Fix higher-order unification incorrectly substituting tparams #16181
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When creating a fresh type lambda for the purpose of higher-order type
inference, we incorrectly substituted references to type parameters
before this commit. We want to construct:
bodyArgs := otherArgs.take(d), T_0, ..., T_k-1
[T_0, ..., T_k-1] =>> otherTycon[bodyArgs]
For this type to be valid, we need the bounds of `T_i` to be the bounds of
the (d+i) type parameter of `otherTycon` after substituting references to
each type parameter of `otherTycon` by the corresponding argument in `bodyArgs`.
The previous implementation incorrectly substituted only the last `k` type
parameters, this was not enough for correctness. It could also lead to a crash
because it called `integrate` which implicitly assumes it is passed a full list
of type parameters (this is now documented).
Fixes scala#15983.
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odersky
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Oct 15, 2022
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When creating a fresh type lambda for the purpose of higher-order type inference, we incorrectly substituted references to type parameters before this commit. We want to construct:
For this type to be valid, we need the bounds of
T_ito be the bounds of the (d+i) type parameter ofotherTyconafter substituting references to each type parameter ofotherTyconby the corresponding argument inbodyArgs.The previous implementation incorrectly substituted only the last
ktype parameters, this was not enough for correctness. It could also lead to a crash because it calledintegratewhich implicitly assumes it is passed a full list of type parameters (this is now documented).Fixes #15983.