax.InterfaceType{MethodList: []*syntax.Field{{Type: 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/cmd/compile/internal/types2/interface.go

@@ -16,6 +16,7 @@ type Interface struct {
 	methods   []*Func       // ordered list of explicitly declared methods
 	embeddeds []Type        // ordered list of explicitly embedded elements
 	embedPos  *[]syntax.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 all fields (except for tset) are set up
 
 	tset *_TypeSet // type set described by this interface, computed lazily
@@ -82,6 +83,9 @@ func (t *Interface) IsComparable() bool { return t.typeSet().IsComparable() }
 // IsMethodSet reports whether the interface t is fully described by its method 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 }
+
 func (t *Interface) Underlying() Type { return t }
 func (t *Interface) String() string   { return TypeString(t, nil) }
 
@@ -102,7 +106,6 @@ func (check *Checker) interfaceType(ityp *Interface, iface *syntax.InterfaceType
 
 	for _, f := range iface.MethodList {
 		if f.Name == nil {
-			// We have an embedded type; possibly a union of types.
 			addEmbedded(posFor(f.Type), parseUnion(check, flattenUnion(nil, f.Type)))
 			continue
 		}

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

@@ -100,9 +100,9 @@ func (t *TypeParam) iface() *Interface {
 	}
 
 	// If we don't have an interface, wrap constraint into an implicit 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/cmd/compile/internal/types2/typestring.go

@@ -190,6 +190,15 @@ 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/cmd/compile/internal/types2/universe.go

@@ -88,7 +88,7 @@ func defPredeclaredTypes() {
 		res := NewVar(nopos, nil, "", Typ[String])
 		sig := NewSignatureType(nil, nil, nil, nil, NewTuple(res), false)
 		err := NewFunc(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, nopos, ityp) // prevent races due to lazy computation of tset
 		typ := NewNamed(obj, ityp, nil)
 		sig.recv = NewVar(nopos, nil, "", typ)
@@ -99,7 +99,7 @@ func defPredeclaredTypes() {
 	{
 		obj := NewTypeName(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)
 	}