[libcxx-commits] [PATCH] D140911: [libc++] Implement P2505R5(Monadic operations for std::expected).

[Louis Dionne via Phabricator via libcxx-commits]

ldionne added inline comments.


================
Comment at: libcxx/include/__expected/expected.h:843
+    using _Gp = remove_cv_t<invoke_result_t<_Func, _Err&&>>;
+    if constexpr (!__valid_std_unexpected<_Gp>::value) {
+      static_assert(__valid_std_unexpected<_Gp>::value,
----------------
yronglin wrote:
> So sorry, this code looks very ugly, it used to simplify the compile error diagnostic message(useful when I write the test case to check the `static_assert message` for LWG3866), do you have more good ideas for this? 
> 
> before:
> ```
> In file included from /Users/yrong/Developer/Toolchain/llvm/trunk/llvm-project/libcxx/test/libcxx/utilities/expected/expected.void/transform_error.mandates.verify.cpp:29:
> In file included from /Users/yrong/Developer/Toolchain/llvm/trunk/rel/include/c++/v1/expected:44:
> /Users/yrong/Developer/Toolchain/llvm/trunk/rel/include/c++/v1/__expected/expected.h:1433:19: error: static assertion failed due to requirement 'integral_constant<bool, false>::value': The result of f(error()) must be a valid template argument for unexpected
>     static_assert(__valid_std_unexpected<_Gp>::value,
>                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> /Users/yrong/Developer/Toolchain/llvm/trunk/llvm-project/libcxx/test/libcxx/utilities/expected/expected.void/transform_error.mandates.verify.cpp:50:7: note: in instantiation of function template specialization 'std::expected<void, int>::transform_error<std::unexpected<int> (&)(int &)>' requested here
>     e.transform_error(return_unexpected); 
>       ^
> In file included from /Users/yrong/Developer/Toolchain/llvm/trunk/llvm-project/libcxx/test/libcxx/utilities/expected/expected.void/transform_error.mandates.verify.cpp:29:
> In file included from /Users/yrong/Developer/Toolchain/llvm/trunk/rel/include/c++/v1/expected:44:
> /Users/yrong/Developer/Toolchain/llvm/trunk/rel/include/c++/v1/__expected/expected.h:954:17: error: static assertion failed due to requirement 'integral_constant<bool, false>::value': [expected.void.general] A program that instantiates expected<T, E> with a E that is not a valid argument for unexpected<E> is ill-formed
>   static_assert(__valid_std_unexpected<_Err>::value,
>                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> /Users/yrong/Developer/Toolchain/llvm/trunk/rel/include/c++/v1/__expected/expected.h:1436:14: note: in instantiation of template class 'std::expected<void, std::unexpected<int>>' requested here
>       return expected<_Tp, _Gp>();
>              ^
> /Users/yrong/Developer/Toolchain/llvm/trunk/llvm-project/libcxx/test/libcxx/utilities/expected/expected.void/transform_error.mandates.verify.cpp:50:7: note: in instantiation of function template specialization 'std::expected<void, int>::transform_error<std::unexpected<int> (&)(int &)>' requested here
>     e.transform_error(return_unexpected); 
>       ^
> In file included from /Users/yrong/Developer/Toolchain/llvm/trunk/llvm-project/libcxx/test/libcxx/utilities/expected/expected.void/transform_error.mandates.verify.cpp:29:
> In file included from /Users/yrong/Developer/Toolchain/llvm/trunk/rel/include/c++/v1/expected:44:
> /Users/yrong/Developer/Toolchain/llvm/trunk/rel/include/c++/v1/__expected/expected.h:1438:12: error: excess elements in struct initializer
>     return expected<_Tp, _Gp>(__expected_construct_unexpected_from_invoke_tag{}, std::forward<_Func>(__f), error());
>            ^                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> /Users/yrong/Developer/Toolchain/llvm/trunk/llvm-project/libcxx/test/libcxx/utilities/expected/expected.void/transform_error.mandates.verify.cpp:50:7: note: in instantiation of function template specialization 'std::expected<void, int>::transform_error<std::unexpected<int> (&)(int &)>' requested here
>     e.transform_error(return_unexpected); 
>       ^
