Question

This code is rejected by (at least) MSVC, ICC, and GCC:

class A {
public:
    A( int ) {  }
};

class B: virtual public A {
public:
    //B(): A( -1 ) {  } // uncomment to make it compilable
    virtual void do_something() = 0;
};

class C: public B {
public:
    C(): A( 1 ) {  }
    virtual void do_something() {  }
};

int main() {
    C c;
    return 0;
}

on the basis of

error : no default constructor exists for class "A"
    class B: virtual public A {
                            ^
            detected during implicit generation of "B::B()" at line 14

Questions:

1. If the code is indeed invalid, how exactly does this follow from
   the standard? AFAICT, 10.4/2 and 1.8/4 taken together imply that B
   cannot be a type of the most derived class, and therefore from
   12.6.2/10 we have that B can never, ever call A’s constructors.
   (The section numbers are for C++11.)

2. If the code is valid, are compilers violating the standard by
   requiring the presence of constructors they could not possibly call?
   Note that not only they want to call A::A() from B::B(), but they
   want to do it while compiling C::C() (double weird).

P.S. This was originally asked on the ICC forum, but posted here due to not being limited to this compiler (and no details forthcoming).

Answer 1

Clang shows the error as:

error: call to implicitly-deleted default constructor of 'B'
    C(): A( 1 ) {  }
    ^

12.1/5 says “A defaulted default constructor for class X is defined as deleted if […] any […] virtual base class […] has class type M […] and […] M has no default constructor […].”