On invalid prefix, throw a TypeError

[odersky]

Previously, the type was set to UnspecifiedErrorType. But that violates the rule that every ErrorType has to be reported. If that does not happen, crashes in pickler will result.

Fixes #18058

[dwijnand]

Should type G = (f: _ <: F) => f.A not be able to compile?

[odersky]

I don't think it should compile. f does not have a type we can use.

[dwijnand]

The test case goes through typedFunctionType's typedDependent, which encodes the function as a RefinedTypeTree, using an approximate AppliedTypeTree refined by the more specific param-dependant type. For f.A we use nonDependentResultApprox to get the "least supertype of resultType that does not contain parameter dependencies". Wouldn't F#A's supertype Any be that supertype? My thinking is perhaps the issue is that <: F should be handled in the typemap's derivedSelect before failing in withPrefix...

Other that that, my change was to avoid a compiler crash caused by an assertion error. I see now that using a TypeError makes the whole typedFunctionType fail and then associate the error message to the function tree, which looks good.

So I think perhaps this should compile, but not crashing in pickling is progress.

[odersky]

We could widen to Any, but that's no enough. The problem is we are writing a dependent function type, which is expanded to a generic type (that's where the avoid map was applied) and also an apply method. The apply method does not have a reasonable type either.

[clavigne]

Thanks for the prompt fix!