[gcc r16-7539] c++: Don't support auto{x} and auto(x) for C++ < 23 in SFINAE contexts [PR120685]

[Jakub Jelinek]

https://gcc.gnu.org/g:f7efb7e62ac96c082ff72549d9b3789f5c955efe

commit r16-7539-gf7efb7e62ac96c082ff72549d9b3789f5c955efe
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.
    
    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.

Diff:
---
 gcc/cp/semantics.cc                        | 10 +++++++---
 gcc/cp/typeck2.cc                          | 10 +++++++---
 gcc/testsuite/g++.dg/cpp23/auto-fncast19.C | 12 ++++++++++++
 3 files changed, 26 insertions(+), 6 deletions(-)

diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index 236bc625c25d..ba5c8fa1b552 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -3874,9 +3874,13 @@ 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)
+	    return error_mark_node;
+	  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 a5567e7de180..664dd0990bf8 100644
--- a/gcc/cp/typeck2.cc
+++ b/gcc/cp/typeck2.cc
@@ -2610,9 +2610,13 @@ 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)
+		return error_mark_node;
+	      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..f14e81486016
--- /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);			// { dg-error "no matching function for call to 'foo<int>\\\(int\\\)'" "" { target c++20_down } }
+}