Previously we had a hard-coded list of allowed combinations in the
grammar, but this suffers from combinatorial explosion, and results
in a vague `Syntax error in input` error for invalid (and missing)
combinations.
This means we now support a number of cases which are valid C++
but weren't supported.
Fixes#302Fixes#2079 (friend constexpr)
Fixes#2474 (virtual explicit)
Fixes:
Undefined symbols for architecture x86_64: "___cxa_deleted_virtual"
which clang issues when a class deletes a method (seems to be when the
function is not one of the compiler's automatically added special member
functions).