go/types, types2: don't crash in selectors referring to the type being declared

In Checker.typInternal, the SelectorExpr case was the only case that
didn't either set or pass along the incoming def *Named type.

Handle this by passing it along to Checker.selector and report a
cycle if one is detected.

Fixes #51509.

Change-Id: I6c2d46835f225aeb4cb25fe0ae55f6180cef038b
Reviewed-on: https://go-review.googlesource.com/c/go/+/390314
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -423,7 +423,7 @@ var cgoPrefixes = [...]string{
 	"_Cmacro_", // function to evaluate the expanded expression
 }
 
-func (check *Checker) selector(x *operand, e *syntax.SelectorExpr) {
+func (check *Checker) selector(x *operand, e *syntax.SelectorExpr, def *Named) {
 	// these must be declared before the "goto Error" statements
 	var (
 		obj      Object
@@ -526,6 +526,12 @@ func (check *Checker) selector(x *operand, e *syntax.SelectorExpr) {
 
 	check.exprOrType(x, e.X, false)
 	switch x.mode {
+	case typexpr:
+		// don't crash for "type T T.x" (was issue #51509)
+		if def != nil && x.typ == def {
+			check.cycleError([]Object{def.obj})
+			goto Error
+		}
 	case builtin:
 		check.errorf(e.Pos(), "cannot select on %s", x)
 		goto Error

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

@@ -1556,7 +1556,7 @@ func (check *Checker) exprInternal(x *operand, e syntax.Expr, hint Type) exprKin
 		return kind
 
 	case *syntax.SelectorExpr:
-		check.selector(x, e)
+		check.selector(x, e, nil)
 
 	case *syntax.IndexExpr:
 		if check.indexExpr(x, e) {

src/cmd/compile/internal/types2/testdata/fixedbugs/issue51509.go

@@ -0,0 +1,7 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type T /* ERROR illegal cycle */ T.x

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

@@ -256,7 +256,7 @@ func (check *Checker) typInternal(e0 syntax.Expr, def *Named) (T Type) {
 
 	case *syntax.SelectorExpr:
 		var x operand
-		check.selector(&x, e)
+		check.selector(&x, e, def)
 
 		switch x.mode {
 		case typexpr:

src/go/types/call.go

@@ -429,7 +429,7 @@ var cgoPrefixes = [...]string{
 	"_Cmacro_", // function to evaluate the expanded expression
 }
 
-func (check *Checker) selector(x *operand, e *ast.SelectorExpr) {
+func (check *Checker) selector(x *operand, e *ast.SelectorExpr, def *Named) {
 	// these must be declared before the "goto Error" statements
 	var (
 		obj      Object
@@ -528,6 +528,12 @@ func (check *Checker) selector(x *operand, e *ast.SelectorExpr) {
 
 	check.exprOrType(x, e.X, false)
 	switch x.mode {
+	case typexpr:
+		// don't crash for "type T T.x" (was issue #51509)
+		if def != nil && x.typ == def {
+			check.cycleError([]Object{def.obj})
+			goto Error
+		}
 	case builtin:
 		// types2 uses the position of '.' for the error
 		check.errorf(e.Sel, _UncalledBuiltin, "cannot select on %s", x)

src/go/types/expr.go

@@ -1533,7 +1533,7 @@ func (check *Checker) exprInternal(x *operand, e ast.Expr, hint Type) exprKind {
 		return kind
 
 	case *ast.SelectorExpr:
-		check.selector(x, e)
+		check.selector(x, e, nil)
 
 	case *ast.IndexExpr, *ast.IndexListExpr:
 		ix := typeparams.UnpackIndexExpr(e)

src/go/types/testdata/fixedbugs/issue51509.go

@@ -0,0 +1,7 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+type T /* ERROR illegal cycle */ T.x

src/go/types/typexpr.go

@@ -254,7 +254,7 @@ func (check *Checker) typInternal(e0 ast.Expr, def *Named) (T Type) {
 
 	case *ast.SelectorExpr:
 		var x operand
-		check.selector(&x, e)
+		check.selector(&x, e, def)
 
 		switch x.mode {
 		case typexpr: