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[4.2][ConstraintSystem] Fix a logic error in computing potential bindings. #17320

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Merged
merged 2 commits into from
Jun 19, 2018
Merged

[4.2][ConstraintSystem] Fix a logic error in computing potential bindings. #17320

merged 2 commits into from
Jun 19, 2018

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rudkx
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@rudkx rudkx commented Jun 19, 2018 •
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  • Explanation: In some cases we're failing to select potential type bindings for type variables due to a change that was made after 4.1 was released. The result is that we fail to type check code that worked before. The fix is a simple check of a condition that is missing in the code that computes potential bindings.
  • Scope: This could potentially hit any project.
  • Issue #: rdar://problem/40810000 / https://bugs.swift.org/browse/SR-7875
  • Risk: Very low.
  • Testing: Standard regression tests plus source compatibility tests.
  • Reviewer: @xedin

rudkx added 2 commits June 18, 2018 22:53
@rudkx
[ConstraintSystem] Fix a logic error in computing potential bindings.
b6d48fb 
A change was made to attempt to use constraints that we have between
type variables to inform potential bindings, such that if we have:
   $T1 <: $T2
we would use $T1's bindings to add to the bindings of $T2. This is
only valid if we're adding bindings where $T1 is the supertype,
though, otherwise we could have the constraints:
  $T1 <: $T2
  $T1 <: X
imply that $T2 is a supertype of X, which doesn't make sense.

Fixes rdar://problem/40810000 (aka https://bugs.swift.org/browse/SR-7875).

(cherry picked from commit 54b2504)
@rudkx
Tweak formatting to match surrounding code.
6373dac 
(cherry picked from commit 48544fb)
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rudkx commented Jun 19, 2018

@swift-ci Please test

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rudkx commented Jun 19, 2018

@swift-ci nominate

@rudkx rudkx merged commit 7e9c8a3 into swiftlang:swift-4.2-branch-06-11-2018 Jun 19, 2018
@rudkx rudkx deleted the fix-sr7875-4.2-06-11 branch June 19, 2018 18:57
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