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>

1 files changed, 5 insertions, 2 deletions

diff --git a/src/go/types/expr.go b/src/go/types/expr.go
index f88b2389c6..d1e892a9b7 100644
--- a/src/go/types/expr.go
+++ b/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
 	}