[PATCH] c++: Fix excessive instantiation inside decltype [PR71879]

[Jason Merrill]

On 1/20/21 11:27 AM, Patrick Palka wrote:
> On Tue, 19 Jan 2021, Jason Merrill wrote:
> 
>> On 1/18/21 12:31 AM, Patrick Palka wrote:
>>> Here after resolving the address of a template-id inside decltype, we
>>> end up instantiating the chosen specialization from the call to
>>> mark_used in resolve_nondeduced_context, even though only its type is
>>> needed.
>>>
>>> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look for
>>> trunk?
>>>
>>> gcc/cp/ChangeLog:
>>>
>>> 	PR c++/71879
>>> 	* semantics.c (finish_decltype_type): Temporarily increment
>>> 	cp_unevaluated_operand during call to resolve_nondeduced_context.
>>>
>>> gcc/testsuite/ChangeLog:
>>>
>>> 	PR c++/71879
>>> 	* g++.dg/cpp0x/decltype-71879.C: New test.
>>> ---
>>>    gcc/cp/semantics.c                          | 2 ++
>>>    gcc/testsuite/g++.dg/cpp0x/decltype-71879.C | 5 +++++
>>>    2 files changed, 7 insertions(+)
>>>    create mode 100644 gcc/testsuite/g++.dg/cpp0x/decltype-71879.C
>>>
>>> diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
>>> index c8a6283b120..cad55665ce8 100644
>>> --- a/gcc/cp/semantics.c
>>> +++ b/gcc/cp/semantics.c
>>> @@ -10098,7 +10098,9 @@ finish_decltype_type (tree expr, bool
>>> id_expression_or_member_access_p,
>>>        /* The type denoted by decltype(e) is defined as follows:  */
>>>    +  ++cp_unevaluated_operand;
>>>      expr = resolve_nondeduced_context (expr, complain);
>>> +  --cp_unevaluated_operand;
>>
>> Hmm, is there a reason not to have cp_unevaluated_operand set through the
>> whole function?  We might stick 'cp_unevaluated u;' at the top of the function
>> and remove the existing messing with cp_unevaluated_operand.  Or assert that
>> it's set and fix the callers to leave it set longer.
> 
> That makes sense to me.  AFAICT the only parts of finish_decltype_type
> tha are sensitive to cp_unevaluated_operand are
> resolve_nondeduced_context and instantiate_nondependent_expr_sfinae,
> but it makes sense to have cp_unevaluated_operand set throughout the
> function.
> 
> The following passes limited testing, does it look OK for trunk after a
> full bootstrap+regtest?

OK.

> -- >8 --
> 
> Subject: [PATCH] c++: Fix excessive instantiation inside decltype [PR71879]
> 
> Here after resolving the address of a template-id inside decltype, we
> end up instantiating the chosen specialization (from the call to
> mark_used in resolve_nondeduced_context), even though only its type is
> needed.
> 
> This patch sets cp_unevaluated_operand throughout finish_decltype_type,
> so that in particular it's set during the call to
> resolve_nondeduced_context within.
> 
> gcc/cp/ChangeLog:
> 
> 	PR c++/71879
> 	* semantics.c (finish_decltype_type): Set up a cp_unevaluated
> 	sentinel at the start of the function.  Remove a now-redundant
> 	manual adjustment of cp_unevaluated_operand.
> 
> gcc/testsuite/ChangeLog:
> 
> 	PR c++/71879
> 	* g++.dg/cpp0x/decltype-71879.C: New test.
> ---
>   gcc/cp/semantics.c                          | 5 +++--
>   gcc/testsuite/g++.dg/cpp0x/decltype-71879.C | 5 +++++
>   2 files changed, 8 insertions(+), 2 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/decltype-71879.C
> 
> diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
> index 244fc70d02d..834885616db 100644
> --- a/gcc/cp/semantics.c
> +++ b/gcc/cp/semantics.c
> @@ -10080,6 +10080,9 @@ finish_decltype_type (tree expr, bool id_expression_or_member_access_p,
>         return error_mark_node;
>       }
>   
> +  /* The operand of decltype is an unevaluated expression.  */
> +  cp_unevaluated u;
> +
>     /* Depending on the resolution of DR 1172, we may later need to distinguish
>        instantiation-dependent but not type-dependent expressions so that, say,
>        A<decltype(sizeof(T))>::U doesn't require 'typename'.  */
> @@ -10095,9 +10098,7 @@ finish_decltype_type (tree expr, bool id_expression_or_member_access_p,
>       }
>     else if (processing_template_decl)
>       {
> -      ++cp_unevaluated_operand;
>         expr = instantiate_non_dependent_expr_sfinae (expr, complain);
> -      --cp_unevaluated_operand;
>         if (expr == error_mark_node)
>   	return error_mark_node;
>       }
> diff --git a/gcc/testsuite/g++.dg/cpp0x/decltype-71879.C b/gcc/testsuite/g++.dg/cpp0x/decltype-71879.C
> new file mode 100644
> index 00000000000..9da4d40ca70
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/decltype-71879.C
> @@ -0,0 +1,5 @@
> +// PR c++/71879
> +// { dg-do compile { target c++11 } }
> +
> +template <class T> void f(T x) { x.fail(); }
> +using R = decltype(&f<int>);
>