Give warning when two type parameters cannot be inequal

[MeAmAnUsername]

Summary
Give warning when two type parameters cannot be inequal

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
Having multiple type parameters is useless if the only possible instantiations give them both the same type, so it should give a warning when detected.

Examples
From data_def_generics.spt. Don't know for sure if these can only be instantiated when T=R, but seems so to me.

data Foo0[ T <: Foo0[ R, R ], R <: Foo0[ T, R ] ] = foreign java Foo {}

data Foo1[T <: Foo1[R, T], R <: Foo1[T, R]] = foreign java Foo {}
data Foo2[T <: Foo2[R, T], R <: Foo2[R, T]] = foreign java Foo {}
data Foo3[T <: Foo3[T, R], R <: Foo3[R, R]] = foreign java Foo {}
data Foo4[R <: Foo4[T, R], T <: Foo4[R, R]] = foreign java Foo {}
data Foo5[T <: Foo5[R, R], R <: Foo5[T, R]] = foreign java Foo {}
data Foo6[T <: Foo6[T, T], R <: Foo6[T, R]] = foreign java Foo {}
data Foo7[T <: Foo7[T, T], R <: Foo7[R, R]] = foreign java Foo {}
data Foo8[T <: Foo8[R, R], R <: Foo8[R, T]] = foreign java Foo {}
data Foo9[T <: Foo9[T, R], R <: Foo9[T, R]] = foreign java Foo {}