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MSVC's implementation of the C++ Standard Library.
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`<__msvc_int128.hpp>`: Fix warning C26437 about slicing #4712

Closed StephanTLavavej closed 3 weeks ago

StephanTLavavej commented 3 weeks ago

In internal MSVC-PR-555396, @MahmoudGSaleh updated our checked-in toolset to VS 2022 17.11 Preview 2 and encountered a newly emitted warning C26437: "Do not slice (es.63)." One affected call:

https://github.com/microsoft/STL/blob/e36ee6c2b9bc6f5b1f70776c18cf5d3a93a69798/stl/inc/__msvc_int128.hpp#L902-L903

This can be avoided, for all callsites, by changing _Base128's _Divide() and _Modulo() to copy their parameters locally. (This is still slicing, of course, just in a way that the compiler won't complain about. :smirk_cat:)

I'm also updating our test coverage to prevent regressions. Note that 17.11 Preview 2 newly warns about this code; 17.11 Preview 1 didn't warn.

AlexGuteniev commented 3 weeks ago

Maybe comment like "slice explicitly to avoid C26437" ?

StephanTLavavej commented 3 weeks ago

The static analysis team has confirmed that this is actually a compiler bug, VSO-2080466 "Code Analysis: False C26437 warnings due to badly constructed AST for assignment operation from base class object for trivially copiable base class". While _Divide was technically slicing here, the compiler was designed to not warn when the derived class adds no additional data members. The fact that it's emitting a warning is a regression. We're going to disable the warning in the internal build, instead of modifying the STL's sources, until the compiler's warning logic is properly fixed.