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operator/(duration, duration) cannot be used #555

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mtnpke opened this issue Jan 28, 2025 · 1 comment · May be fixed by #557
Open

operator/(duration, duration) cannot be used #555

mtnpke opened this issue Jan 28, 2025 · 1 comment · May be fixed by #557

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@mtnpke
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mtnpke commented Jan 28, 2025

Trying to invoke operator/(duration, duration) yields to compile errors on MSVC because it tries to instantiate the other overload - operator/(duration, number) - and fails in determining the common_type of the arguments. This failure is fatal because EASTL includes the original C++11 common_type definition that is not SFINAE-friendly. This was later acknowledged in N3843 and fixed for C++17 in LWG2408.

(This is irrespective of and a separate issue to #554)

Example code:

seconds s(5);
milliseconds ms(10);
auto result{s / ms}; // added as line 107 in TestChrono.cpp; see below

Error message:

C:\_GIT\EASTL\include\EASTL\internal\type_compound.h(640): error C2446: ':': no conversion from 'T' to 'T'
          with
          [
              T=eastl::chrono::milliseconds
          ]
          and
          [
              T=__int64
          ]
  C:\_GIT\EASTL\include\EASTL/internal/type_compound.h(640): note: the template instantiation context (the oldest one first) is
  C:\_GIT\EASTL\test\source\TestChrono.cpp(107): note: see reference to function template instantiation 'eastl::chrono::duration<eastl::common_type<Rep1,Rep2>::type,Period1> eastl::chrono::operator /(const eastl::chrono::duration<Rep,Period> &,const Rep2 &)' being compiled
  C:\_GIT\EASTL\test\source\TestChrono.cpp(107): note: see reference to class template instantiation 'eastl::common_type<__int64,eastl::chrono::milliseconds>' being compiled
C:\_GIT\EASTL\include\EASTL\internal\type_compound.h(640): error C2062: type 'unknown-type' unexpected
C:\_GIT\EASTL\include\EASTL\internal\type_compound.h(640): error C2238: unexpected token(s) preceding ';'

Compiler version: Microsoft (R) C/C++ Optimizing Compiler Version 19.40.33817 for x64
@mtnpke
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mtnpke commented Jan 28, 2025 •
edited
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Further research: While SFINAE seems to fix this problem, the root cause is likely that EASTL is missing an enable_if. The standard says regarding operator/(duration, number): "This operator shall not participate in overload resolution unless Rep2 is implicitly convertible to CR(Rep1, Rep2) and Rep2 is not a specialization of duration." - this is not guarded in EASTL

@mtnpke mtnpke changed the title operator/(duration, duration) cannot be used because common_type is SFINAE-unfriendly operator/(duration, duration) cannot be used Jan 28, 2025
mtnpke added a commit to mtnpke/EASTL that referenced this issue Jan 28, 2025
@mtnpke
Fix chrono duration operator* return type and enable_if
5716a7f 
The division of two durations does not return in a duration, but a plain
number.
Also, the (duration, number) overloads of operator/ and operator%
shall not participate in overload resolution if Rep2 is a duration.

Fixes electronicarts#554, electronicarts#555.
mtnpke added a commit to mtnpke/EASTL that referenced this issue Jan 28, 2025
@mtnpke
Fix chrono duration operator/ and % enable_if
1e83906 
The (duration, number) overloads of operator/ and operator%
shall not participate in overload resolution if Rep2 is a duration.

Fixes electronicarts#555.
This was referenced Jan 28, 2025
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Update common_type to C++17 for SFINAE mtnpke/EASTL
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