c++: Give better placeholder diagnostic

We were saying 'auto parameter not permitted' in a place where 'auto' is in
fact permitted in C++20, but a class template placeholder is not.

gcc/cp/ChangeLog:

	* decl.c (grokdeclarator): Improve diagnostic for
	disallowed CTAD placeholder.

gcc/testsuite/ChangeLog:

	* g++.dg/other/pr88187.C: Adjust expected error.
	* g++.dg/cpp2a/class-deduction-abbrev1.C: New test.

diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 1e2bae4..0cf84a0 100644 (file)
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -13145,6 +13145,16 @@ grokdeclarator (const cp_declarator *declarator,
              if (decl_context == PARM && AUTO_IS_DECLTYPE (auto_node))
                error_at (typespec_loc,
                          "cannot declare a parameter with %<decltype(auto)%>");
+             else if (tree c = CLASS_PLACEHOLDER_TEMPLATE (auto_node))
+               {
+                 auto_diagnostic_group g;
+                 error_at (typespec_loc,
+                           "class template placeholder %qE not permitted "
+                           "in this context", c);
+                 if (decl_context == PARM && cxx_dialect >= cxx20)
+                   inform (typespec_loc, "use %<auto%> for an "
+                           "abbreviated function template");
+               }
              else
                error_at (typespec_loc,
                          "%<auto%> parameter not permitted in this context");

diff --git a/gcc/testsuite/g++.dg/cpp2a/class-deduction-abbrev1.C b/gcc/testsuite/g++.dg/cpp2a/class-deduction-abbrev1.C
new file mode 100644 (file)
index 0000000..f931009
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/class-deduction-abbrev1.C
@@ -0,0 +1,13 @@
+ // { dg-do compile { target c++20 } }
+
+template <class T> struct A { };
+template <class T> concept is_A = requires { A(T()); };
+
+void f(auto);      // OK
+void f(is_A auto); // OK
+void f(A);        // { dg-error "placeholder" }
+
+int main()
+{
+  f(A<int>());
+}

diff --git a/gcc/testsuite/g++.dg/other/pr88187.C b/gcc/testsuite/g++.dg/other/pr88187.C
index ebdafdd..13466d3 100644 (file)
--- a/gcc/testsuite/g++.dg/other/pr88187.C
+++ b/gcc/testsuite/g++.dg/other/pr88187.C
@@ -4,4 +4,4 @@
 template <int> struct A;
 void f (A ()); // { dg-error "6:variable or field 'f' declared void" "" { target c++14_down } }
                // { dg-error "missing template arguments before '\\(' token" "" { target c++14_down } .-1 }
-               // { dg-error "'auto' parameter not permitted in this context" "" { target c++17 } .-2 }
+               // { dg-error "placeholder .A. not permitted in this context" "" { target c++17 } .-2 }