Compiler: type safety for type-match/pattern-match overloads

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It is not enforced that all type-match or pattern-match overloads return the 
same type. This can cause bogosity. Add type safety checks to ensure either:

 - All overloads return the same type

OR

 - Overloads are grouped by return type and the appropriate group is selected at compile time

Original issue reported on code.google.com by [email protected] on 5 Sep 2014 at 5:28