go/types, types2: be sure to type-check wrong methods in missingMethod

In the case of a wrong method, we were not ensuring that it was
type-checked before passing it to funcString.

Formatting the missing method error message requires a fully set-up
signature.

Fixes #59848

Change-Id: I1467e036afbbbdd00899bfd627a945500dc709c2
Reviewed-on: https://go-review.googlesource.com/c/go/+/494615
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Robert Findley <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>

Author: findleyr

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

@@ -387,6 +387,10 @@ func (check *Checker) missingMethod(V, T Type, static bool, equivalent func(x, y
 					obj, _, _ = lookupFieldOrMethodImpl(V, false, m.pkg, m.name, true /* fold case */)
 					f, _ = obj.(*Func)
 					if f != nil {
+						// This method is formatted in funcString below, so must be type-checked.
+						if check != nil {
+							check.objDecl(f, nil)
+						}
 						state = wrongName
 					}
 				}

src/go/types/lookup.go

@@ -0,0 +1,10 @@
+// Copyright 2023 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 struct{}
+type I interface{ M() }
+var _ I = T /* ERROR "missing method M" */ {} // must not crash
+func (T) m() {}