Protocol requirements don't support default arguments. Although this is
a "semantic" diagnostics, we currently do this for 'func' and 'init' in
Parser. So for fixing a crash, let's to it for 'subscript' in Parser
too.
rdar://problem/73159041
Most tests were using %swift or similar substitutions, which did not
include the target triple and SDK. The driver was defaulting to the
host OS. Thus, we could not run the tests when the standard library was
not built for OS X.
Swift SVN r24504
We were never handling this correctly, and default arguments are checked along with the method body.
In some cases where no further validation was necessary (such as when the default argument was a literal value),
the compiler would let this slip through but in others this would cause a crash. (rdar://problem/16476405)
Swift SVN r15736
