[libcxx-commits] [PATCH] D96683: [libcxx] adds concept `std::common_with`

Louis Dionne via Phabricator via libcxx-commits libcxx-commits at lists.llvm.org
Fri Feb 19 14:26:09 PST 2021

ldionne added inline comments.

Comment at: libcxx/include/concepts:195
+  common_reference_with<
+    add_lvalue_reference_t<common_type_t<_Tp, _Up>>,
+    common_reference_t<
I see this follows the spec closely, but I'm curious to understand. Here, I would have expected the check to be:

    common_type_t<_Tp, _Up> const&,
        const _Tp&,
        const _Up&>>;

instead (with `add_lvalue_reference_t` simplified naively for legibility). In other words, why are we looking for a common reference between the non-const common type and the common-ref between the `const`-qualified types? I'm sure that'll highlight a misunderstanding I have about the concept.

Comment at: libcxx/test/std/concepts/lang/common.compile.pass.cpp:19
+constexpr bool CheckCommonWith() noexcept {
+  static_assert(std::common_with<T, U&>);
+  static_assert(std::common_with<T, const U&>);
Same comment about testing with `static_assert(std::common_with<T, U&> == std::common_with<T, U>)` as in the other patch, if you think it makes sense.

Comment at: libcxx/test/std/concepts/lang/common.compile.pass.cpp:257
+struct BadBasicCommonType {
+  // This test is ill-formed, NDR. If it ever blows up in our faces: that's a good thing.
+  // In the meantime, the test should be included. If compiler support is added, then an include guard
I'm not sure I follow this. Can you explain?

  rG LLVM Github Monorepo



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