c++: Treat alignas align_expr and aligned attribute's operand as manifestly constant...

c++: Treat alignas align_expr and aligned attribute's operand as manifestly constant evaluation [PR105233]

The following testcase fails, because we only constant evaluate the
alignas argument as non-manifestly constant-evaluated and as
__builtin_is_constant_evaluated appears, we make it non-constant
(the reason is that we often try to evaluate some expression without
manifestly_const_eval perhaps even multiple times before actually
evaluating it with manifestly_const_eval (e.g. when folding for warnings
and in many other places), and we don't want __builtin_is_constant_evaluated
to evaluate to false in those cases, because we could get a different
result from when we actually evaluate it with manifestly_const_eval
set).
Now, for alignas the standard seems to be clear, it says the
argument is constant-expression, which means we should
manifestly-constant-eval it.
Attributes are a fuzzy area, they are extensions and various attributes
take e.g. identifiers, or string literals etc. as arguments.

Either we can just treat alignas as manifestly-const-eval, for that
we'd need some way how to differentiate between alignas and gnu::aligned
or aligned attribute.

Another possibility is what the patch below implements, treat
both alignas and gnu::aligned and aligned attribute's argument as
manifestly-const-eval and not do that for other attributes.

Another is to go through all attributes and figure out for which
such treatment is useful (e.g. those that expect INTEGER_CST as argument),
and either add a new column in the attribute table or have another table
in the C++ FE to find out which attribute needs that.

Another is do that for all the attribute arguments that are EXPR_P
and see what breaks (bet that it could be quite risky this late in
GCC 12 cycle and especially for backporting).

2022-04-13  Jakub Jelinek  <jakub@redhat.com>

	PR c++/105233
	* decl2.cc (cp_check_const_attributes): For aligned attribute
	pass manifestly_const_eval=true to fold_non_dependent_expr.

	* g++.dg/cpp2a/is-constant-evaluated13.C: New test.