c++: P2448 - Relaxing some constexpr restrictions [PR106649]

This patch implements C++23 P2448, which lifts more restrictions on the
constexpr keyword.  It's effectively going the way of being just a hint
(hello, inline!).

This gist is relatively simple: in C++23, a constexpr function's return
type/parameter type doesn't have to be a literal type; and you can have
a constexpr function for which no invocation satisfies the requirements
of a core constant expression.  For example,

  void f(int& i); // not constexpr

  constexpr void g(int& i) {
    f(i); // unconditionally calls a non-constexpr function
  }

is now OK, even though there isn't an invocation of 'g' that would be
a constant expression.  Maybe 'f' will be made constexpr soon, or maybe
this depends on the version of C++ used, and similar.  The patch is
unfortunately not that trivial.  The important bit is to use the new
require_potential_rvalue_constant_expression_fncheck in
maybe_save_constexpr_fundef (and where appropriate).  It has a new flag
that says that we're checking the body of a constexpr function, and in
that case it's OK to find constructs that aren't a constant expression.

Since it's useful to be able to check for problematic constructs even
in C++23, this patch implements a new warning, -Winvalid-constexpr,
which is a pedwarn turned on by default in C++20 and earlier, and which
can be turned on in C++23 as well, in which case it's an ordinary warning.
This I implemented by using the new function constexpr_error, used in
p_c_e_1 and friends.  (In some cases I believe fundef_p will be always
false (= hard error), but it made sense to me to be consistent and use
constexpr_error throughout p_c_e_1.)

While working on this I think I found a bug, see constexpr-nonlit15.C
and <https://gcc.gnu.org/PR107598>.  This patch doesn't address that.

This patch includes changes to diagnose the problem if the user doesn't
use -Winvalid-constexpr and calls a constexpr function that in fact isn't
constexpr-ready yet: maybe_save_constexpr_fundef registers the function
if warn_invalid_constexpr is 0 and explain_invalid_constexpr_fn then
gives the diagnostic.

	PR c++/106649

gcc/c-family/ChangeLog:

	* c-cppbuiltin.cc (c_cpp_builtins): Update value of __cpp_constexpr for
	C++23.
	* c-opts.cc (c_common_post_options): Set warn_invalid_constexpr
	depending on cxx_dialect.
	* c.opt (Winvalid-constexpr): New option.

gcc/cp/ChangeLog:

	* constexpr.cc (constexpr_error): New function.
	(is_valid_constexpr_fn): Use constexpr_error.
	(maybe_save_constexpr_fundef): Call
	require_potential_rvalue_constant_expression_fncheck rather than
	require_potential_rvalue_constant_expression.  Register the
	function if -Wno-invalid-constexpr was specified.
	(explain_invalid_constexpr_fn): Don't return early if a function marked
	'constexpr' that isn't actually a constant expression was called.
	(non_const_var_error): Add a bool parameter.  Use constexpr_error.
	(inline_asm_in_constexpr_error): Likewise.
	(cxx_eval_constant_expression): Adjust calls to non_const_var_error
	and inline_asm_in_constexpr_error.
	(potential_constant_expression_1): Add a bool parameter.  Use
	constexpr_error.
	(require_potential_rvalue_constant_expression_fncheck): New function.
	* cp-tree.h (require_potential_rvalue_constant_expression_fncheck):
	Declare.
	* method.cc (struct comp_info): Call
	require_potential_rvalue_constant_expression_fncheck rather than
	require_potential_rvalue_constant_expression.

gcc/ChangeLog:

