Re: [PATCH] c++: noexcept and copy elision [PR109030]

[Jason Merrill <jason@redhat.com>]

On 3/9/23 14:32, Patrick Palka wrote:
> On Mon, 6 Mar 2023, Marek Polacek via Gcc-patches wrote:
> 
>> When processing a noexcept, constructors aren't elided: build_over_call
>> has
>> 	 /* It's unsafe to elide the constructor when handling
>> 	    a noexcept-expression, it may evaluate to the wrong
>> 	    value (c++/53025).  */
>> 	 && (force_elide || cp_noexcept_operand == 0))
>> so the assert I added recently needs to be relaxed a little bit.
>>
>> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>>
>> 	PR c++/109030
>>
>> gcc/cp/ChangeLog:
>>
>> 	* constexpr.cc (cxx_eval_call_expression): Relax assert.
>>
>> gcc/testsuite/ChangeLog:
>>
>> 	* g++.dg/cpp0x/noexcept77.C: New test.
>> ---
>>   gcc/cp/constexpr.cc                     | 6 +++++-
>>   gcc/testsuite/g++.dg/cpp0x/noexcept77.C | 9 +++++++++
>>   2 files changed, 14 insertions(+), 1 deletion(-)
>>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept77.C
>>
>> diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
>> index 364695b762c..5384d0e8e46 100644
>> --- a/gcc/cp/constexpr.cc
>> +++ b/gcc/cp/constexpr.cc
>> @@ -2869,7 +2869,11 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
>>   
>>     /* We used to shortcut trivial constructor/op= here, but nowadays
>>        we can only get a trivial function here with -fno-elide-constructors.  */
>> -  gcc_checking_assert (!trivial_fn_p (fun) || !flag_elide_constructors);
>> +  gcc_checking_assert (!trivial_fn_p (fun)
>> +		       || !flag_elide_constructors
>> +		       /* We don't elide constructors when processing
>> +			  a noexcept-expression.  */
>> +		       || cp_noexcept_operand);
> 
> It seems weird that we're performing constant evaluation within an
> unevaluated operand.  Would it make sense to also fix this a second way
> by avoiding constant evaluation from maybe_constant_init when
> cp_unevaluated_operand && !manifestly_const_eval, like in maybe_constant_value?

Sounds good.

> IIUC since we could still have an evaluated subexpression withis
> noexcept, the two fixes would be complementary.
> 
>>   
>>     bool non_constant_args = false;
>>     new_call.bindings
>> diff --git a/gcc/testsuite/g++.dg/cpp0x/noexcept77.C b/gcc/testsuite/g++.dg/cpp0x/noexcept77.C
>> new file mode 100644
>> index 00000000000..16db8eb79ee
>> --- /dev/null
>> +++ b/gcc/testsuite/g++.dg/cpp0x/noexcept77.C
>> @@ -0,0 +1,9 @@
>> +// PR c++/109030
>> +// { dg-do compile { target c++11 } }
>> +
>> +struct foo { };
>> +
>> +struct __as_receiver {
>> +  foo empty_env;
>> +};
>> +void sched(foo __fun) noexcept(noexcept(__as_receiver{__fun})) { }
>>
>> base-commit: dfb14cdd796ad9df6b5f2def047ef36b29385902
>> -- 
>> 2.39.2
>>
>>
>