12 files changed, 215 insertions, 13 deletions
diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index 8f79f04..d962b29 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -3134,6 +3134,20 @@ finish_compound_literal (tree type, tree compound_literal,
       if (type == error_mark_node)
 	return error_mark_node;
     }
+  /* C++23 auto{x}.  */
+  else if (is_auto (type)
+	   && !AUTO_IS_DECLTYPE (type)
+	   && CONSTRUCTOR_NELTS (compound_literal) == 1)
+    {
+      if (cxx_dialect < cxx23)
+	pedwarn (input_location, OPT_Wc__23_extensions,
+		 "%<auto{x}%> only available with "
+		 "%<-std=c++2b%> or %<-std=gnu++2b%>");
+      type = do_auto_deduction (type, compound_literal, type, complain,
+				adc_variable_type);
+      if (type == error_mark_node)
+	return error_mark_node;
+    }
 
   /* Used to hold a copy of the compound literal in a template.  */
   tree orig_cl = NULL_TREE;
diff --git a/gcc/cp/typeck2.c b/gcc/cp/typeck2.c
index e98fbf7..3fb651a 100644
--- a/gcc/cp/typeck2.c
+++ b/gcc/cp/typeck2.c
@@ -2201,19 +2201,29 @@ build_functional_cast_1 (location_t loc, tree exp, tree parms,
 
   if (tree anode = type_uses_auto (type))
     {
-      if (!CLASS_PLACEHOLDER_TEMPLATE (anode))
+      tree init;
+      if (CLASS_PLACEHOLDER_TEMPLATE (anode))
+	init = parms;
+      /* C++23 auto(x).  */
+      else if (!AUTO_IS_DECLTYPE (anode)
+	       && list_length (parms) == 1)
 	{
-	  if (complain & tf_error)
-	    error_at (loc, "invalid use of %qT", anode);
-	  return error_mark_node;
+	  init = TREE_VALUE (parms);
+	  if (cxx_dialect < cxx23)
+	    pedwarn (loc, OPT_Wc__23_extensions,
+		     "%<auto(x)%> only available with "
+		     "%<-std=c++2b%> or %<-std=gnu++2b%>");
 	}
       else
 	{
-	  type = do_auto_deduction (type, parms, anode, complain,
-				    adc_variable_type);
-	  if (type == error_mark_node)
-	    return error_mark_node;
+	  if (complain & tf_error)
+	    error_at (loc, "invalid use of %qT", anode);
+	  return error_mark_node;
 	}
+      type = do_auto_deduction (type, init, anode, complain,
+				adc_variable_type);
+      if (type == error_mark_node)
+	return error_mark_node;
     }
 
   if (processing_template_decl)
diff --git a/gcc/testsuite/g++.dg/cpp0x/auto25.C b/gcc/testsuite/g++.dg/cpp0x/auto25.C
index 19d51bc..3af0899 100644
--- a/gcc/testsuite/g++.dg/cpp0x/auto25.C
+++ b/gcc/testsuite/g++.dg/cpp0x/auto25.C
@@ -3,10 +3,10 @@
 
 template<int> struct A
 {
-  int a[auto(1)]; // { dg-error "9:invalid use of" }
+  int a[auto(1)]; // { dg-error "9:only available" "" { target c++20_down } }
 };
 
