go/types: improve error message for pointer receiver errors

The compiler produces high quality error messages when an interface is
implemented by *T, rather than T. This change improves the analogous
error messages in go/types, from "missing method X" to "missing method
X (X has pointer receiver)".

I am open to improving this message further - I didn't copy the compiler
error message exactly because, at one of the call sites of
(*check).missingMethod, we no longer have access to the name of the
interface.

Fixes golang/go#36336

Change-Id: Ic4fc38b13fff9e5d9a69cc750c21e0b0c34d85a8
Reviewed-on: https://go-review.googlesource.com/c/go/+/229801
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/expr.go

@@ -1566,10 +1566,13 @@ func (check *Checker) typeAssertion(pos token.Pos, x *operand, xtyp *Interface,
 	if method == nil {
 		return
 	}
-
 	var msg string
 	if wrongType != nil {
-		msg = fmt.Sprintf("wrong type for method %s (have %s, want %s)", method.name, wrongType.typ, method.typ)
+		if check.identical(method.typ, wrongType.typ) {
+			msg = fmt.Sprintf("missing method %s (%s has pointer receiver)", method.name, method.name)
+		} else {
+			msg = fmt.Sprintf("wrong type for method %s (have %s, want %s)", method.name, wrongType.typ, method.typ)
+		}
 	} else {
 		msg = "missing method " + method.name
 	}

src/go/types/lookup.go

@@ -271,8 +271,10 @@ func MissingMethod(V Type, T *Interface, static bool) (method *Func, wrongType b
 // The receiver may be nil if missingMethod is invoked through
 // an exported API call (such as MissingMethod), i.e., when all
 // methods have been type-checked.
-// If the type has the correctly names method, but with the wrong
+// If the type has the correctly named method, but with the wrong
 // signature, the existing method is returned as well.
+// To improve error messages, also report the wrong signature
+// when the method exists on *V instead of V.
 func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method, wrongType *Func) {
 	check.completeInterface(T)
 
@@ -302,6 +304,15 @@ func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method,
 	for _, m := range T.allMethods {
 		obj, _, _ := check.rawLookupFieldOrMethod(V, false, m.pkg, m.name)
 
+		// Check if *V implements this method of T.
+		if obj == nil {
+			ptr := NewPointer(V)
+			obj, _, _ = check.rawLookupFieldOrMethod(ptr, false, m.pkg, m.name)
+			if obj != nil {
+				return m, obj.(*Func)
+			}
+		}
+
 		// we must have a method (not a field of matching function type)
 		f, _ := obj.(*Func)
 		if f == nil {

src/go/types/operand.go

@@ -266,7 +266,12 @@ func (x *operand) assignableTo(check *Checker, T Type, reason *string) bool {
 		if m, wrongType := check.missingMethod(V, Ti, true); m != nil /* Implements(V, Ti) */ {
 			if reason != nil {
 				if wrongType != nil {
-					*reason = fmt.Sprintf("wrong type for method %s (have %s, want %s)", m.Name(), wrongType.typ, m.typ)
+					if check.identical(m.typ, wrongType.typ) {
+						*reason = fmt.Sprintf("missing method %s (%s has pointer receiver)", m.name, m.name)
+					} else {
+						*reason = fmt.Sprintf("wrong type for method %s (have %s, want %s)", m.Name(), wrongType.typ, m.typ)
+					}
+
 				} else {
 					*reason = "missing method " + m.Name()
 				}

src/go/types/testdata/issues.src

@@ -130,6 +130,9 @@ func issue10260() {
 		t2 *T2
 	)
 
+	var x I1 = T1 /* ERROR cannot use .*: missing method foo \(foo has pointer receiver\) */ {}
+	_ = x /* ERROR .* cannot have dynamic type T1 \(missing method foo \(foo has pointer receiver\)\) */ .(T1)
+
 	_ = i2 /* ERROR i2 .* cannot have dynamic type \*T1 \(wrong type for method foo \(have func\(\), want func\(x int\)\)\) */ .(*T1)
 
 	i1 = i0 /* ERROR cannot use .* missing method foo */
@@ -355,4 +358,4 @@ func issue35895() {
 	// Because both t1 and t2 have the same global package name (template),
 	// qualify packages with full path name in this case.
 	var _ t1.Template = t2 /* ERROR cannot use .* \(value of type "html/template".Template\) as "text/template".Template */ .Template{}
-}
+}