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

[Jason Merrill]

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.

Jason

> Barry also noticed that we allow designated initializers for
> non-aggregate types in the template-argument argument context, i.e. this
> 
>    struct S {
>      unsigned a;
>      unsigned b;
>      constexpr S(unsigned _a, unsigned _b) noexcept: a{_a}, b{_b} { }
>    };
> 
>    template<S s> struct X { };
> 
>    void f()
>    {
>      X<{.a = 1, .b = 2}> x;
>    }
> 
> probably should not compile.  But I'm not too sure about it, and don't
> know how I would fix it anyway, so I'm not dealing with it in this
> patch.
> 
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> 
> gcc/cp/ChangeLog:
> 
> 	PR c++/95369
> 	* pt.c (convert_nontype_argument): In C++20, reshape and digest
> 	a braced-init-list if the type is an aggregate.
> 
> gcc/testsuite/ChangeLog:
> 
> 	PR c++/95369
> 	* g++.dg/cpp2a/nontype-class38.C: New test.
> ---
>   gcc/cp/pt.c                                  | 13 +++++++++
>   gcc/testsuite/g++.dg/cpp2a/nontype-class38.C | 30 ++++++++++++++++++++
>   2 files changed, 43 insertions(+)
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/nontype-class38.C
> 
> diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
> index 90dafff3aa7..adb7593f77d 100644
> --- a/gcc/cp/pt.c
> +++ b/gcc/cp/pt.c
> @@ -7133,6 +7133,19 @@ convert_nontype_argument (tree type, tree expr, tsubst_flags_t complain)
>   	return error_mark_node;
>       }
>   
> +  /* For a { } template argument, like in X<{ 1, 2 }>, we need to digest
> +     here so that build_converted_constant_expr below is able to convert
> +     it to TYPE.  */
> +  if (cxx_dialect >= cxx20
> +      && BRACE_ENCLOSED_INITIALIZER_P (expr)
> +      && CP_AGGREGATE_TYPE_P (type))
> +    {
> +      expr = reshape_init (type, expr, complain);
> +      expr = digest_init (type, expr, complain);
> +      if (expr == error_mark_node)
> +	return error_mark_node;
> +    }
> +
>     /* If we are in a template, EXPR may be non-dependent, but still
>        have a syntactic, rather than semantic, form.  For example, EXPR
>        might be a SCOPE_REF, rather than the VAR_DECL to which the
> 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: 3d8d5ddb539a5254c7ef83414377f4c74c7701d4
> 

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