Reducer

A type map that tries to reduce (part of) the result type of the type lambda tycon with the given args(some of which are wildcard arguments represented by type bounds). Non-wildcard arguments are substituted everywhere as usual. A wildcard argument >: L <: H is substituted for a type lambda parameter X only under certain conditions.

1. If Mode.AllowLambdaWildcardApply is set: The wildcard argument is substituted only if X appears in a toplevel application of the form

   C[..., X, ...]

and there are no other occurrences of X in the reduced type. In that case the refinement above is replaced by

  C[..., ? >: L <: H, ...]

The allReplaced field indicates whether all occurrences of type lambda parameters in the reduced type have been replaced with arguments.

2. If Mode.AllowLambdaWildcardApply is not set: All X arguments are replaced by:

   ? >: L <: H

Any other occurrence of X in tycon is replaced by U, if the occurrence of X in tycon is covariant, or nonvariant, or by L, if the occurrence is contravariant.

The idea is that the AllowLambdaWildcardApply mode is used to check whether a type can be soundly reduced, and to give an error or warning if that is not the case. By contrast, the default mode, with AllowLambdaWildcardApply not set, reduces all applications even if this yields a different type, so its postcondition is that no type parameters of tycon appear in the result type. Using this mode, we can guarantee that appliedTo will never produce a higher-kinded application with a type lambda as type constructor.