Re: [PATCH] c++: decltype of by-ref capture proxy of ref [PR115504]

[Jason Merrill <jason@redhat.com>]

On 6/25/24 11:03, Patrick Palka wrote:
> On Mon, 24 Jun 2024, Jason Merrill wrote:
> 
>> On 6/24/24 21:00, Patrick Palka wrote:
>>> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK
>>> for trunk/14?
>>>
>>> -- >8 --
>>>
>>> The capture proxy handling in finish_decltype_type added in r14-5330
>>> was stripping the reference type of a capture proxy's captured variable,
>>> which is desirable for a by-value capture, but not for a by-ref capture
>>> (of a reference).
>>
>> I'm not sure why we would want it for by-value, either; regardless of the
>> capture kind, decltype(x) is int&.
> 
> Ah, makes sense.  But I guess that means
> 
>    void f(int& x) {
>      [x]() {
>        decltype(auto) a = x;
>      }
>    }
> 
> is ill-formed since decltype(x) is int& but the corresponding closure
> member is const?  It works if we make the lambda mutable.

Yes, and clang agrees.  Let's also test that case.

> Like so?  Bootstrapped and regtested on x86_64-pc-linux-gnu.
> 
> 
> -- >8 --
> 
> Subject: [PATCH] c++: decltype of capture proxy of ref [PR115504]
> 
> The capture proxy handling in finish_decltype_type added in r14-5330
> was stripping the reference type of a capture proxy's captured variable.
> 
> 	PR c++/115504
> 
> gcc/cp/ChangeLog:
> 
> 	* semantics.cc (finish_decltype_type): Don't strip the reference
> 	type (if any) of a capture proxy's captured variable.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp1y/decltype-auto8.C: New test.
> ---
>   gcc/cp/semantics.cc                         |  1 -
>   gcc/testsuite/g++.dg/cpp1y/decltype-auto8.C | 18 ++++++++++++++++++
>   2 files changed, 18 insertions(+), 1 deletion(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp1y/decltype-auto8.C
> 
> diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
> index 6c1813d37c6..6a383c0f7f9 100644
> --- a/gcc/cp/semantics.cc
> +++ b/gcc/cp/semantics.cc
> @@ -12071,7 +12071,6 @@ finish_decltype_type (tree expr, bool id_expression_or_member_access_p,
>   		{
>   		  expr = DECL_CAPTURED_VARIABLE (expr);
>   		  type = TREE_TYPE (expr);
> -		  type = non_reference (type);
>   		}
>   	      else
>   		{
> diff --git a/gcc/testsuite/g++.dg/cpp1y/decltype-auto8.C b/gcc/testsuite/g++.dg/cpp1y/decltype-auto8.C
> new file mode 100644
> index 00000000000..1100b94a5b7
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp1y/decltype-auto8.C
> @@ -0,0 +1,18 @@
> +// PR c++/115504
> +// { dg-do compile { target c++14 } }
> +
> +void f(int& x) {
> +  [&x]() {
> +    decltype(auto) a = x;
> +    using type = decltype(x);
> +    using type = decltype(a);
> +    using type = int&; // not 'int'
> +  };
> +
> +  [x]() mutable {
> +    decltype(auto) a = x;
> +    using type = decltype(x);
> +    using type = decltype(a);
> +    using type = int&;
> +  };
> +}