Suggested resolution:

Change in 6.7.7 [class.temporary] bullet 6.6 as follows:

• ...
• a
  • const_cast (7.6.1.11 [expr.const.cast]),
  • static_cast (7.6.1.9 [expr.static.cast]),
  • dynamic_cast (7.6.1.7 [expr.dynamic.cast]), or
  • reinterpret_cast (7.6.1.10 [expr.reinterpret.cast])
  converting, without a user-defined conversion, a glvalue operand that is one of these expressions to a glvalue that refers to the object designated by the operand, or to its complete object or a subobject thereof, where
  • the cast does not invoke a user-defined conversion, or
  • the cast invokes a user-defined conversion that yields a prvalue, and the glvalue result of the cast is bound directly (9.4.4 [dcl.init.ref]) to that prvalue,
• ...

Consider:

  struct S { S(int) {} };

  const S& r1 = 0; // #1
  const S& r2 = static_cast<const S&>(S(0)); // #2
  const S& r3 = static_cast<const S&>(0); // #3
  // Are r1, r2, and r3 dangling past this point?

For #1, a temporary object is materialized and the reference binds to the result of the materialization conversion. This results in lifetime extension per 6.7.7 [class.temporary] bullet 6.1.

For #2, 6.7.7 [class.temporary] bullet 6.6 requires the cast to convert "without a user-defined conversion", but S(0) is such.

For #3, a user-defined conversion is clearly required.

However, implementations treat all three cases the same and do extend the lifetime of the temporary.