[Bug c++/53903] New: [C++11] Incompatible exception-specification allowed if member explicitly-defaulted after first declaration

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53903

             Bug #: 53903
           Summary: [C++11] Incompatible exception-specification allowed
                    if member explicitly-defaulted after first declaration
    Classification: Unclassified
           Product: gcc
           Version: 4.7.1
            Status: UNCONFIRMED
          Keywords: accepts-invalid
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: r...@gcc.gnu.org
                CC: ja...@gcc.gnu.org


struct T {
    T() noexcept(false) { }
    ~T() noexcept(false) { }
};

struct A
{
    A() noexcept;
    ~A() noexcept;

    T t;
};

A::A() noexcept = default;
A::~A() noexcept = default;

This is accepted, but I think it shouldn't be.

[dcl.fct.def.default]/2

    An explicitly-defaulted function [...] may have an explicit
    exception-specification only if it is compatible (15.4) with
    the exception specification on the implicit declaration."

The implicit declarations for A::A() and A::~A() would be noexcept(false) which
is not compatible.

If the functions are defaulted on their first declaration they get rejected:

struct T {
    T() noexcept(false) { }
    ~T() noexcept(false) { }
};

struct A
{
    A() noexcept = default;
    ~A() noexcept = default;

    T t;
};

ex.cc:8:5: error: function 'A::A()' defaulted on its first declaration with an
exception-specification that differs from the implicit declaration 'A::A()'
     A() noexcept = default;
     ^
ex.cc:9:5: error: function 'A::~A()' defaulted on its first declaration with an
exception-specification that differs from the implicit declaration 'A::~A()'
     ~A() noexcept = default;
     ^

The diagnostic specifically refers to defaulted on first declaration, implying
that's significant, but the [dcl.fct.def.default] wording above doesn't
distinguish between explicitly defaulting on the first declaration or after it.