[SR-3350] Constraint solver may return "Solved" even when the solution contains ErrorType

[benlangmuir]

Previous ID	SR-3350
Radar	None
Original Reporter	@benlangmuir
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, TypeChecker
Assignee	None
Priority	Medium

md5: 6c8887670e6b4267fda881d7fabbb358

Issue Description:

For example, in the following code-completion:

func c{case
let a#^A^#}

We will try to solve an unresolved_pattern expression, which CSGen will mark as ErrorType. The solver then returns this as a solution. Either the solver should notice the ErrorType and return Error, or the code outside the solver should be adapted to ensure that we don't blow up in applySolution.

[benlangmuir]

This may not actually produce any bugs in the compiler itself today, but it seems like a hole that we can ever have a solution containing ErrorType, but have applySolution unable to handle that.