c++: Extend std::is_constant_evaluated in if warning [PR100995]

Jakub pointed me at
<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p1938r3.html#compiler-warnings>
which shows that our existing warning could be extended to handle more
cases.  This patch implements that.

A minor annoyance was handling macros, in libstdc++ we have

  reference operator[](size_type __pos) {
      __glibcxx_assert(__pos <= size());
      ...
  }

wherein __glibcxx_assert expands to

  if (__builtin_is_constant_evaluated() && !bool(__pos <= size())
    ...

but I'm of a mind to not warn on that.

Once consteval if makes it in, we should tweak this warning one more
time.

	PR c++/100995

gcc/cp/ChangeLog:

	* constexpr.c (maybe_constexpr_fn): New.
	* cp-tree.h (maybe_constexpr_fn): Declare.
	* semantics.c (find_std_constant_evaluated_r): New.
	(maybe_warn_for_constant_evaluated): New.
	(finish_if_stmt_cond): Call it.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/is-constant-evaluated9.C: Add dg-warning.
	* g++.dg/cpp2a/is-constant-evaluated12.C: New test.

1 files changed, 10 insertions, 0 deletions

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 297f207..01b0c42 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -5121,6 +5121,16 @@ var_in_constexpr_fn (tree t)
 	  && DECL_DECLARED_CONSTEXPR_P (ctx));
 }
 
+/* True if a function might be constexpr: either a function that was
+   declared constexpr, or a C++17 lambda op().  */
+
+bool
+maybe_constexpr_fn (tree t)
+{
+  return (DECL_DECLARED_CONSTEXPR_P (t)
+	  || (cxx_dialect >= cxx17 && LAMBDA_FUNCTION_P (t)));
+}
+
 /* True if T was declared in a function that might be constexpr: either a
    function that was declared constexpr, or a C++17 lambda op().  */