Title
Constant conversions to bool
Status
cd4
Section
14.5 [except.spec]
Submitter
Richard Smith

Created on 2014-11-09.00:00:00 last changed 49 months ago

Messages

Date: 2015-09-15.00:00:00

Proposed resolution (September, 2015):

  1. Change 7.7 [expr.const] paragraph 4 as follows:

  2. ...binds directly. [Note: such expressions may be used in new expressions (7.6.2.8 [expr.new]), as case expressions (8.5.3 [stmt.switch]), as enumerator initializers if the underlying type is fixed (9.7.1 [dcl.enum]), as array bounds (9.3.4.5 [dcl.array]), and as non-type template arguments (13.4 [temp.arg]). —end note] A contextually converted constant expression of type bool is an expression, contextually converted to bool (Clause 7.3 [conv]), where the converted expression is a constant expression and the conversion sequence contains only the conversions above.
  3. Change Clause 9 [dcl.dcl] paragraph 6 as follows:

  4. In a static_assert-declaration, the constant-expression shall be a contextually converted constant expression of type bool (7.7 [expr.const]) that can be contextually converted to bool (Clause 7.3 [conv]). If the value of the expression when so converted is true...
  5. Change 14.5 [except.spec] paragraph 1 as follows:

  6. In a noexcept-specification, the constant-expression, if supplied, shall be a contextually converted constant expression of type bool (7.7 [expr.const]) that is contextually converted to bool (Clause 7.3 [conv]). A ( token that follows noexcept...
Date: 2016-02-15.00:00:00

[Adopted at the February, 2016 meeting.]

According to 14.5 [except.spec] paragraph 1,

In a noexcept-specification, the constant-expression, if supplied, shall be a constant expression (7.7 [expr.const]) that is contextually converted to bool (Clause 7.3 [conv]).

This allows the expression to have any type that can be converted to bool, which is too lenient; it should instead say something like “a converted constant expression of type bool (7.7 [expr.const]).” This would include the conversion to bool in the determination of whether the expression is constant or not and would also disallow narrowing conversions.

A similar consideration applies to static_assert (Clause 9 [dcl.dcl] paragraph 6), which should probably be recast to something like, “an expression that is a constant expression (7.7 [expr.const]) after contextual conversion to bool (Clause 7.3 [conv]).”

History
Date User Action Args
2017-02-06 00:00:00adminsetstatus: tentatively ready -> cd4
2015-11-10 00:00:00adminsetmessages: + msg5616
2015-11-10 00:00:00adminsetstatus: drafting -> tentatively ready
2014-11-09 00:00:00admincreate