c++: don't default -frange-for-ext-temps in -std=gnu++20 [PR188574]

Since -frange-for-ext-temps has been causing trouble, let's not enable it
by default in pre-C++23 GNU modes for GCC 15, and also allow disabling it in
C++23 and up.

	PR c++/188574

gcc/c-family/ChangeLog:

	* c-opts.cc (c_common_post_options): Only enable
	-frange-for-ext-temps by default in C++23.

gcc/ChangeLog:

	* doc/invoke.texi: Adjust -frange-for-ext-temps documentation.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/range-for3.C: Use -frange-for-ext-temps.
	* g++.dg/cpp23/range-for4.C: Adjust expected result.

libgomp/ChangeLog:

	* testsuite/libgomp.c++/range-for-4.C: Adjust expected result.

gcc/c-family/c-opts.cc

@@ -1213,20 +1213,9 @@ c_common_post_options (const char **pfilename)
   if (cxx_dialect >= cxx20)
     flag_concepts = 1;
 
-  /* Enable lifetime extension of range based for temporaries for C++23.
-     Diagnose -std=c++23 -fno-range-for-ext-temps.  */
-  if (cxx_dialect >= cxx23)
-    {
-      if (OPTION_SET_P (flag_range_for_ext_temps)
-	  && !flag_range_for_ext_temps)
-	error ("%<-fno-range-for-ext-temps%> is incompatible with C++23");
-      flag_range_for_ext_temps = 1;
-    }
-  /* Otherwise default to enabled in GNU modes but allow user to override.  */
-  else if (cxx_dialect >= cxx11
-	   && !flag_iso
-	   && !OPTION_SET_P (flag_range_for_ext_temps))
-    flag_range_for_ext_temps = 1;
+  /* Enable lifetime extension of range based for temporaries for C++23.  */
+  SET_OPTION_IF_UNSET (&global_options, &global_options_set,
+		       flag_range_for_ext_temps, cxx_dialect >= cxx23);
 
   /* -fimmediate-escalation has no effect when immediate functions are not
      supported.  */

gcc/doc/invoke.texi

@@ -3548,9 +3548,8 @@ easier, you can use @option{-fno-pretty-templates} to disable them.
 Enable lifetime extension of C++ range based for temporaries.
 With @option{-std=c++23} and above this is part of the language standard,
 so lifetime of the temporaries is extended until the end of the loop
-regardless of this option.  This option allows enabling that behavior also
-in earlier versions of the standard and is enabled by default in the
-GNU dialects, from @option{-std=gnu++11} until @option{-std=gnu++20}.
+by default.  This option allows enabling that behavior also
+in earlier versions of the standard.
 
 @opindex fno-rtti
 @opindex frtti

gcc/testsuite/g++.dg/cpp23/range-for3.C

@@ -1,7 +1,7 @@
 // P2718R0 - Wording for P2644R1 Fix for Range-based for Loop
 // { dg-do run { target c++11 } }
-// Verify -frange-for-ext-temps is set by default in -std=gnu++* modes.
-// { dg-options "" }
+// Verify -frange-for-ext-temps works in earlier standards.
+// { dg-additional-options "-frange-for-ext-temps" }
 
 #define RANGE_FOR_EXT_TEMPS 1
 #include "range-for1.C"

gcc/testsuite/g++.dg/cpp23/range-for4.C

@@ -1,7 +1,7 @@
 // P2718R0 - Wording for P2644R1 Fix for Range-based for Loop
 // { dg-do run { target c++11 } }
-// Verify -frange-for-ext-temps is set by default in -std=gnu++* modes.
+// Verify -frange-for-ext-temps is not set by default in -std=gnu++* modes.
 // { dg-options "" }
 
-#define RANGE_FOR_EXT_TEMPS 1
+#define RANGE_FOR_EXT_TEMPS 0
 #include "range-for2.C"

libgomp/testsuite/libgomp.c++/range-for-4.C

@@ -3,5 +3,5 @@
 // { dg-additional-options "-std=gnu++17" }
 // { dg-require-effective-target tls_runtime }
 
-#define RANGE_FOR_EXT_TEMPS 1
+#define RANGE_FOR_EXT_TEMPS 0
 #include "range-for-1.C"