C++ PATCH to improve diagnostic for non-type template parameters
Marek Polacek
polacek@redhat.com
Fri May 10 23:10:00 GMT 2019
When we have
template<auto>
struct S { };
then in
S<int()> s;
"int()" is resolved to a type-id, as per [temp.arg]/2, causing this program to
fail to compile. This can be rather confusing so I think we want to improve the
diagnostic a bit.
Bootstrapped/regtested on x86_64-linux, ok for trunk?
2019-05-10 Marek Polacek <polacek@redhat.com>
* pt.c (convert_template_argument): Add a diagnostic for the
[temp.arg]/2 ambiguity case.
* g++.dg/cpp2a/nontype-class17.C: New test.
diff --git gcc/cp/pt.c gcc/cp/pt.c
index 08da94ae0c9..b38e65d7f7e 100644
--- gcc/cp/pt.c
+++ gcc/cp/pt.c
@@ -7961,10 +7961,22 @@ convert_template_argument (tree parm,
"parameter list for %qD",
i + 1, in_decl);
if (is_type)
- inform (input_location,
- " expected a constant of type %qT, got %qT",
- TREE_TYPE (parm),
- (DECL_P (arg) ? DECL_NAME (arg) : orig_arg));
+ {
+ /* The template argument is a type, but we're expecting
+ an expression. */
+ inform (input_location,
+ " expected a constant of type %qT, got %qT",
+ TREE_TYPE (parm),
+ (DECL_P (arg) ? DECL_NAME (arg) : orig_arg));
+ /* [temp.arg]/2: "In a template-argument, an ambiguity
+ between a type-id and an expression is resolved to a
+ type-id, regardless of the form of the corresponding
+ template-parameter." So give the user a clue. */
+ if (TREE_CODE (arg) == FUNCTION_TYPE)
+ inform (input_location, " template argument for "
+ "non-type template parameter is treated as "
+ "function type");
+ }
else if (requires_tmpl_type)
inform (input_location,
" expected a class template, got %qE", orig_arg);
diff --git gcc/testsuite/g++.dg/cpp2a/nontype-class17.C gcc/testsuite/g++.dg/cpp2a/nontype-class17.C
new file mode 100644
index 00000000000..ca5f68e1611
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/nontype-class17.C
@@ -0,0 +1,17 @@
+// { dg-do compile { target c++2a } }
+
+template<auto>
+struct S { };
+
+struct R { };
+
+void
+g (void)
+{
+ S<R()> s; // { dg-error "mismatch" }
+// { dg-message "treated as function" "note" { target *-*-* } .-1 }
+ S<R{}> s2;
+ S<int()> s3; // { dg-error "mismatch" }
+// { dg-message "treated as function" "note" { target *-*-* } .-1 }
+ S<int{}> s4;
+}
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