From: Jakub Jelinek <jakub@redhat.com>
Date: Wed, 19 Nov 2025 10:02:13 +0000 (+0100)
Subject: c++, libstdc++: Implement CWG1670 and LWG4468
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=3d000da53203c9f17b29dd9328d12dfcbe8d03aa;p=thirdparty%2Fgcc.git

c++, libstdc++: Implement CWG1670 and LWG4468

The following patch attempts to implement CWG1670 and related LWG4468.

2025-11-19  Jakub Jelinek  <jakub@redhat.com>

gcc/cp/
	* parser.cc (cp_parser_conversion_type_id): Implement C++ DR1670
	- auto as conversion-type-id.  Pedwarn on conversion operators
	with placeholder return type.
gcc/testsuite/
	* g++.dg/DRs/dr1670-1.C: New test.
	* g++.dg/DRs/dr1670-2.C: New test.
	* g++.dg/DRs/dr1670-3.C: New test.
	* g++.dg/modules/auto-1_a.H: Use dg-options instead of
	dg-additional-options.
	* g++.dg/modules/auto-1_b.C: Likewise.
	* g++.dg/cpp1y/auto-fn12.C: Likewise.
	* g++.dg/cpp1y/auto-fn13.C: Add empty dg-options.
	* g++.dg/cpp1y/auto-fn22.C: Likewise.
	* g++.dg/cpp1y/constexpr-assert2.C: Likewise.
	* g++.dg/cpp1y/auto-fn44.C: Add dg-options -Wpedantic and expect
	further warnings.
	* g++.dg/cpp1y/auto-fn50.C: Likewise.
	* g++.dg/cpp0x/auto9.C: Expect two errors always rather than just
	for C++11.
libstdc++-v3/
	* include/std/type_traits (constant_wrapper conversion operator):
	Use decltype(value) instead of decltype(auto).  Resolves LWG4468.
---

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 06cba31ada6..dc26b10f39b 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -18779,9 +18779,14 @@ cp_parser_conversion_type_id (cp_parser* parser)
 	  error ("invalid use of %<auto%> in conversion operator");
 	  return error_mark_node;
 	}
-      else if (template_parm_scope_p ())
-	warning (0, "use of %<auto%> in member template "
-		 "conversion operator can never be deduced");
+      else
+	{
+	  pedwarn (input_location, OPT_Wpedantic,
+		   "invalid use of %<auto%> in conversion operator");
+	  if (template_parm_scope_p ())
+	    warning (0, "use of %<auto%> in member template "
+			"conversion operator can never be deduced");
+	}
     }
 
   return type_specified;
diff --git a/gcc/testsuite/g++.dg/DRs/dr1670-1.C b/gcc/testsuite/g++.dg/DRs/dr1670-1.C
new file mode 100644
index 00000000000..1c8430a6558
--- /dev/null
+++ b/gcc/testsuite/g++.dg/DRs/dr1670-1.C
@@ -0,0 +1,9 @@
+// DR 1670 - auto as conversion-type-id
+// { dg-do compile { target c++14 } }
+
+struct S {
+  operator auto () { return 0; }		// { dg-error "invalid use of 'auto' in conversion operator" }
+};
+struct T {
+  operator decltype (auto) () { return 0; }	// { dg-error "invalid use of 'auto' in conversion operator" }
+};
diff --git a/gcc/testsuite/g++.dg/DRs/dr1670-2.C b/gcc/testsuite/g++.dg/DRs/dr1670-2.C
new file mode 100644
index 00000000000..570a04ad796
--- /dev/null
+++ b/gcc/testsuite/g++.dg/DRs/dr1670-2.C
@@ -0,0 +1,10 @@
+// DR 1670 - auto as conversion-type-id
+// { dg-do compile { target c++14 } }
+// { dg-options "-Wpedantic" }
+
+struct S {
+  operator auto () { return 0; }		// { dg-warning "invalid use of 'auto' in conversion operator" }
+};
+struct T {
+  operator decltype (auto) () { return 0; }	// { dg-warning "invalid use of 'auto' in conversion operator" }
+};
diff --git a/gcc/testsuite/g++.dg/DRs/dr1670-3.C b/gcc/testsuite/g++.dg/DRs/dr1670-3.C
new file mode 100644
index 00000000000..9d138757229
--- /dev/null
+++ b/gcc/testsuite/g++.dg/DRs/dr1670-3.C
@@ -0,0 +1,10 @@
+// DR 1670 - auto as conversion-type-id
+// { dg-do compile { target c++14 } }
+// { dg-options "" }
+
+struct S {
+  operator auto () { return 0; }
+};
+struct T {
+  operator decltype (auto) () { return 0; }
+};
diff --git a/gcc/testsuite/g++.dg/cpp0x/auto9.C b/gcc/testsuite/g++.dg/cpp0x/auto9.C
index a3a6caf0a66..2f4de051b01 100644
--- a/gcc/testsuite/g++.dg/cpp0x/auto9.C
+++ b/gcc/testsuite/g++.dg/cpp0x/auto9.C
@@ -15,8 +15,8 @@ const std::type_info &t2 = typeid (auto *);	// { dg-error "auto" }
 
 struct A
 {
-  operator auto ();				// { dg-error "auto" "" { target { ! c++14 } } }
-  operator auto *();				// { dg-error "auto" "" { target { ! c++14 } } }
+  operator auto ();				// { dg-error "auto" }
+  operator auto *();				// { dg-error "auto" }
 };
 
