[C/C++ PATCH] Allow __atomic_always_lock_free in a static assert (PR c/62024)

[Joseph S. Myers]

On Mon, 25 Aug 2014, Marek Polacek wrote:

> PR62024 reports that we can't use __atomic_always_lock_free in
> a static assert, as the FEs say it's not a constant expression.  Yet the
> docs say that the result of __atomic_always_lock_free is a compile time
> constant.
> We can fix this pretty easily.  While fold folds __atomic_always_lock_free
> to a constant, that constant is wrapped in NOP_EXPR - and static assert
> code is unhappy.
> I think we can just STRIP_TYPE_NOPS - we don't expect an lvalue in the
> static assert code.  This is done in both C and C++ FEs.  What do you think?
> In C, we'd still pedwarn on such code, and in C++ we'd still reject
> non-constexpr functions that are not builtin functions.

Is this NOP_EXPR (for C) the one left by c_fully_fold to carry 
TREE_NO_WARNING information?

If so, the C front-end part of this patch is OK, but at least in principle 
this issue could affect various other places that give a 
pedwarn-if-pedantic for something that's not an integer constant 
expression but folds to one.

-- 
Joseph S. Myers
joseph@codesourcery.com