c++: Reject alias CTAD in C++17 [PR99008]

Here, in C++17 mode, we only pedwarn about the use of alias CTAD and
then later ICE from alias_ctad_tweaks when attempting to constrain
the guides.  Since the construction of the guides of an alias template
effectively relies on concepts, we shouldn't be permissive about alias
CTAD in C++17 mode, so this patch turns the pertinent pedwarn in
do_class_deduction into an error.

In order to get a consistent diagnostic for B() vs the other forms in
the added testcase, I had to remove the special handling of CTAD with
empty initializer in build_functional_cast_1 so that we always pass
'complain' to do_auto_deduction.

gcc/cp/ChangeLog:

	PR c++/99008
	* pt.c (do_class_deduction): Reject alias CTAD in C++17 mode
	rather than issuing a pedwarn.
	* typeck2.c (build_functional_cast_1): Handle CTAD uniformly
	for consistent diagnostics.

gcc/testsuite/ChangeLog:

	PR c++/99008
	* g++.dg/parse/template2.C: Adjust expected diagnostic.
	* g++.dg/template/error8.C: Likewise.
	* g++.dg/cpp1z/class-deduction84.C: New test.

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 6b63edda60d26a34f2507f3feec979c17f2a46b7..106dfe556f7fd7c9ff2af2cbfbf7f9254316b3cf 100644 (file)
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -29305,10 +29305,10 @@ do_class_deduction (tree ptype, tree tmpl, tree init,
     }
   else if (cxx_dialect < cxx20 && DECL_ALIAS_TEMPLATE_P (tmpl))
     {
-      /* This doesn't affect conforming C++17 code, so just pedwarn.  */
-      if (complain & tf_warning_or_error)
-       pedwarn (input_location, 0, "alias template deduction only available "
-                "with %<-std=c++20%> or %<-std=gnu++20%>");
+      if (complain & tf_error)
+       error ("alias template deduction only available "
+              "with %<-std=c++20%> or %<-std=gnu++20%>");
+      return error_mark_node;
     }
 
   tree type = TREE_TYPE (tmpl);

diff --git a/gcc/cp/typeck2.c b/gcc/cp/typeck2.c
index a58d397ca3fd8e24c5ea6f44934decc61c802af4..4e9632f6a7d4279653859d1b66bbab3cf5b6728e 100644 (file)
--- a/gcc/cp/typeck2.c
+++ b/gcc/cp/typeck2.c
@@ -2197,24 +2197,13 @@ build_functional_cast_1 (location_t loc, tree exp, tree parms,
            error_at (loc, "invalid use of %qT", anode);
          return error_mark_node;
        }
-      else if (!parms)
+      else
        {
-         /* Even if there are no parameters, we might be able to deduce from
-            default template arguments.  Pass TF_NONE so that we don't
-            generate redundant diagnostics.  */
-         type = do_auto_deduction (type, parms, anode, tf_none,
+         type = do_auto_deduction (type, parms, anode, complain,
                                    adc_variable_type);
          if (type == error_mark_node)
-           {
-             if (complain & tf_error)
-               error_at (loc, "cannot deduce template arguments "
-                         "for %qT from %<()%>", anode);
-             return error_mark_node;
-           }
+           return error_mark_node;
        }
-      else
-       type = do_auto_deduction (type, parms, anode, complain,
-                                 adc_variable_type);
     }
 
   if (processing_template_decl)

diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction84.C b/gcc/testsuite/g++.dg/cpp1z/class-deduction84.C
new file mode 100644 (file)
index 0000000..29f25e5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction84.C
@@ -0,0 +1,9 @@
+// PR c++/99008
+// { dg-do compile { target c++17 } }
+
+template <class> struct A { A(int = 0); };
+template <class = void> using B = A<int>;
+auto x = B{};  // { dg-error "alias template deduction only available with" "" { target c++17_only } }
+auto y = B();  // { dg-error "alias template deduction only available with" "" { target c++17_only } }
+auto z = B{1}; // { dg-error "alias template deduction only available with" "" { target c++17_only } }
+auto w = B(1); // { dg-error "alias template deduction only available with" "" { target c++17_only } }

diff --git a/gcc/testsuite/g++.dg/parse/template2.C b/gcc/testsuite/g++.dg/parse/template2.C
index 3cb27a85c007df09b97363e7b721f781862df664..3cafd9f1f01a38700b7b646c32bd1be44c2899a1 100644 (file)
--- a/gcc/testsuite/g++.dg/parse/template2.C
+++ b/gcc/testsuite/g++.dg/parse/template2.C
@@ -3,6 +3,6 @@ namespace N {
 }
 
 int main() {
-  N::C(); // { dg-error "6:cannot deduce template arguments" "" { target c++17 } }
+  N::C(); // { dg-error "8:class template argument deduction failed|no match" "" { target c++17 } }
   // { dg-error "7:missing template arguments" "" { target c++14_down } .-1 }
 }

diff --git a/gcc/testsuite/g++.dg/template/error8.C b/gcc/testsuite/g++.dg/template/error8.C
index 6cae360aa161ac65c9c3a81ecde6d39d6d7d8ac2..de1534cc6373ab9bbb83649c64ac468bc73799aa 100644 (file)
--- a/gcc/testsuite/g++.dg/template/error8.C
+++ b/gcc/testsuite/g++.dg/template/error8.C
@@ -3,6 +3,6 @@
 template <typename T> struct S {};
 
 void f() {
-  throw S (); // { dg-error "9:cannot deduce template arguments" "" { target c++17 } }
+  throw S (); // { dg-error "12:class template argument deduction failed|no match" "" { target c++17 } }
   // { dg-error "11:missing template arguments" "" { target c++14_down } .-1 }
 }