 template<int> void foo()
 {
-  int a[auto(1)]; // { dg-error "9:invalid use of" }
+  int a[auto(1)]; // { dg-error "9:only available" "" { target c++20_down } }
 }
diff --git a/gcc/testsuite/g++.dg/cpp0x/auto9.C b/gcc/testsuite/g++.dg/cpp0x/auto9.C
index 0e80c30..a3a6caf 100644
--- a/gcc/testsuite/g++.dg/cpp0x/auto9.C
+++ b/gcc/testsuite/g++.dg/cpp0x/auto9.C
@@ -45,7 +45,7 @@ foo ()
   C<int> c;
   dynamic_cast<auto> (c);			// { dg-error "auto" }
   reinterpret_cast<auto> (c);			// { dg-error "auto" }
-  int i = auto (0);				// { dg-error "auto" }
+  int i = auto (0);				// { dg-error "auto" "" { target c++20_down } }
   auto p1 = new (auto);				// { dg-error "auto" }
   auto p2 = new (auto) (42);			// { dg-error "invalid use of|deduce" }
   offsetof (auto, fld);				// { dg-error "auto" }
diff --git a/gcc/testsuite/g++.dg/cpp23/auto-fncast1.C b/gcc/testsuite/g++.dg/cpp23/auto-fncast1.C
new file mode 100644
index 0000000..25e53c4
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/auto-fncast1.C
@@ -0,0 +1,14 @@
+// PR c++/103049
+// P0849R8 - auto(x)
+// { dg-do compile { target c++23 } }
+// Testcase from P0849R8.
+
+struct A {};
+void f(A&) = delete;  // #1
+void f(A&&); // #2
+A& g();
+void h() {
+//  f(g());      // calls #1
+  f(A(g()));     // calls #2 with a temporary object
+  f(auto(g()));  // calls #2 with a temporary object
+}
diff --git a/gcc/testsuite/g++.dg/cpp23/auto-fncast2.C b/gcc/testsuite/g++.dg/cpp23/auto-fncast2.C
new file mode 100644
index 0000000..327a448
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/auto-fncast2.C
@@ -0,0 +1,62 @@
+// PR c++/103049
+// P0849R8 - auto(x)
+// { dg-do compile { target c++23 } }
+
+struct X { };
+X& fn ();
+X&& fnr ();
+
+void h()
+{
+  double v[] = { 1.2, 3.4 };
+  +auto(v);
+  +auto{v};
+  static_assert (__is_same_as (decltype (auto(v)), double *));
+  static_assert (__is_same_as (decltype (auto{v}), double *));
+  auto a1 = fn ();
+  static_assert (__is_same_as (decltype (auto(fn())), decltype (a1)));
+  static_assert (__is_same_as (decltype (auto{fn()}), decltype (a1)));
+  auto a2 = fnr ();
+  static_assert (__is_same_as (decltype (auto(fnr())), decltype (a2)));
+  static_assert (__is_same_as (decltype (auto{fnr()}), decltype (a2)));
+  +auto(1);
+  new auto(1);
+  +auto{1};
+  new auto{1};
+}
+
+template<typename T>
+void baz (T t, const T &tr, T &&trr)
+{
+  +auto(t);
+  +auto{t};
+  +auto(tr);
+  +auto{tr};
+  +auto(trr);
+  +auto{trr};
+  static_assert (__is_same_as (decltype (auto(t)), T));
+  static_assert (__is_same_as (decltype (auto{t}), T));
+  static_assert (__is_same_as (decltype (auto(tr)), T));
+  static_assert (__is_same_as (decltype (auto{tr}), T));
+  static_assert (__is_same_as (decltype (auto(trr)), T));
+  static_assert (__is_same_as (decltype (auto{trr}), T));
+}
+
+template<typename = decltype(auto(1))>
+void foo ()
+{
+}
+
+template<int = auto(1)>
+void bar ()
+{
+}
+
+void
+g()
+{
+  foo<>();
+  bar<>();
+  int i = 42;
+  baz (1, i, 42);
+}
diff --git a/gcc/testsuite/g++.dg/cpp23/auto-fncast3.C b/gcc/testsuite/g++.dg/cpp23/auto-fncast3.C
new file mode 100644
index 0000000..1204458
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/auto-fncast3.C
@@ -0,0 +1,21 @@
+// PR c++/103049
+// P0849R8 - auto(x)
+// { dg-do compile { target c++23 } }
+// Test invalid use.
+
+void
+f ()
+{
+  char x[] = "foo";
+  +decltype(auto){x}; // { dg-error "invalid use of .decltype\\(auto\\)." }
+  +decltype(auto)(x); // { dg-error "invalid use of .decltype\\(auto\\)." }
+
+  +auto(); // { dg-error "invalid use of .auto." }
+  new auto(); // { dg-error "requires exactly one element" }
+  +auto{}; // { dg-error "invalid use of .auto." }
+  new auto{}; // { dg-error "requires exactly one element" }
+  +auto(1, 2); // { dg-error "invalid use of .auto." }
+  new auto(1, 2); // { dg-error "requires exactly one element" }
+  +auto{1, 2}; // { dg-error "too many initializers" }
