Sema: Don't attempt deep equality if we're also attempting optional-to-optional #78958
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I had to revert swiftlang#78301 to fix rdar://143338891. This now brings back the change, but without deleting code; instead, it's guarded by the -disable-optimized-restrictions flag. Once the flag is removed, I will once again completely delete OneWayEqual constraints.
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When solving a conversion between two optional types, for example
Optional<$T1>andOptional<$T2>, we would introduce a disjunction with three choices:$T1and$T2$T1to$T2$T2toOptional<$T1>.However, the first one is actually redundant, because any situation where it leads to a solution, the second choice will also lead to a solution. This is inefficient because it results in repeated work in the solver. The first choice was "favored", meaning if it succeeds we would skip the others, which partially mitigated the overhead. However, if we're exploring a dead end in the solution space, we would attempt all three choices, when it would suffice to attempt two.
A new
-disable-optimized-restrictionsflag turns off the new behavior. I plan on removing this flag as soon as possible. I will also make a similar change to array conversions shortly.