Vfrdtyky

Consider the following code:

int x;

int& f() {

  return x ? x : throw 0;

}

With gcc version 7.3.0 (Ubuntu 7.3.0-27ubuntu1~18.04) I get the following compilation error:

cannot bind non-const lvalue reference of type ‘int&’ to an rvalue of type ‘int’

Note that this compiles just fine in clang. Here is (what I believe to be) the relevant statement from the standard:

N4659 [8.16.2.1] (Conditional Operator):

The second or the third operand (but not both) is a (possibly parenthesized) throw-expression (8.17); the result is of the type and value category of the other.

As far as I understand, x is an lvalue, so it seems to me that clang is right. Am I wrong?

clang is correct here, the old behavior was to unconditionally convert the value to a prvalue which it looks like gcc still implements.

This was the subject of DR 1560 which was fixed by the resolution of DR 1550. DR 1560 says:

A glvalue appearing as one operand of a conditional-expression in
which the other operand is a throw-expression is converted to a
prvalue, regardless of how the conditional-expression is used:

If either the second or the third operand has type void, then the lvalue-to-rvalue (7.1 [conv.lval]), array-to-pointer (7.2
[conv.array]), and function-to-pointer (7.3 [conv.func]) standard
conversions are performed on the second and third operands, and one of
the following shall hold:

• The second or the third operand (but not both) is a throw-expression (18.1 [except.throw]); the result is of the type of
  the other and is a prvalue.

This seems to be gratuitous and surprising.

and DR 1550 changed the wording in [expr.cond] to what we have now:

The second or the third operand (but not both) is a (possibly parenthesized) throw-expression; the result is of the type and value category of the other. The conditional-expression is a bit-field if that operand is a bit-field.

So it looks like gcc is implementing the old behavior while clang is implementing the DR.

This is the patch that applied DR 1560 to clang. It added the following test:

namespace DR1560 { // dr1560: 3.5

  void f(bool b, int n) {

    (b ? throw 0 : n) = (b ? n : throw 0) = 0;

  }

  class X { X(const X&); };

  const X &get();

  const X &x = true ? get() : throw 0;

}

which on godbolt we can see this fails for gcc due to:

error: lvalue required as left operand of assignment

4 |     (b ? throw 0 : n) = (b ? n : throw 0) = 0;

  |                    

We have a gcc bug report for a very similar issue which has the following reduced test case:

I wanted to bump this bug and supply a simpler test-case:

void blah(int&) {}



int main() {

    int i{};

    blah(true ? i : throw);

}

result with gcc 6.0:

prog.cc: In function 'int main()':

prog.cc:6:15: error: invalid initialization of non-const reference of type 'int&' from an rvalue of type 'int'

     blah(true ? i : throw 0);

          ~~~~~^~~~~~~~~~~~~