Value category of conditional operator

问题

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?

If I had to take a guess, the l-to-rvalue conversion is occurring because the two expressions in the conditional don't have the same type, but because the second is a throw this conversion should be preempted. I'm not familiar with submitting bug reports, but perhaps that would be a better forum for this.

Here are some (probably) more helpful questions about the conditional operator:
Why does this function return an lvalue reference given rvalue arguments?
Error: lvalue required in this simple C code? (Ternary with assignment?)

回答1:

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);
          ~~~~~^~~~~~~~~~~~~

来源：https://stackoverflow.com/questions/53328407/value-category-of-conditional-operator

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c++

g++

language-lawyer

conditional-operator

value-categories