[gcc r15-10903] c++: Don't support auto{x} and auto(x) for C++ < 23 in SFINAE contexts [PR120685]

[Jakub Jelinek]

https://gcc.gnu.org/g:daa030592bdd082c8a4452df5fddc98686d51003

commit r15-10903-gdaa030592bdd082c8a4452df5fddc98686d51003
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Tue Feb 17 08:59:06 2026 +0100

    c++: Don't support auto{x} and auto(x) for C++ < 23 in SFINAE contexts [PR120685]
    
    The following testcase ICEs in C++ 20 and older, because during
    diagnostics dump_template_bindings attempts to tsubst with tf_none
    something and we try to emit a pedwarn during that.
    While the pedwarn could be just guarded with if (complain & tf_warning),
    I thought we shouldn't support extensions for tf_none and should
    instead return error_mark_node.
    
    The following patch does that.
    
    This is a backport version which instead of not supporting it just
    doesn't emit the pedwarn in SFINAE contexts for C++ < 23 but still
    supports it.
    
    2026-02-17  Jakub Jelinek  <jakub@redhat.com>
    
            PR c++/120685
            * typeck2.cc (build_functional_cast_1): For C++23 auto(x)
            without tf_warning or tf_error return error_mark_node instead of
            emitting pedwarn and handling it.
            * semantics.cc (finish_compound_literal): Similarly for C++26
            auto{x}.
    
            * g++.dg/cpp23/auto-fncast19.C: New test.
    
    (cherry picked from commit f7efb7e62ac96c082ff72549d9b3789f5c955efe)

Diff:
---
 gcc/cp/semantics.cc                        |  9 ++++++---
 gcc/cp/typeck2.cc                          |  9 ++++++---
 gcc/testsuite/g++.dg/cpp23/auto-fncast19.C | 12 ++++++++++++
 3 files changed, 24 insertions(+), 6 deletions(-)

diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index 0b5584c10e7c..46e7baaea1f7 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -3834,9 +3834,12 @@ finish_compound_literal (tree type, tree compound_literal,
 	  return error_mark_node;
 	}
       else if (cxx_dialect < cxx23)
-	pedwarn (input_location, OPT_Wc__23_extensions,
-		 "%<auto{x}%> only available with "
-		 "%<-std=c++23%> or %<-std=gnu++23%>");
+	{
+	  if ((complain & tf_warning_or_error) != 0)
+	    pedwarn (input_location, OPT_Wc__23_extensions,
+		     "%<auto{x}%> only available with "
+		     "%<-std=c++23%> or %<-std=gnu++23%>");
+	}
       type = do_auto_deduction (type, compound_literal, type, complain,
 				adc_variable_type);
       if (type == error_mark_node)
diff --git a/gcc/cp/typeck2.cc b/gcc/cp/typeck2.cc
index 45edd1801739..1ed20ce921c0 100644
--- a/gcc/cp/typeck2.cc
+++ b/gcc/cp/typeck2.cc
@@ -2509,9 +2509,12 @@ build_functional_cast_1 (location_t loc, tree exp, tree parms,
 	      return error_mark_node;
 	    }
 	  else if (cxx_dialect < cxx23)
-	    pedwarn (loc, OPT_Wc__23_extensions,
-		     "%<auto(x)%> only available with "
-		     "%<-std=c++23%> or %<-std=gnu++23%>");
+	    {
+	      if ((complain & tf_warning_or_error) != 0)
+		pedwarn (loc, OPT_Wc__23_extensions,
+			 "%<auto(x)%> only available with "
+			 "%<-std=c++23%> or %<-std=gnu++23%>");
+	    }
 	}
       else
 	{
diff --git a/gcc/testsuite/g++.dg/cpp23/auto-fncast19.C b/gcc/testsuite/g++.dg/cpp23/auto-fncast19.C
new file mode 100644
index 000000000000..05a0dbd2f39e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/auto-fncast19.C
@@ -0,0 +1,12 @@
+// PR c++/120685
+// { dg-do compile { target c++11 } }
+// { dg-options "" }
+
+template <typename T>
+void foo (decltype (auto (T ())) x) {}	// { dg-warning "'auto\\\(x\\\)' only available with" "" { target c++20_down } }
+
+int
+main ()
+{
+  foo <int> (1);
+}