Doug Gregor d9095b1079 [Conformance checking] Don't suppress substitution failures during checking. ...

When checking whether a particular protocol conformance satisfies all of
the protocol's requirements, we were suppressing substitution failures.
In some cases, this would mean that we marked a conformance "invalid"
without ever emitting a diagnostic, which would lead to downstream crashes.

Instead, treat substitution failures somewhat more lazily. If we encounter
one while performing the checking, put the conformance into a "delayed" list
rather than failing immediately. Teach the top-level type checking
loop to re-check these conformances, emitting a diagnostic if they
fail the second time around.

Fixes rdar://problem/35082483 and likely other issues that slipped
through the type checker or blew up in unpredictable ways.

2017-12-18 16:43:59 -08:00

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