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
diff --git a/src/cmd/gc/typecheck.c b/src/cmd/gc/typecheck.c
index f21f07f..eeb46e2 100644
--- a/src/cmd/gc/typecheck.c
+++ b/src/cmd/gc/typecheck.c
@@ -735,14 +735,20 @@
}
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;
diff --git a/test/interface/explicit.go b/test/interface/explicit.go
index 7822b88..eb81156 100644
--- a/test/interface/explicit.go
+++ b/test/interface/explicit.go
@@ -15,6 +15,10 @@
var t *T
+type X int
+
+func (x *X) M() {}
+
type I interface {
M()
}
@@ -66,6 +70,8 @@
var _ = m.(Int) // ERROR "impossible type assertion"
+var _ = m.(X) // ERROR "pointer receiver"
+
var ii int
var jj Int
