The restriction that default arguments be disallowed from accessing
`@usableFromInline` decls is overbearing for library developers who need to
write non-trivial code to compute a default value, since it forces them to
either write a verbose closure inline in the function signature or expose a
`public` helper function which unnecessarily expands API surface. A
`@usableFromInline` function a more reasonable way to encapsulate a verbose
default value computation.
This reverses the semantic changes included in https://github.com/apple/swift/pull/15666.
Resolves rdar://112093794.
We previously allowed these closures to default to (), but be inferred
as other types as well, which means that we will find some expressions
to be ambiguous because we end up finding multiple viable solutions
where there is really only one reasonable solution.
Fixes: rdar://problem/42337247
We parse default argument expressions before building a
FuncDecl, so we might see GenericTypeParamDecls that have
not yet been re-parented to the FuncDecl's context.
Fixes <https://bugs.swift.org/browse/SR-5559>,
<rdar://problem/33539464>.
