C++ PATCH for c++/87372, __func__ constexpr evaluation

[Jakub Jelinek]

On Thu, Sep 20, 2018 at 11:56:37PM -0400, Marek Polacek wrote:
> The patch for P0595R1 - is_constant_evaluated had this hunk:
> 
> @@ -5279,7 +5315,9 @@ maybe_constant_init_1 (tree t, tree decl, bool allow_non_constant)
>    else if (CONSTANT_CLASS_P (t) && allow_non_constant)
>      /* No evaluation needed.  */;
>    else
> -    t = cxx_eval_outermost_constant_expr (t, allow_non_constant, false, decl);
> +    t = cxx_eval_outermost_constant_expr (t, allow_non_constant,
> +                     !allow_non_constant,
> +                     pretend_const_required, decl);
>    if (TREE_CODE (t) == TARGET_EXPR)
>      {
>        tree init = TARGET_EXPR_INITIAL (t);
> 
> the false -> !allow_non_constant change means that when calling
> cxx_constant_init strict will be true because cxx_constant_init does not allow
> non constants.  That means that for VAR_DECLs such as __func__ we'll call
> decl_really_constant_value instead of decl_constant_value.  But only the latter
> can evaluate __func__ to "foo()".
> 
> Jakub, was there a specific reason for this change?  Changing it back still
> regtests cleanly and the attached test compiles again.

It just didn't feel right that cxx_constant_init which looks like a function
that requires strict conformance still passes false as strict.
If there is a reason to pass false, I think we need a comment that explains
it.

> Bootstrapped/regtested on x86_64-linux, ok for trunk?
> 
> 2018-09-20  Marek Polacek  <polacek@redhat.com>
> 
> 	PR c++/87372 - __func__ constexpr evaluation.
> 	* constexpr.c (maybe_constant_init_1): Pass false for strict down to
> 	cxx_eval_outermost_constant_expr.
> 
> 	* g++.dg/cpp1y/func_constexpr2.C: New test.
> 
> diff --git gcc/cp/constexpr.c gcc/cp/constexpr.c
> index fdea769faa9..6436b2f832d 100644
> --- gcc/cp/constexpr.c
> +++ gcc/cp/constexpr.c
> @@ -5361,7 +5361,7 @@ maybe_constant_init_1 (tree t, tree decl, bool allow_non_constant,
>      /* No evaluation needed.  */;
>    else
>      t = cxx_eval_outermost_constant_expr (t, allow_non_constant,
> -					  !allow_non_constant,
> +					  /*strict*/false,
>  					  pretend_const_required, decl);
>    if (TREE_CODE (t) == TARGET_EXPR)
>      {

	Jakub