> In file included from /Users/yrong/Developer/Toolchain/llvm/trunk/llvm-project/libcxx/test/libcxx/utilities/expected/expected.void/transform_error.mandates.verify.cpp:29:
> In file included from /Users/yrong/Developer/Toolchain/llvm/trunk/rel/include/c++/v1/expected:44:
> ```
> after:
> ```
> In file included from /Users/yrong/Developer/Toolchain/llvm/trunk/llvm-project/libcxx/test/libcxx/utilities/expected/expected.expected/transform_error.mandates.verify.cpp:29:
> In file included from /Users/yrong/Developer/Toolchain/llvm/trunk/rel/include/c++/v1/expected:44:
> /Users/yrong/Developer/Toolchain/llvm/trunk/rel/include/c++/v1/__expected/expected.h:814:19: error: static assertion failed due to requirement 'integral_constant<bool, false>::value': The result of f(error()) must be a valid template argument for unexpected
>     static_assert(__valid_std_unexpected<_Gp>::value,
>                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> /Users/yrong/Developer/Toolchain/llvm/trunk/llvm-project/libcxx/test/libcxx/utilities/expected/expected.expected/transform_error.mandates.verify.cpp:45:7: note: in instantiation of function template specialization 'std::expected<int, int>::transform_error<std::unexpected<int> (&)(int)>' requested here
>     e.transform_error(return_unexpected<int&>); // expected-error-re@*:* {{{{(static_assert|static assertion)}} failed {{.*}} The result of {{.*}} must be a valid template argument for unexpected}}
>       ^
> In file included from /Users/yrong/Developer/Toolchain/llvm/trunk/llvm-project/libcxx/test/libcxx/utilities/expected/expected.expected/transform_error.mandates.verify.cpp:29:
> In file included from /Users/yrong/Developer/Toolchain/llvm/trunk/rel/include/c++/v1/expected:44:
> /Users/yrong/Developer/Toolchain/llvm/trunk/rel/include/c++/v1/__expected/expected.h:814:19: error: static assertion failed due to requirement 'integral_constant<bool, false>::value': The result of f(error()) must be a valid template argument for unexpected
>     static_assert(__valid_std_unexpected<_Gp>::value,
>                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> /Users/yrong/Developer/Toolchain/llvm/trunk/llvm-project/libcxx/test/libcxx/utilities/expected/expected.expected/transform_error.mandates.verify.cpp:46:7: note: in instantiation of function template specialization 'std::expected<int, int>::transform_error<int &(&)(int)>' requested here
>     e.transform_error(return_no_object<int&>); // expected-error-re@*:* {{{{(static_assert|static assertion)}} failed {{.*}} The result of {{.*}} must be a valid template argument for unexpected}}
>       ^
> In file included from /Users/yrong/Developer/Toolchain/llvm/trunk/llvm-project/libcxx/test/libcxx/utilities/expected/expected.expected/transform_error.mandates.verify.cpp:29:
> In file included from /Users/yrong/Developer/Toolchain/llvm/trunk/rel/include/c++/v1/expected:44:
> /Users/yrong/Developer/Toolchain/llvm/trunk/rel/include/c++/v1/__expected/expected.h:829:19: error: static assertion failed due to requirement 'integral_constant<bool, false>::value': The result of f(error()) must be a valid template argument for unexpected
>     static_assert(__valid_std_unexpected<_Gp>::value,
>                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> /Users/yrong/Developer/Toolchain/llvm/trunk/llvm-project/libcxx/test/libcxx/utilities/expected/expected.expected/transform_error.mandates.verify.cpp:52:7: note: in instantiation of function template specialization 'std::expected<int, int>::transform_error<std::unexpected<int> (&)(int)>' requested here
>     e.transform_error(return_unexpected<const int &>); // expected-error-re@*:* {{{{(static_assert|static assertion)}} failed {{.*}} The result of {{.*}} must be a valid template argument for unexpected}}
>       ^
> 
> ```
I'm not sure I see how the "after" is better than the "before"? They look very similar, don't they?


Repository:
  rG LLVM Github Monorepo

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