Fix #8111: Use better algorithms to infer parameter types #8232
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odersky
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A maximized type variable could be instantiated to a type that contains more type variables. These have to be instantiated in turn.
When we reach out to get the callee type, we might get an error, e.g. if the callee is overloaded. Example: ``` ch => sb.append(ch) ``` Here we might want to call calleeType to find the argument type of `sb.append` in order to use this type as the type of `ch`. But this fails since `append` is overloaded on StringBuffer. Prevously this did not matter since we called calleeType only as a last effort, so if it failed the whole typing failed. But with the changes forseen for fiing scala#8111 we need to be able to compute calleeType and retract it if it does not work.
An inferred parameter type I has two passible sources: - the type S known from the context - the type T known from the callee `f` if the lambda is of a form like `x => f(x)` If `T` exists, we know that `S <: I <: T`. In this commit we make use of this information to be more intelligent how the type I is inferred.
odersky
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We need to split `notBottom` into two states - flipBottom the old noBottom: when encountering bottom, maximimze instead - failBottom when encountering bottom, return false That way, we can have an inferredParamType which is closer to the old one. Try first to instantiate the formal type known from the context, but fail on bottom. If that fails, compute target type and proceed as before.
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An inferred parameter type I has two possible sources:
fif the lambda is of a form likex => f(x)If
Texists, we know thatS <: I <: T. In this commit we make use of this information in orderto be more intelligent how the type I is inferred.
This change required two other fixes.
First, we need to be more precise in
IsFullyDefinedAccumulator. Previously, a maximized type variable could be instantiated to a type that contains more type variables. These have to be instantiatedin turn, but were not. 5e59f87
Second, we need to make
calleeTyperetractable since it is no longer just a part of last-ditch effort with no alternatives remaining.. Previously, it was not. 4c84ee8