cmd/gc: Suggest *T in error for x.(T) if it would work.

Accomplished by synchronizing the formatting of conversion errors between typecheck.c and subr.c

Fixes #3984.

R=golang-dev, remyoudompheng, rsc
CC=golang-dev
https://golang.org/cl/6500064

src/cmd/gc/typecheck.c

@@ -735,14 +735,20 @@ reswitch:
 		}
 		if(n->type != T && n->type->etype != TINTER)
 		if(!implements(n->type, t, &missing, &have, &ptr)) {
-			if(have)
-				yyerror("impossible type assertion: %lN cannot have dynamic type %T"
-					" (wrong type for %S method)\n\thave %S%hT\n\twant %S%hT",
-					l, n->type, missing->sym, have->sym, have->type,
-					missing->sym, missing->type);
+			if(have && have->sym == missing->sym)
+				yyerror("impossible type assertion:\n\t%T does not implement %T (wrong type for %S method)\n"
+					"\t\thave %S%hhT\n\t\twant %S%hhT", n->type, t, missing->sym,
+					have->sym, have->type, missing->sym, missing->type);
+			else if(ptr)
+				yyerror("impossible type assertion:\n\t%T does not implement %T (%S method requires pointer receiver)",
+					n->type, t, missing->sym);
+			else if(have)
+				yyerror("impossible type assertion:\n\t%T does not implement %T (missing %S method)\n"
+					"\t\thave %S%hhT\n\t\twant %S%hhT", n->type, t, missing->sym,
+					have->sym, have->type, missing->sym, missing->type);
 			else
-				yyerror("impossible type assertion: %lN cannot have dynamic type %T"
-					" (missing %S method)", l, n->type, missing->sym);
+				yyerror("impossible type assertion:\n\t%T does not implement %T (missing %S method)",
+					n->type, t, missing->sym);
 			goto error;
 		}
 		goto ret;

test/interface/explicit.go

@@ -15,6 +15,10 @@ type T struct {
 
 var t *T
 
+type X int
+
+func (x *X) M() {}
+
 type I interface {
 	M()
 }
@@ -66,6 +70,8 @@ func (Int) M(float64) {}
 
 var _ = m.(Int) // ERROR "impossible type assertion"
 
+var _ = m.(X) // ERROR "pointer receiver"
+
 var ii int
 var jj Int