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

[Marek Polacek via Gcc-patches]

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.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

        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.
---
 gcc/cp/semantics.cc                        | 1 +
 gcc/cp/typeck2.cc                          | 1 +
 gcc/testsuite/g++.dg/cpp23/auto-fncast12.C | 8 ++++++++
 3 files changed, 10 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp23/auto-fncast12.C

diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index a2c0eb050e6..5129b12f00f 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -3148,6 +3148,7 @@ finish_compound_literal (tree type, tree compound_literal,
   /* C++23 auto{x}.  */
   else if (is_auto (type)
           && !AUTO_IS_DECLTYPE (type)
+          && !is_constrained_auto (type)
           && CONSTRUCTOR_NELTS (compound_literal) == 1)
     {
       if (cxx_dialect < cxx23)
diff --git a/gcc/cp/typeck2.cc b/gcc/cp/typeck2.cc
index 39d03e4c3c4..c9314bbeb6f 100644
--- a/gcc/cp/typeck2.cc
+++ b/gcc/cp/typeck2.cc
@@ -2305,6 +2305,7 @@ build_functional_cast_1 (location_t loc, tree exp, tree 
parms,
        init = parms;
       /* C++23 auto(x).  */
       else if (!AUTO_IS_DECLTYPE (anode)
+              && !is_constrained_auto (anode)
               && list_length (parms) == 1)
        {
          init = TREE_VALUE (parms);
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..f513f7c9325
--- /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 "invalid use" }
+auto z = C auto{1};   // { dg-error "invalid use" }

base-commit: 8977f4bec650bb6975792772245b07b722ee9843
-- 
2.35.1