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<p class="MsoNormal">There exists a unit test in llvm/unittests/Support/CrashRecoveryTest.cpp that has the following line:<o:p></o:p></p>
<p class="MsoNormal" style="text-indent:.5in">EXPECT_FALSE(CRC.RunSafely(abort));<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The CRC object (CrashRecoveryContext), declares RunSafely like this:<o:p></o:p></p>
<p class="MsoNormal" style="text-indent:.5in">bool RunSafely(function_ref<void()> Fn);<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">A recent commit (b953a01b) added to the function_ref class a check in a constructor to determine if a given Callable returns the expected type ‘Ret’.<o:p></o:p></p>
<p class="MsoNormal">To do this, the commit utilized std::result_of_t.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">+ // This is not the copy-constructor.<o:p></o:p></p>
<p class="MsoNormal"> std::enable_if_t<<o:p></o:p></p>
<p class="MsoNormal"> !std::is_same<std::remove_cv_t<std::remove_reference_t<Callable>>,<o:p></o:p></p>
<p class="MsoNormal">- function_ref>::value> * = nullptr)<o:p></o:p></p>
<p class="MsoNormal">+ function_ref>::value> * = nullptr,<o:p></o:p></p>
<p class="MsoNormal">+ // Functor must be callable and return a suitable type.<o:p></o:p></p>
<p class="MsoNormal">+ std::enable_if_t<std::is_void<Ret>::value ||<o:p></o:p></p>
<p class="MsoNormal">+ std::is_convertible<<o:p></o:p></p>
<p class="MsoNormal">+ std::result_of_t<Callable(Params...)>, Ret>::value><o:p></o:p></p>
<p class="MsoNormal">+ * = nullptr)<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">In libc++, std::result_of_t uses some internal checks to ensure that the type is invokable before inferring the returned type.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">In particular, prior to commit 11a18a79, these checks included a static complete-type detection structure called ‘__check_complete’. The implementation had at its core:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">template <class _Tp><o:p></o:p></p>
<p class="MsoNormal">struct __check_complete<_Tp><o:p></o:p></p>
<p class="MsoNormal">{<o:p></o:p></p>
<p class="MsoNormal"> static_assert(sizeof(_Tp) > 0, "Type must be complete.");<o:p></o:p></p>
<p class="MsoNormal">};<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Our team’s downstream compiler built using a libc++ prior to 11a18a79 (particularly, __libcpp_version contains 5000) errors on the CRC.RunSafely invocation I began with. This is because we instantiate function_ref with the void (*)() type
and attempt to call the constructor modified in commit b953a01b using the function pointer for ‘abort’. This triggers the std::result_of_t trigger that then later involves __check_complete. When we get to the static assert, the compiler errors: Calling sizeof
on a function type is not allowed.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Is this an instance of us needing to upgrade libc++ on the machines because our version is not supported? We’ve confirmed that with the release where __libcpp_version is 8000, the changes from 11a18a79 have been implemented and the file
compiles correctly.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">JB<o:p></o:p></p>
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