[llvm-bugs] [Bug 43454] New: Hard error when checking validity of protected default constructor in SFINAE context

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43454

            Bug ID: 43454
           Summary: Hard error when checking validity of protected default
                    constructor in SFINAE context
           Product: clang
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: ldionne at apple.com
                CC: llvm-bugs at lists.llvm.org, neeilans at live.com,
                    richard-llvm at metafoo.co.uk

The following code causes a hard compilation error on Clang trunk in all
standard modes, except C++17:

    #include <utility>
    #include <type_traits>

    struct Base { };
    struct Derived : Base { protected: Derived() = default; };


    template <class _Tp>
    void test_implicit_default_constructible(_Tp);

    template <class _Tp, class = void>
    struct is_implicitly_default_constructible
        : std::false_type
    { };

    template <class _Tp>
    struct is_implicitly_default_constructible<_Tp,
decltype(test_implicit_default_constructible<_Tp const&>({}))>
        : std::is_default_constructible<_Tp>
    { };
    static_assert(!is_implicitly_default_constructible<Derived>::value, "");

You can reproduce with:

    $ cat <file above> | clang++ -xc++ - -std=c++2a -fsyntax-only

    <stdin>:17:110: error: calling a protected constructor of class 'Derived'
        struct is_implicitly_default_constructible<_Tp,
decltype(test_implicit_default_constructible<_Tp const&>({}))>
                                                                               
                                 ^
    <stdin>:20:20: note: in instantiation of template class
'is_implicitly_default_constructible<Derived, void>' requested
          here
        static_assert(!is_implicitly_default_constructible<Derived>::value,
"");
                       ^
    <stdin>:5:40: note: declared protected here
        struct Derived : Base { protected: Derived() = default; };
                                           ^
    1 error generated.

Live repro: https://godbolt.org/z/e9-G8b


Instead, I would expect the static_assert to pass since Derived isn't default
constructible (because of access checking), and we check that inside a SFINAE
context (so no hard error should happen).

Note that Clang produces an error in C++14 and C++2a modes, but produces no
error in C++17 mode. GCC trunk, in comparison, produces no error in C++14,
C++17 and C++2a mode.

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