Doug Gregor 2f6cb21079 [AST] Prefer 'synthesized' conformances to 'implied' ones consistently. ...

When determining which declaration context should own a particular
protocol conformance that was not explicitly spelled out, prefer
"synthesized" contexts (i.e., which is always the nominal type itself)
for automatically-generated conformances (such as a raw-valued enum's
conformance to RawRepresentable) to conformances that are "implied" by
conformance to a more-refined protocol. Previously, we biased the
other way---but because conformances due to more-refined protocols can
be discovered later, we could get into a problem where two files
disagreed on which context would own the conformance---and neither
would emit the corresponding witness table.

Biasing toward "synthesized" contexts, which are always trivially
discoverable from the nominal type declaration itself, eliminates the
issue.

Fixes SR-6839 / rdar://problem/36911943.

2018-02-02 21:46:22 -08:00

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