[PATCH v2] c++: Make braced-init-list as template arg work with aggr init [PR95369]

[Jason Merrill]

On 6/5/20 3:13 PM, Marek Polacek wrote:
> On Fri, May 29, 2020 at 03:17:51PM -0400, Jason Merrill via Gcc-patches wrote:
>> On 5/28/20 7:23 PM, Marek Polacek wrote:
>>> Barry pointed out to me that our braced-init-list as a template-argument
>>> extension doesn't work as expected when we aggregate-initialize.  Thus
>>> fixed by calling digest_init in convert_nontype_argument so that we can
>>> actually convert the CONSTRUCTOR.
>>>
>>> I don't think we can call digest_init any earlier, and it needs to
>>> happen before the call to build_converted_constant_expr.
>>
>> Or we could fix build_converted_constant_expr to allow it; aggregate
>> list-initialization is a user-defined conversion sequence, so I tihnk it
>> should be allowed as part of a converted constant expression.
> 
> Ah, nice.  It works just fine.  Here's the complete patch:
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

OK.

> -- >8 --
> Barry pointed out to me that our braced-init-list as a template-argument
> extension doesn't work as expected when we aggregate-initialize.  Since
> aggregate list-initialization is a user-defined conversion sequence, we
> allow it as part of a converted constant expression.
> 
> Co-authored-by: Jason Merrill <jason@redhat.com>
> 
> gcc/cp/ChangeLog:
> 
> 	PR c++/95369
> 	* call.c (build_converted_constant_expr_internal): Allow
> 	list-initialization.
> 
> gcc/testsuite/ChangeLog:
> 
> 	PR c++/95369
> 	* g++.dg/cpp2a/nontype-class38.C: New test.
> ---
>   gcc/cp/call.c                                |  4 ++-
>   gcc/testsuite/g++.dg/cpp2a/nontype-class38.C | 30 ++++++++++++++++++++
>   2 files changed, 33 insertions(+), 1 deletion(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/nontype-class38.C
> 
> diff --git a/gcc/cp/call.c b/gcc/cp/call.c
> index 2b393f96e5b..3c97b9846e2 100644
> --- a/gcc/cp/call.c
> +++ b/gcc/cp/call.c
> @@ -4348,7 +4348,7 @@ build_converted_constant_expr_internal (tree type, tree expr,
>        and where the reference binding (if any) binds directly.  */
>   
>     for (conversion *c = conv;
> -       conv && c->kind != ck_identity;
> +       c && c->kind != ck_identity;
>          c = next_conversion (c))
>       {
>         switch (c->kind)
> @@ -4356,6 +4356,8 @@ build_converted_constant_expr_internal (tree type, tree expr,
>   	  /* A conversion function is OK.  If it isn't constexpr, we'll
>   	     complain later that the argument isn't constant.  */
>   	case ck_user:
> +	  /* List-initialization is OK.  */
> +	case ck_aggr:
>   	  /* The lvalue-to-rvalue conversion is OK.  */
>   	case ck_rvalue:
>   	  /* Array-to-pointer and function-to-pointer.  */
> diff --git a/gcc/testsuite/g++.dg/cpp2a/nontype-class38.C b/gcc/testsuite/g++.dg/cpp2a/nontype-class38.C
> new file mode 100644
> index 00000000000..5b440fd1c9e
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/nontype-class38.C
> @@ -0,0 +1,30 @@
> +// PR c++/95369
> +// { dg-do compile { target c++20 } }
> +
> +struct S {
> +  int a;
> +  int b;
> +};
> +
> +struct W {
> +  int i;
> +  S s;
> +};
> +
> +template <S p>
> +void fnc()
> +{
> +}
> +
> +template<S s> struct X { };
> +template<W w> struct Y { };
> +
> +void f()
> +{
> +  fnc<{ .a = 10, .b = 20 }>();
> +  fnc<{ 10, 20 }>();
> +  X<{ .a = 1, .b = 2 }> x;
> +  X<{ 1, 2 }> x2;
> +  // Brace elision is likely to be allowed.
> +  Y<{ 1, 2, 3 }> x3;
> +}
> 
> base-commit: 640e05e02b567fa5ccf4c207e6fc6c3e9a93b17c
>