Give warnings when type parameters do not have multiple solutions

[MeAmAnUsername]

Summary
Give warnings when no solutions or only one solution exists for the constraints on type parameters

Todo

• Figure out general rules when constraints do not have multiple solutions
  • Find more examples of places where there is only one solution
  • Minimize examples
  • Figure out general rules when constraints do not have multiple solutions
• Write tests (use minimized examples)
• Implement

Reason
Using type parameters is useless if there is only one possible instantiation, so it should give a warning when detected.

Examples

// don't know for sure if this is an example, but I think it is
data Fruit[A <: Fruit[B, A], B :> Fruit[B, A]] = foreign java Fruit {}