[llvm-bugs] [Bug 25736] New: constexpr allows uninitialized variables

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=25736

            Bug ID: 25736
           Summary: constexpr allows uninitialized variables
           Product: clang
           Version: 3.7
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++14
          Assignee: unassignedclangbugs at nondot.org
          Reporter: tomilovanatoliy at gmail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Given a code:

    #include <type_traits>

    struct A { int i; };

    template< typename type >
    struct C { constexpr C() = default; type a; };

    struct V { V() = default; C< A > d; };

    constexpr bool test()
    {
        V v;
        static_assert(std::is_trivially_default_constructible< V >::value,
"!");
        return (v.d.a.i == 0);
    }

    static_assert(test(), "!");

It compiles without errors by `clang`. Compilation by `g++` failed with error:

main.cpp: In function 'constexpr bool test()':
main.cpp:12:11: error: uninitialized variable 'v' in 'constexpr' function
         V v;
           ^
main.cpp:8:12: note: 'struct V' has no user-provided default constructor
     struct V { V() = default; C< A > d; };
            ^
main.cpp:8:16: note: constructor is not user-provided because it is explicitly
defaulted in the class body
     struct V { V() = default; C< A > d; };
                ^
main.cpp:3:20: note: and the implicitly-defined constructor does not initialize
'int A::i'
     struct A { int i; };
                    ^
main.cpp: At global scope:
main.cpp:17:5: error: non-constant condition for static assertion
     static_assert(test(), "!");
     ^
main.cpp:17:23: error: 'constexpr bool test()' called in a constant expression
     static_assert(test(), "!");
                       ^

If I remove `template`s and substitute `A` directly, then `clang` sees
uninitialized during default construction variables.
If I remove `constexpr` specifier of `C::C()` default constructor, then `clang`
also sees wrong things.

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