[gcc r11-8155] c++: Reject alias CTAD in C++17 [PR99008]

[gcc r11-8155] c++: Reject alias CTAD in C++17 [PR99008]

[Patrick Palka]

https://gcc.gnu.org/g:8913b2c2bcded39427ff27e6dfc276ae8555f6b8

commit r11-8155-g8913b2c2bcded39427ff27e6dfc276ae8555f6b8
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue Apr 13 12:35:33 2021 -0400

    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:
---
 gcc/cp/pt.c                                    |  8 ++++----
 gcc/cp/typeck2.c                               | 17 +++--------------
 gcc/testsuite/g++.dg/cpp1z/class-deduction84.C |  9 +++++++++
 gcc/testsuite/g++.dg/parse/template2.C         |  2 +-
 gcc/testsuite/g++.dg/template/error8.C         |  2 +-
 5 files changed, 18 insertions(+), 20 deletions(-)

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 6b63edda60d..106dfe556f7 100644
--- 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 a58d397ca3f..4e9632f6a7d 100644
--- 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
index 00000000000..29f25e50c9e
--- /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 3cb27a85c00..3cafd9f1f01 100644
--- 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 6cae360aa16..de1534cc637 100644
--- 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 }
 }