[PATCH] Implement sane variant converting constructor (P0608R3)

[Jonathan Wakely]

On 17/05/19 15:55 +0200, Rainer Orth wrote:
>Hi Jonathan,
>
>> 	* include/std/variant (__overload_set): Remove.
>> 	(_Arr): New helper.
>> 	(_Build_FUN): New class template to define a single FUN overload,
>> 	with specializations to prevent unwanted conversions, as per P0608R3.
>> 	(_Build_FUNs): New class template to build an overload set of FUN.
>> 	(_FUN_type): New alias template to perform overload resolution.
>> 	(__accepted_type): Use integer_constant base for failure case. Use
>> 	_FUN_type for successful case.
>> 	(variant::__accepted_index): Use _Tp instead of _Tp&&.
>> 	(variant::variant(_Tp&&)): Likewise.
>> 	(variant::operator=(_Tp&&)): Likewise.
>>
>> Tested powerpc64le-linux, committed to trunk.
>[...]
>> diff --git a/libstdc++-v3/testsuite/20_util/variant/compile.cc b/libstdc++-v3/testsuite/20_util/variant/compile.cc
>> index c6b18d08258..4560f774452 100644
>> --- a/libstdc++-v3/testsuite/20_util/variant/compile.cc
>> +++ b/libstdc++-v3/testsuite/20_util/variant/compile.cc
>> @@ -142,6 +142,11 @@ void arbitrary_ctor()
>>    static_assert(noexcept(variant<int, DefaultNoexcept>(int{})));
>>    static_assert(!noexcept(variant<int, Empty>(Empty{})));
>>    static_assert(noexcept(variant<int, DefaultNoexcept>(DefaultNoexcept{})));
>> +
>> +  // P0608R3 disallow narrowing conversions and boolean conversions
>> +  static_assert(!is_constructible_v<variant<int>, long>);
>> +  static_assert(!is_constructible_v<variant<bool>, int>);
>> +  static_assert(!is_constructible_v<variant<bool>, void*>);
>
>this test (which didn't make it into the ChangeLog, btw.) FAILs on

Drat, not sure how I missed the two test changes out, sorry.

>32-bit targets (seen on i386-pc-solaris2.11, sparc-sun-solaris.11, and
>x86_64-pc-linux-gnu -m32):
>
>+FAIL: 20_util/variant/compile.cc (test for excess errors)
>
>Excess errors:
>/vol/gcc/src/hg/trunk/local/libstdc++-v3/testsuite/20_util/variant/compile.cc:147: error: static assertion failed

I keep forgetting 32-bit exists ;-)

Of course if sizeof(int) == sizeof(long) then it's not a lossy
conversion, and so is allowed.

This patch makes it always a narrowing conversion, irrespective of
target. I'll commit it shortly.


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