+  new auto{1, 2}; // { dg-error "requires exactly one element" }
+}
diff --git a/gcc/testsuite/g++.dg/cpp23/auto-fncast4.C b/gcc/testsuite/g++.dg/cpp23/auto-fncast4.C
new file mode 100644
index 0000000..0e26bf2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/auto-fncast4.C
@@ -0,0 +1,26 @@
+// PR c++/103049
+// P0849R8 - auto(x)
+// { dg-do compile { target c++23 } }
+
+class cmdline_parser
+{
+    public:
+    cmdline_parser(char const*);
+
+    auto add_option(char const*, char const*) & -> cmdline_parser &;
+    auto add_option(char const*, char const*) && -> cmdline_parser &&;
+
+    void parse(int, char**);
+};
+
+int main(int argc, char *argv[])
+{
+    auto cmdline = cmdline_parser("driver");
+
+    cmdline.add_option("-h", "show help messages")
+           .add_option("-v", "show version");
+
+    auto internal = auto(cmdline).add_option("--logging-level", "set logging level to 1-3")
+                                 .add_option("--dump-full", "do not minimize dump");
+    internal.parse(argc, argv);
+}
diff --git a/gcc/testsuite/g++.dg/cpp23/auto-fncast5.C b/gcc/testsuite/g++.dg/cpp23/auto-fncast5.C
new file mode 100644
index 0000000..b29901f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/auto-fncast5.C
@@ -0,0 +1,39 @@
+// PR c++/103049
+// P0849R8 - auto(x)
+// { dg-do compile { target c++23 } }
+
+struct X {
+  X() = default;
+  X(const X&) = delete;
+};
+
+void
+g ()
+{
+  X x;
+  +X(x); // { dg-error "use of deleted function" }
+  +auto(x); // { dg-error "use of deleted function" }
+}
+
+class A;
+void f(A);
+
+class A {
+    int x;
+
+public:
+    A();
+
+    auto run() {
+        f(A(*this));
+        f(auto(*this));
+    }
+
+protected:
+    A(const A&);
+};
+
+void z () {
+  A a;
+  a.run ();
+}
diff --git a/gcc/testsuite/g++.dg/cpp23/auto-fncast6.C b/gcc/testsuite/g++.dg/cpp23/auto-fncast6.C
new file mode 100644
index 0000000..6b7858d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/auto-fncast6.C
@@ -0,0 +1,14 @@
+// PR c++/103049
+// P0849R8 - auto(x)
+// { dg-do compile { target c++20 } }
+
+void f (int);
+
+void
+g ()
+{
+  auto a1 = auto(f); // { dg-error "only available with" "" { target c++20_only } }
+  auto a2 = auto{f}; // { dg-error "only available with" "" { target c++20_only } }
+  static_assert (__is_same_as (decltype (a1), void(*)(int)));
+  static_assert (__is_same_as (decltype (a2), void(*)(int)));
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-pr84979-2.C b/gcc/testsuite/g++.dg/cpp2a/concepts-pr84979-2.C
index 290aaf8..025bbf3 100644
--- a/gcc/testsuite/g++.dg/cpp2a/concepts-pr84979-2.C
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-pr84979-2.C
@@ -6,7 +6,8 @@ void foo1(T& t) {
   typename T::template C<void> tcv = t;
   typename T::template C<auto> u = tcv; // { dg-error "not permitted" "" { target c++20 } }
   T::template C<auto>::f (tcv, u); // { dg-error "incomplete|not permitted" }
-  (typename T::template D<auto> (t)); // { dg-error "invalid|not permitted" }
+  (typename T::template D<auto> (t)); // { dg-error "invalid|not permitted|unable" }
+// { dg-warning "only available" "" { target c++17_down } .-1 }
 }
 
 struct T1 {
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-pr84979-3.C b/gcc/testsuite/g++.dg/cpp2a/concepts-pr84979-3.C
index d612327..80a3884 100644
--- a/gcc/testsuite/g++.dg/cpp2a/concepts-pr84979-3.C
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-pr84979-3.C
@@ -10,7 +10,8 @@ void foo1(T& t) {
   typename T::template C<void> tcv = t;
   typename T::template C<auto> u = tcv; // { dg-error "not permitted" "" { target c++20 } }
   T::template C<auto>::f (tcv, u); // { dg-error "incomplete|not permitted" }
-  (typename T::template D<auto> (t)); // { dg-error "invalid|not permitted" }
+  (typename T::template D<auto> (t)); // { dg-error "invalid|not permitted|no class" }
+// { dg-warning "only available" "" { target c++17_down } .-1 }
 }
 
 struct T1 {