	* doc/invoke.texi: Document -Winvalid-constexpr.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-ctor2.C: Expect an error in c++20_down only.
	* g++.dg/cpp0x/constexpr-default-ctor.C: Likewise.
	* g++.dg/cpp0x/constexpr-diag3.C: Likewise.
	* g++.dg/cpp0x/constexpr-ex1.C: Likewise.
	* g++.dg/cpp0x/constexpr-friend.C: Likewise.
	* g++.dg/cpp0x/constexpr-generated1.C: Likewise.
	* g++.dg/cpp0x/constexpr-ice5.C: Likewise.
	* g++.dg/cpp0x/constexpr-ice6.C: Likewise.
	* g++.dg/cpp0x/constexpr-memfn1.C: Likewise.
	* g++.dg/cpp0x/constexpr-neg2.C: Likewise.
	* g++.dg/cpp0x/constexpr-non-const-arg.C: Likewise.
	* g++.dg/cpp0x/constexpr-reinterpret1.C: Likewise.
	* g++.dg/cpp0x/pr65327.C: Likewise.
	* g++.dg/cpp1y/constexpr-105050.C: Likewise.
	* g++.dg/cpp1y/constexpr-89285-2.C: Likewise.
	* g++.dg/cpp1y/constexpr-89285.C: Likewise.
	* g++.dg/cpp1y/constexpr-89785-2.C: Likewise.
	* g++.dg/cpp1y/constexpr-neg1.C: Likewise.
	* g++.dg/cpp1y/constexpr-nsdmi7b.C: Likewise.
	* g++.dg/cpp1y/constexpr-throw.C: Likewise.
	* g++.dg/cpp23/constexpr-nonlit3.C: Remove dg-error.
	* g++.dg/cpp23/constexpr-nonlit6.C: Call the test functions.
	* g++.dg/cpp23/feat-cxx2b.C: Adjust the expected value of
	__cpp_constexpr.
	* g++.dg/cpp2a/consteval3.C: Remove dg-error.
	* g++.dg/cpp2a/constexpr-new7.C: Expect an error in c++20_down only.
	* g++.dg/cpp2a/constexpr-try5.C: Remove dg-error.
	* g++.dg/cpp2a/spaceship-constexpr1.C: Expect an error in c++20_down
	only.
	* g++.dg/cpp2a/spaceship-eq3.C: Likewise.
	* g++.dg/diagnostic/constexpr1.C: Remove dg-error.
	* g++.dg/gomp/pr79664.C: Use -Winvalid-constexpr -pedantic-errors.
	* g++.dg/ubsan/vptr-4.C: Likewise.
	* g++.dg/cpp23/constexpr-nonlit10.C: New test.
	* g++.dg/cpp23/constexpr-nonlit11.C: New test.
	* g++.dg/cpp23/constexpr-nonlit12.C: New test.
	* g++.dg/cpp23/constexpr-nonlit13.C: New test.
	* g++.dg/cpp23/constexpr-nonlit14.C: New test.
	* g++.dg/cpp23/constexpr-nonlit15.C: New test.
	* g++.dg/cpp23/constexpr-nonlit16.C: New test.
	* g++.dg/cpp23/constexpr-nonlit8.C: New test.
	* g++.dg/cpp23/constexpr-nonlit9.C: New test.

1 files changed, 27 insertions, 1 deletions

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 96f0b6d08bd..31d031cd25c 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -256,7 +256,7 @@ in the following sections.
 -Wno-deprecated-enum-enum-conversion -Wno-deprecated-enum-float-conversion @gol
 -Weffc++  -Wno-exceptions -Wextra-semi  -Wno-inaccessible-base @gol
 -Wno-inherited-variadic-ctor  -Wno-init-list-lifetime @gol
--Winvalid-imported-macros @gol
+-Winvalid-constexpr -Winvalid-imported-macros @gol
 -Wno-invalid-offsetof  -Wno-literal-suffix @gol
 -Wmismatched-new-delete -Wmismatched-tags @gol
 -Wmultiple-inheritance  -Wnamespaces  -Wnarrowing @gol
@@ -3766,6 +3766,32 @@ the variable declaration statement.
 
 @end itemize
 
+@item -Winvalid-constexpr
+@opindex Winvalid-constexpr
+@opindex Wno-invalid-constexpr
+
+Warn when a function never produces a constant expression.  In C++20
+and earlier, for every @code{constexpr} function and function template,
+there must be at least one set of function arguments in at least one
+instantiation such that an invocation of the function or constructor
+could be an evaluated subexpression of a core constant expression.
+C++23 removed this restriction, so it's possible to have a function
+or a function template marked @code{constexpr} for which no invocation
+satisfies the requirements of a core constant expression.
+
+This warning is enabled as a pedantic warning by default in C++20 and
+earlier.  In C++23, @option{-Winvalid-constexpr} can be turned on, in
+which case it will be an ordinary warning.  For example:
+
+@smallexample
+void f (int& i);
+constexpr void
+g (int& i)
+@{
+  f(i); // warns by default in C++20, in C++23 only with -Winvalid-constexpr
+@}
+@end smallexample
+
 @item -Winvalid-imported-macros
 @opindex Winvalid-imported-macros
 @opindex Wno-invalid-imported-macros