internal/analysisinternal: check for interface recievers

typesinternal.ReceiverNamed(R) may return nil on a interface
method receiver R. Document this, and update
analysisinternal.IsMethodNamed to allow for this this.

Fixes golang/go#70399

Change-Id: I4f0542f2f9495cc373ad2d772736152b29dcc254
Cq-Include-Trybots: luci.golang.try:x_tools-gotip-linux-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/tools/+/641177
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: timothy-king

internal/analysisinternal/analysis.go

@@ -336,7 +336,8 @@ func IsMethodNamed(obj types.Object, pkgPath string, typeName string, names ...s
 	if fn, ok := obj.(*types.Func); ok {
 		if recv := fn.Type().(*types.Signature).Recv(); recv != nil {
 			_, T := typesinternal.ReceiverNamed(recv)
-			return IsTypeNamed(T, pkgPath, typeName) &&
+			return T != nil &&
+				IsTypeNamed(T, pkgPath, typeName) &&
 				slices.Contains(names, fn.Name())
 		}
 	}

internal/typesinternal/recv.go

@@ -12,7 +12,8 @@ import (
 // type of recv, which may be of the form N or *N, or aliases thereof.
 // It also reports whether a Pointer was present.
 //
-// The named result may be nil in ill-typed code.
+// The named result may be nil if recv is from a method on an
+// anonymous interface or struct types or in ill-typed code.
 func ReceiverNamed(recv *types.Var) (isPtr bool, named *types.Named) {
 	t := recv.Type()
 	if ptr, ok := types.Unalias(t).(*types.Pointer); ok {