US195 18.05.2 Remove concept "boolean" and use convertible_to<bool> instead P1934 P1964

[wg21bot]

The boolean concept is over-complicated and fails to capture what it intends because doing so would require it to be recursive (i.e., b satisfies boolean iff the expression b && b also satisfies boolean, etc.). LEWG decided in Cologne that the boolean concept should be removed and all uses of it in the library be replaced with convertible_to<bool>.

Proposed change:
Remove the boolean concept and replace all uses of it with convertible_to<bool>.

[tituswinters]

LEWG in Belfast: Forward to LWG for C++20.
(Prioritization on this vs. other paper-sized NB comments will be shared with LWG chair when LEWG finishes NB comment processing.)

[dietmarkuehl]

LWG small group looked at that on Wednesday morning: there is paper P1934 cplusplus/papers#680. There is also alternative D1964 (attached to the LEWG wiki). We probably want to have LEWG look at that before following the recommendation.

[jensmaurer]

See also P1964 Casting convertible_to<bool> considered harmful cplusplus/papers#698

[JeffGarland]

Reviewed Monday afternoon full group in Prague -- P1964 will be reviewed later.

[JeffGarland]

LWG Tuesday afternoon in Prague - accept 1964 for C++20

[jensmaurer]

Accepted with modification. See paper P1964R1.