[5.4][CSBindings] Adjust type variable viability condition #37097
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To be viable to be attempted next type variable should have at least one of the following: - a direct binding (from relational constraint); - a default (from `Defaultable` constraint); - a literal protocol conformance with a default type described by `LiteralConformsTo` constraint; Third condition was under-defined in 5.4 since it would consider _all_ protocol requirements, but that discounts the fact that only literal conformances could be binding sources, which leads to problems where type variables with incomplete set of bindings could be picked to be attempted. Resolves: rdar://77233864
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airspeedswift
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May 3, 2021
hborla
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May 17, 2021
xedin
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tkremenek
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May 18, 2021
AnthonyLatsis
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swift 5.4
🍒 release cherry pick
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Fixes an issue where valid code might fail due to incorrect inference.
To be viable to be attempted next type variable should have
at least one of the following:
Defaultableconstraint);described by
LiteralConformsToconstraint;Third condition was under-defined in 5.4 since it would consider
all protocol requirements, but that discounts the fact that
only literal conformances could be binding sources, which leads
to problems where type variables with incomplete set of bindings
could be picked to be attempted.
Scope: Expressions with ternary operator that has literals on both sides passed as an argument to a generic function with conformance requirements to literal protocol(s).
Resolves: rdar://78035555
Risk: Very Low
Reviewed By: @hborla
Testing: Regression tests added to the suite
Resolves: rdar://78035555