[dev.go2go] go/types: don't panic if receiver type arguments cannot be inferred

This can happen due to earlier errors. This change avoids the panic
but reports a follow-up error. Not ideal, but good enough for now.

Fixes #40056.

Change-Id: I500d4fc2b058cdc70f28883ff8d004df4b43fe4e
Reviewed-on: https://go-review.googlesource.com/c/go/+/241130
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/call.go

@@ -522,7 +522,7 @@ func (check *Checker) selector(x *operand, e *ast.SelectorExpr) {
 			// The method may have a pointer receiver, but the actually provided receiver
 			// may be a (hopefully addressable) non-pointer value, or vice versa. Here we
 			// only care about inferring receiver type parameters; to make the inferrence
-			// work, match up pointer-ness of reveiver and argument.
+			// work, match up pointer-ness of receiver and argument.
 			arg := x
 			if ptrRecv := isPointer(sig.recv.typ); ptrRecv != isPointer(arg.typ) {
 				copy := *arg
@@ -533,12 +533,13 @@ func (check *Checker) selector(x *operand, e *ast.SelectorExpr) {
 				}
 				arg = &copy
 			}
-			targs := check.infer(sig.recv.pos, sig.rparams, NewTuple(sig.recv), []*operand{arg})
+			targs := check.infer(sig.rparams[0].Pos(), sig.rparams, NewTuple(sig.recv), []*operand{arg})
 			//check.dump("### inferred targs = %s", targs)
-			if len(targs) == 0 {
-				// TODO(gri) Provide explanation as to why we can't possibly
-				//           reach here (consider invalid receivers, etc.).
-				panic("internal error: receiver type parameter inference failed")
+			if targs == nil {
+				// We may reach here if there were other errors (see issue #40056).
+				// check.infer will report a follow-up error.
+				// TODO(gri) avoid the follow-up error or provide better explanation.
+				goto Error
 			}
 			// Don't modify m. Instead - for now - make a copy of m and use that instead.
 			// (If we modify m, some tests will fail; possibly because the m is in use.)

src/go/types/fixedbugs/issue40056.go2

@@ -0,0 +1,15 @@
+// Copyright 2020 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 _() {
+	NewS() /* ERROR cannot infer T */ .M()
+}
+
+type S struct {}
+
+func NewS(type T)() *S
+
+func (_ *S /* ERROR S is not a generic type */ (T /* ERROR cannot infer T */ )) M()