go/types, types2: better error if there's a field with the name of a missing method

Fixes #51025.

Change-Id: I469a705e7da059e7ac0b12b05beb9ed5d3617396
Reviewed-on: https://go-review.googlesource.com/c/go/+/438856
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>

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

@@ -395,6 +395,11 @@ func (check *Checker) missingMethodCause(V, T Type, m, alt *Func) string {
 		return "(" + check.interfacePtrError(T) + ")"
 	}
 
+	obj, _, _ := lookupFieldOrMethod(V, true /* auto-deref */, m.pkg, m.name, false)
+	if fld, _ := obj.(*Var); fld != nil {
+		return check.sprintf("(%s.%s is a field, not a method)", V, fld.Name())
+	}
+
 	return check.sprintf("(missing %s)", mname)
 }
 

src/go/types/lookup.go

@@ -395,6 +395,11 @@ func (check *Checker) missingMethodCause(V, T Type, m, alt *Func) string {
 		return "(" + check.interfacePtrError(T) + ")"
 	}
 
+	obj, _, _ := lookupFieldOrMethod(V, true /* auto-deref */, m.pkg, m.name, false)
+	if fld, _ := obj.(*Var); fld != nil {
+		return check.sprintf("(%s.%s is a field, not a method)", V, fld.Name())
+	}
+
 	return check.sprintf("(missing %s)", mname)
 }
 

src/internal/types/testdata/fixedbugs/issue51025.go

@@ -0,0 +1,38 @@
+// 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
+
+var _ interface{ m() } = struct /* ERROR m is a field, not a method */ {
+	m func()
+}{}
+
+var _ interface{ m() } = & /* ERROR m is a field, not a method */ struct {
+	m func()
+}{}
+
+var _ interface{ M() } = struct /* ERROR missing method M */ {
+	m func()
+}{}
+
+var _ interface{ M() } = & /* ERROR missing method M */ struct {
+	m func()
+}{}
+
+// test case from issue
+type I interface{ m() }
+type T struct{ m func() }
+type M struct{}
+
+func (M) m() {}
+
+func _() {
+	var t T
+	var m M
+	var i I
+
+	i = m
+	i = t // ERROR m is a field, not a method
+	_ = i
+}