C++ Poll Answer for December 29th 2018

Prior to defect report 1550 and defect report 1560 the answer would have been 1 because if the second or third operand of a conditional-expression was a throw-expression the result would be prvalue which would mean a temporary would be created. DR 1560 refers to this as gratuitous and surprising:

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.

This potential incompatibility is also covered in [diff.cpp11.expr]:

Change: A conditional expression with a throw expression as its second or third operand keeps the type and value category of the other operand.
Rationale: Formerly mandated conversions (lvalue-to-rvalue, array-to-pointer, and function-to-pointer standard conversions), especially the creation of the temporary due to lvalue-to-rvalue conversion, were considered gratuitous and surprising.
Effect on original feature: Valid C++ 2011 code that relies on the conversions may behave differently in this International Standard:
struct S {
   int x = 1;
   void mf() { x = 2; }
 };
 int f(bool cond) {
   S s;
   (cond ? s : throw 0).mf();
   return s.x;
 }
In C++ 2011, f(true) returns 1. In this International Standard, it returns 2.
sizeof(true ? "" : throw 0)
In C++ 2011, the expression yields sizeof(const char*). In this International Standard, it yields sizeof(const char[1]).

clang applies this all the way back to C++98 mode see live godbolt but it looks like gcc has a bug and this currently does not compile on trunk.

Shafik Yaghmour