 struct A2
diff --git a/gcc/testsuite/g++.dg/cpp1y/auto-fn12.C b/gcc/testsuite/g++.dg/cpp1y/auto-fn12.C
index 98885799de9..1ee1755bd00 100644
--- a/gcc/testsuite/g++.dg/cpp1y/auto-fn12.C
+++ b/gcc/testsuite/g++.dg/cpp1y/auto-fn12.C
@@ -1,6 +1,6 @@
 // { dg-do compile { target c++14 } }
 // { dg-final { scan-assembler "_ZN1AIiEcvDaEv" } }
-// { dg-additional-options -fno-implicit-constexpr }
+// { dg-options -fno-implicit-constexpr }
 
 template <class T>
 struct A {
diff --git a/gcc/testsuite/g++.dg/cpp1y/auto-fn13.C b/gcc/testsuite/g++.dg/cpp1y/auto-fn13.C
index 96b4b0c64aa..bb4f3ca94ee 100644
--- a/gcc/testsuite/g++.dg/cpp1y/auto-fn13.C
+++ b/gcc/testsuite/g++.dg/cpp1y/auto-fn13.C
@@ -1,4 +1,5 @@
 // { dg-do compile { target c++14 } }
+// { dg-options "" }
 
 struct A {
   template <class T>
diff --git a/gcc/testsuite/g++.dg/cpp1y/auto-fn22.C b/gcc/testsuite/g++.dg/cpp1y/auto-fn22.C
index b16a7644c0f..68d8d853b3c 100644
--- a/gcc/testsuite/g++.dg/cpp1y/auto-fn22.C
+++ b/gcc/testsuite/g++.dg/cpp1y/auto-fn22.C
@@ -1,4 +1,5 @@
 // { dg-do compile { target c++14 } }
+// { dg-options "" }
 
 struct A
 {
diff --git a/gcc/testsuite/g++.dg/cpp1y/auto-fn44.C b/gcc/testsuite/g++.dg/cpp1y/auto-fn44.C
index e35215d64c7..4439f47abf5 100644
--- a/gcc/testsuite/g++.dg/cpp1y/auto-fn44.C
+++ b/gcc/testsuite/g++.dg/cpp1y/auto-fn44.C
@@ -1,11 +1,12 @@
 // PR c++/79474
 // { dg-do compile { target c++14 } }
+// { dg-options "-Wpedantic" }
 
 struct Funject
 {  
-  operator auto() { return +[](bool b) {return b;}; }
+  operator auto() { return +[](bool b) {return b;}; }		   // { dg-warning "invalid use of 'auto' in conversion operator" }
   operator auto() { return +[](bool b, bool, bool) {return b;}; }  // { dg-error "cannot be overloaded" }
-};
+};								   // { dg-warning "invalid use of 'auto' in conversion operator" "" { target *-*-* } .-1 }
 
 Funject fun;
 auto bbb = fun(true);
diff --git a/gcc/testsuite/g++.dg/cpp1y/auto-fn50.C b/gcc/testsuite/g++.dg/cpp1y/auto-fn50.C
index a2c7e40755b..8a9010521ad 100644
--- a/gcc/testsuite/g++.dg/cpp1y/auto-fn50.C
+++ b/gcc/testsuite/g++.dg/cpp1y/auto-fn50.C
@@ -1,10 +1,11 @@
 // PR c++/84906
 // { dg-do compile { target c++14 } }
+// { dg-options "-Wpedantic" }
 
 extern "C" int puts(const char*);
 
 struct aa {
-  operator auto() {
+  operator auto() {		// { dg-warning "invalid use of 'auto' in conversion operator" }
     puts("auto");
     return false;
   }
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-assert2.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-assert2.C
index def05f5fb25..58f9a311351 100644
--- a/gcc/testsuite/g++.dg/cpp1y/constexpr-assert2.C
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-assert2.C
@@ -1,6 +1,7 @@
 // PR c++/65985
 // { dg-do compile { target c++14 } }
 // { dg-skip-if "requires hosted libstdc++ for cassert" { ! hostedlib } }
+// { dg-options "" }
 
 #include <cassert>
 
diff --git a/gcc/testsuite/g++.dg/modules/auto-1_a.H b/gcc/testsuite/g++.dg/modules/auto-1_a.H
index 4d3bc77dbb1..e9cbd00aceb 100644
--- a/gcc/testsuite/g++.dg/modules/auto-1_a.H
+++ b/gcc/testsuite/g++.dg/modules/auto-1_a.H
@@ -1,4 +1,4 @@
-// { dg-additional-options -fmodule-header }
+// { dg-options -fmodule-header }
 // { dg-module-cmi {} }
 
 #include "auto-1.h"
diff --git a/gcc/testsuite/g++.dg/modules/auto-1_b.C b/gcc/testsuite/g++.dg/modules/auto-1_b.C
index 96350662fed..f6422339ae5 100644
--- a/gcc/testsuite/g++.dg/modules/auto-1_b.C
+++ b/gcc/testsuite/g++.dg/modules/auto-1_b.C
@@ -1,4 +1,4 @@
-// { dg-additional-options "-fmodules-ts -fno-module-lazy -fdump-lang-module-alias" }
+// { dg-options "-fmodules-ts -fno-module-lazy -fdump-lang-module-alias" }
 
 #include "auto-1.h"
 import "auto-1_a.H";
diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
index d28b077398b..5c09fac8662 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -4682,7 +4682,7 @@ template<typename _Ret, typename _Fn, typename... _Args>
       }
 
     constexpr
-    operator decltype(auto)() const noexcept
+    operator decltype(value)() const noexcept
     { return value; }
   };