[PATCH] c++: Add std::to_underlying to the set of stdlib functions that are always folded

[Ville Voutilainen]

This function is yet another stdlib function that is just a simple
cast, so having
it appear while debugging is arguably not useful. So add it to the
existing handling
that always-folds some stdlib functions.

Add std::to_underlying to the set of stdlib functions that are always folded

gcc/cp/ChangeLog:
        * cp-gimplify.cc (cp_fold): Add to_underlying.

gcc/testsuite/ChangeLog:
        * g++.dg/opt/pr96780_cpp23.C: New.

libstdc++/ChangeLog:
        * include/std/utility (to_underlying): Add the
__always_inline__ attribute.

diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
index d2423fd1848..542c2f5a35e 100644
--- a/gcc/cp/cp-gimplify.cc
+++ b/gcc/cp/cp-gimplify.cc
@@ -3343,13 +3343,15 @@ cp_fold (tree x, fold_flags_t flags)
 		|| id_equal (DECL_NAME (callee), "addressof")
 		/* This addressof equivalent is used heavily in libstdc++.  */
 		|| id_equal (DECL_NAME (callee), "__addressof")
+		|| id_equal (DECL_NAME (callee), "to_underlying")
 		|| id_equal (DECL_NAME (callee), "as_const")))
 	  {
 	    r = CALL_EXPR_ARG (x, 0);
 	    /* Check that the return and argument types are sane before
 	       folding.  */
-	    if (INDIRECT_TYPE_P (TREE_TYPE (x))
-		&& INDIRECT_TYPE_P (TREE_TYPE (r)))
+	    if (id_equal (DECL_NAME (callee), "to_underlying")
+		|| (INDIRECT_TYPE_P (TREE_TYPE (x))
+		    && INDIRECT_TYPE_P (TREE_TYPE (r))))
 	      {
 		if (!same_type_p (TREE_TYPE (x), TREE_TYPE (r)))
 		  r = build_nop (TREE_TYPE (x), r);
diff --git a/gcc/testsuite/g++.dg/opt/pr96780_cpp23.C b/gcc/testsuite/g++.dg/opt/pr96780_cpp23.C
new file mode 100644
index 00000000000..a9c41b55984
--- /dev/null
+++ b/gcc/testsuite/g++.dg/opt/pr96780_cpp23.C
@@ -0,0 +1,22 @@
+// PR c++/96780
+// Verify calls to std::move/forward are folded away by the frontend.
+// { dg-do compile { target c++23 } }
+// { dg-additional-options "-ffold-simple-inlines -fdump-tree-gimple" }
+
+#include <utility>
+
+enum class A : char {a};
+
+extern A& x;
+
+void f() {
+  auto&& x1 = std::to_underlying(x);
+}
+
+// { dg-final { scan-tree-dump-not "= std::move" "gimple" } }
+// { dg-final { scan-tree-dump-not "= std::forward" "gimple" } }
+// { dg-final { scan-tree-dump-not "= std::addressof" "gimple" } }
+// { dg-final { scan-tree-dump-not "= std::__addressof" "gimple" } }
+// { dg-final { scan-tree-dump-not "= std::as_const" "gimple" } }
+// { dg-final { scan-tree-dump-not "= std::forward_like" "gimple" } }
+// { dg-final { scan-tree-dump-not "= std::to_underlying" "gimple" } }
diff --git a/libstdc++-v3/include/std/utility b/libstdc++-v3/include/std/utility
index 1c15c75f92b..8a85ccfd09b 100644
--- a/libstdc++-v3/include/std/utility
+++ b/libstdc++-v3/include/std/utility
@@ -201,7 +201,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #ifdef __cpp_lib_to_underlying // C++ >= 23
   /// Convert an object of enumeration type to its underlying type.
   template<typename _Tp>
-    [[nodiscard]]
+    [[nodiscard, __gnu__::__always_inline__]]
     constexpr underlying_type_t<_Tp>
     to_underlying(_Tp __value) noexcept
     { return static_cast<underlying_type_t<_Tp>>(__value); }