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Fix spurious invariant propagation for measures on type synonyms - #2660
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Replace tcUnifyTy with tcMatchTy in goodInvs to prevent measure invariants from being applied to binders whose type arguments are too general. Previously, a measure on [Diff [a]] (via a type synonym Hunk a) would have its Nat invariant (ms v >= 0) spuriously attached to plain [a] binders, causing elaboration failures. The fix uses one-directional matching so that an invariant's type args must be matchable *from* the target type args (i.e., the target must be an instance of the invariant's expected type). This prevents a type variable in the target from absorbing a structured invariant arg.
ninioArtillero
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ninioArtillero
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LGTM! Thanks for addressing this!
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Replace tcUnifyTy with tcMatchTy in goodInvs to prevent measure invariants from being applied to binders whose type arguments are too general. Previously, a measure on [Diff [a]] (via a type synonym Hunk a) would have its Nat invariant (ms v >= 0) spuriously attached to plain [a] binders, causing elaboration failures.
The fix uses one-directional matching so that an invariant's type args must be matchable from the target type args (i.e., the target must be an instance of the invariant's expected type). This prevents a type variable in the target from absorbing a structured invariant arg.