c++: fix parsing with auto(x) [PR112410]

Here we are wrongly parsing

  int y(auto(42));

which uses the C++23 cast-to-prvalue feature, and initializes y to 42.
However, we were treating the auto as an implicit template parameter.

Fixing the auto{42} case is easy, but when auto is followed by a (,
I found the fix to be much more involved.  For instance, we cannot
use cp_parser_expression, because that can give hard errors.  It's
also necessary to disambiguate 'auto(i)' as 'auto i', not a cast.
auto(), auto(int), auto(f)(int), auto(*), auto(i[]), auto(...), etc.
are all function declarations.

This patch rectifies that by undoing the implicit function template
modification.  In the test above, we should notice that the parameter
list is ill-formed, and since we've synthesized an implicit template
parameter, we undo it by calling abort_fully_implicit_template.  Then,
we'll parse the "(auto(42))" as an initializer.

	PR c++/112410

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_direct_declarator): Maybe call
	abort_fully_implicit_template if it turned out the parameter list was
	ill-formed.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/auto-fncast13.C: New test.
	* g++.dg/cpp23/auto-fncast14.C: New test.

gcc/cp/parser.cc

@@ -23594,6 +23594,19 @@ cp_parser_direct_declarator (cp_parser* parser,
 	      /* Consume the `)'.  */
 	      parens.require_close (parser);
 
+	      /* For code like
+		  int x(auto(42));
+		  A a(auto(i), 42);
+		 we have synthesized an implicit template parameter and marked
+		 what we thought was a function as an implicit function template.
+		 But now, having seen the whole parameter list, we know it's not
+		 a function declaration, so undo that.  */
+	      if (cp_parser_error_occurred (parser)
+		  && parser->fully_implicit_function_template_p
+		  /* Don't do this for the inner ().  */
+		  && parser->default_arg_ok_p)
+		abort_fully_implicit_template (parser);
+
 	      /* If all went well, parse the cv-qualifier-seq,
 		 ref-qualifier and the exception-specification.  */
 	      if (member_p || cp_parser_parse_definitely (parser))

gcc/testsuite/g++.dg/cpp23/auto-fncast13.C

@@ -0,0 +1,61 @@
+// PR c++/112410
+// { dg-do compile { target c++23 } }
+
+int f1 (auto(int) -> char);
+int f2 (auto x);
+int f3 (auto);
+int f4 (auto(i));
+
+int v1 (auto(42));
+int v2 (auto{42});
+int e1 (auto{i}); // { dg-error "not declared" }
+int i;
+int v3 (auto{i});
+int v4 (auto(i + 1));
+int v5 (auto(+i));
+int v6 (auto(i = 4));
+
+int f5 (auto(i));
+int f6 (auto());
+int f7 (auto(int));
+int f8 (auto(f)(int));
+int f9 (auto(...) -> char);
+// FIXME: ICEs (PR c++/89867)
+//int f10 (auto(__attribute__((unused)) i));
+int f11 (auto((i)));
+int f12 (auto(i[]));
+int f13 (auto(*i));
+int f14 (auto(*));
+
+int e2 (auto{}); // { dg-error "invalid use of .auto." }
+int e3 (auto(i, i)); // { dg-error "invalid use of .auto." }
+
+char bar (int);
+char baz ();
+char qux (...);
+
+void
+g (int i)
+{
+  f1 (bar);
+  f2 (42);
+  f3 (42);
+  f4 (42);
+  f5 (42);
+  f6 (baz);
+  f7 (bar);
+  f8 (bar);
+  f9 (qux);
+//  f10 (42);
+  f11 (42);
+  f12 (&i);
+  f13 (&i);
+  f14 (&i);
+
+  v1 = 1;
+  v2 = 2;
+  v3 = 3;
+  v4 = 4;
+  v5 = 5;
+  v6 = 6;
+}

gcc/testsuite/g++.dg/cpp23/auto-fncast14.C

@@ -0,0 +1,9 @@
+// PR c++/112410
+// { dg-do compile { target c++23 } }
+
+struct A {
+   A(int,int);
+};
+
+int a;
+A b1(auto(a), 42);