go/types, types2: report an error for x.sel where x is a built-in

In case of a selector expression x.sel where x is a built-in
we didn't report an error because the type of built-ins is
invalid and we surpress errors on operands of invalid types,
assuming that an error has been reported before.

Add a corresponding check for this case.

Review all places where we call Checker.exprOrType to ensure
(invalid) built-ins are reported.

Adjusted position for index error in types2.

Fixes #51360.

Change-Id: I24693819c729994ab79d31de8fa7bd370b3e8469
Reviewed-on: https://go-review.googlesource.com/c/go/+/388054
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -525,7 +525,11 @@ func (check *Checker) selector(x *operand, e *syntax.SelectorExpr) {
 	}
 
 	check.exprOrType(x, e.X, false)
-	if x.mode == invalid {
+	switch x.mode {
+	case builtin:
+		check.errorf(e.Pos(), "cannot select on %s", x)
+		goto Error
+	case invalid:
 		goto Error
 	}
 

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

@@ -182,7 +182,7 @@ func (check *Checker) indexExpr(x *operand, e *syntax.IndexExpr) (isFuncInst boo
 	}
 
 	if !valid {
-		check.errorf(x, invalidOp+"cannot index %s", x)
+		check.errorf(e.Pos(), invalidOp+"cannot index %s", x)
 		x.mode = invalid
 		return false
 	}

src/cmd/compile/internal/types2/testdata/fixedbugs/issue51360.go

@@ -0,0 +1,13 @@
+// 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
+
+func _() {
+	len. /* ERROR cannot select on len */ Println
+	len. /* ERROR cannot select on len */ Println()
+	_ = len. /* ERROR cannot select on len */ Println
+	_ = len[ /* ERROR cannot index len */ 0]
+	_ = *len /* ERROR cannot indirect len */
+}

src/go/types/call.go

@@ -527,7 +527,12 @@ func (check *Checker) selector(x *operand, e *ast.SelectorExpr) {
 	}
 
 	check.exprOrType(x, e.X, false)
-	if x.mode == invalid {
+	switch x.mode {
+	case builtin:
+		// types2 uses the position of '.' for the error
+		check.errorf(e.Sel, _UncalledBuiltin, "cannot select on %s", x)
+		goto Error
+	case invalid:
 		goto Error
 	}
 

src/go/types/index.go

@@ -183,6 +183,7 @@ func (check *Checker) indexExpr(x *operand, e *typeparams.IndexExpr) (isFuncInst
 	}
 
 	if !valid {
+		// types2 uses the position of '[' for the error
 		check.invalidOp(x, _NonIndexableOperand, "cannot index %s", x)
 		x.mode = invalid
 		return false

src/go/types/testdata/fixedbugs/issue51360.go

@@ -0,0 +1,13 @@
+// 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
+
+func _() {
+	len.Println /* ERROR cannot select on len */
+	len.Println /* ERROR cannot select on len */ ()
+	_ = len.Println /* ERROR cannot select on len */
+	_ = len /* ERROR cannot index len */ [0]
+	_ = *len /* ERROR cannot indirect len */
+}