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Improve type inference for functions like fold #18780

Merged
merged 4 commits into from
Nov 14, 2023
Merged

Improve type inference for functions like fold #18780

merged 4 commits into from
Nov 14, 2023

Commits on Nov 13, 2023

  1. Improve type inference for functions like fold 

    When calling a fold with an accumulator like `Nil` or `List()` one used to have add
an explicit type ascription. This is now no longer necessary. When instantiating
type variables that occur invariantly in the expected type of a lambda, we now replace
covariant occurrences of `Nothing` in the (possibly widened) type of the accumulator
with fresh type variables.

The idea is that a fresh type variable in such places is always better than Nothing. For
module values such as `Nil` we widen to `List[<fresh var>]`. This does possibly cause a new
type error if the fold really wanted a `Nil` instance. But that case seems very rare,
so it looks like a good bet in general to do the widening.
    @odersky
    odersky committed Nov 13, 2023
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  2. Address review comments

    @odersky
    odersky committed Nov 13, 2023
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  3. Put back test in instantiate

    @odersky
    odersky committed Nov 13, 2023
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  4. Update compiler/src/dotty/tools/dotc/core/Types.scala

    @odersky
    odersky authored Nov 13, 2023
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