CWG2712 [over.match.oper] Unnecessary restriction on built-in assignment operator candidates

[t3nsor]

Full name of submitter: Brian Bi

Reference (section label): [over.match.oper]

Issue description: As pointed out by Jan Tušil on Stack Overflow, the first bullet in paragraph 5, which (according to my understanding) specifies that a built-in assignment operator candidate is not viable if the implicit conversion sequence required for the left operand would create a temporary, appears to be unnecessary; the second bullet already bans user-defined conversions, and the left parameter (of some type "vq T&", where vq is either "volatile" or empty) will never bind to a temporary in any standard conversion sequence.

Suggested resolution: Edit [over.match.oper]/5:

For the first parameter of the built-in assignment operators, only standard conversion sequences ([over.ics.scs]) are considered. conversions of the left operand are restricted as follows:

• no temporaries are introduced to hold the left operand, and
• no user-defined conversions are applied to the left operand to achieve a type match with the left-most
  parameter of a built-in candidate.

[jensmaurer]

CWG2712