If the if statement is of the form if constexpr, the value of the
condition shall be a contextually converted constant expression of type
[...] A contextually converted constant expression of type bool is an
expression, contextually converted to bool (Clause4), where the
converted expression is a constant expression and the conversion
sequence contains only the conversions above. [...]
Contextually converting result of an expression e to a Boolean value
requires bool t(e) to be well-formed.
An explicit conversion function is only considered as a user-defined
conversion for direct-initialization, which is essentially what
contextually converted to bool requires.
Also, fixes PR28470.