Revert noexcept deduction in favour of SFINAE lambda function matching #677
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noexcept deduction, added in PR pybind#555, doesn't work with clang's -std=c++1z; and while it works with g++, it isn't entirely clear to me that it is required to work in C++17. What should work, however, is that C++17 allows implicit conversion of a `noexcept(true)` function pointer to a `noexcept(false)` (i.e. default, noexcept-not-specified) function pointer. That was breaking in pybind11 because the cpp_function template used for lambdas provided a better match (i.e. without requiring an implicit conversion), but it then failed. This commit takes a different approach of using SFINAE on the lambda function to prevent it from matching a non-lambda object, which then gets implicit conversion from a `noexcept` function pointer to a `noexcept(false)` function pointer. This much nicer solution also gets rid of the C++17 NOEXCEPT macros, and works in both clang and g++.
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That's a much nicer solution, and it makes this quite readable again -- thank you! |
This was referenced Mar 3, 2017
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noexceptdeduction, added in PR #555, doesn't work with clang's-std=c++1z; and while it works with g++, it isn't entirely clear to me that it is required to work in C++17.What should work, however, is C++17's umplicit conversion of a
noexcept(true)function pointer to anoexcept(false)one (i.e. default,noexcept-specifier-omitted). That was breaking in C++17 mode in pybind11 before #555 because thecpp_functiontemplate used for lambdas provided a better match (i.e. without requiring an implicit conversion), but it then failed when trying to treat the matched function pointer as a lambda object.This commit takes a different approach of using SFINAE on the lambda function to prevent it from matching a non-lambda object, which then gets implicit conversion from a
noexceptfunction pointer to anoexcept(false)function pointer. This much nicer solution also gets rid of the C++17 NOEXCEPT macros, and works in both clang and g++.With this, pybind11 builds and tests successfully with clang 4.0 and libc++ 4.0 on linux under
-std=c++1z.