[gcc r12-7600] c++: Don't allow type-constraint auto(x) [PR104752]

[Marek Polacek <mpolacek@gcc.gnu.org>]

https://gcc.gnu.org/g:97f76b5fc4b637033229e53033b4f8b6dc23472c

commit r12-7600-g97f76b5fc4b637033229e53033b4f8b6dc23472c
Author: Marek Polacek <polacek@redhat.com>
Date:   Wed Mar 2 12:12:33 2022 -0500

    c++: Don't allow type-constraint auto(x) [PR104752]
    
    104752 points out that
    
      template<class T>
      concept C = true;
      auto y = C auto(1);
    
    is ill-formed as per [dcl.type.auto.deduct]: "For an explicit type conversion,
    T is the specified type, which shall be auto." which doesn't allow
    type-constraint auto.
    
            PR c++/104752
    
    gcc/cp/ChangeLog:
    
            * semantics.cc (finish_compound_literal): Disallow auto{x} for
            is_constrained_auto.
            * typeck2.cc (build_functional_cast_1): Likewise.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp23/auto-fncast12.C: New test.

Diff:
---
 gcc/cp/semantics.cc                        | 8 +++++++-
 gcc/cp/typeck2.cc                          | 8 +++++++-
 gcc/testsuite/g++.dg/cpp23/auto-fncast12.C | 8 ++++++++
 3 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index 799ce943279..21740064d3d 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -3150,7 +3150,13 @@ finish_compound_literal (tree type, tree compound_literal,
 	   && !AUTO_IS_DECLTYPE (type)
 	   && CONSTRUCTOR_NELTS (compound_literal) == 1)
     {
-      if (cxx_dialect < cxx23)
+      if (is_constrained_auto (type))
+	{
+	  if (complain & tf_error)
+	    error ("%<auto{x}%> cannot be constrained");
+	  return error_mark_node;
+	}
+      else if (cxx_dialect < cxx23)
 	pedwarn (input_location, OPT_Wc__23_extensions,
 		 "%<auto{x}%> only available with "
 		 "%<-std=c++2b%> or %<-std=gnu++2b%>");
diff --git a/gcc/cp/typeck2.cc b/gcc/cp/typeck2.cc
index 39d03e4c3c4..a4c825fc34d 100644
--- a/gcc/cp/typeck2.cc
+++ b/gcc/cp/typeck2.cc
@@ -2308,7 +2308,13 @@ build_functional_cast_1 (location_t loc, tree exp, tree parms,
 	       && list_length (parms) == 1)
 	{
 	  init = TREE_VALUE (parms);
-	  if (cxx_dialect < cxx23)
+	  if (is_constrained_auto (anode))
+	    {
+	      if (complain & tf_error)
+		error_at (loc, "%<auto(x)%> cannot be constrained");
+	      return error_mark_node;
+	    }
+	  else if (cxx_dialect < cxx23)
 	    pedwarn (loc, OPT_Wc__23_extensions,
 		     "%<auto(x)%> only available with "
 		     "%<-std=c++2b%> or %<-std=gnu++2b%>");
diff --git a/gcc/testsuite/g++.dg/cpp23/auto-fncast12.C b/gcc/testsuite/g++.dg/cpp23/auto-fncast12.C
new file mode 100644
index 00000000000..2856c2846fc
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/auto-fncast12.C
@@ -0,0 +1,8 @@
+// PR c++/104752
+// { dg-do compile { target c++23 } }
+
+template<class T>
+concept C = true;
+auto x = auto(1);     // valid (P0849R8)
+auto y = C auto(1);   // { dg-error "cannot be constrained" }
+auto z = C auto{1};   // { dg-error "cannot be constrained" }