Get concrete bounds of TypeRepr

[s5bug]

After my success with https://scastie.scala-lang.org/O4YhUozxSnO2PdXGKwivwQ, I want to create a macro which automatically rejects cases where the match would be non-exhaustive.

For A =:!= B, this means getting a concrete AL and AU s.t. A >: AL <: AU, and likewise for B. How do I do this? I don't see any method to grab TypeBounds off of a TypeRepr. I currently have

  inline given notSubMacro: [A, B] => <:!<[A, B] =
    ${ NotSubPlatform.impl[A, B] }

Consider the case where someone writes

def foo[T <: CharSequence] = summon[String <:!< T]

This should fail, as T = String is a valid possibility. However,

def bar[T <: String] = summon[CharSequence <:!< T]

should succeed, as CharSequence is never going to be widenable to T.

It's impossible to implement this with just subtype checks on T. Consider foo[CharSequence]:

test -- TypeRef(ThisType(TypeRef(NoPrefix,module class lang)),class String) <:!< TypeRef(NoPrefix,type T)
TypeRef(ThisType(TypeRef(NoPrefix,module class lang)),class String) <:< TypeRef(NoPrefix,type T) -- false
TypeRef(NoPrefix,type T) <:< TypeRef(ThisType(TypeRef(NoPrefix,module class lang)),class String) -- false

This false, false result, when summoning the <:!< should fail, is the exact same as a succeeding case:

test -- TypeRef(TermRef(ThisType(TypeRef(NoPrefix,module class <root>)),object scala),class Int) <:!< TypeRef(ThisType(TypeRef(NoPrefix,module class lang)),class String)
TypeRef(TermRef(ThisType(TypeRef(NoPrefix,module class <root>)),object scala),class Int) <:< TypeRef(ThisType(TypeRef(NoPrefix,module class lang)),class String) -- false
TypeRef(ThisType(TypeRef(NoPrefix,module class lang)),class String) <:< TypeRef(TermRef(ThisType(TypeRef(NoPrefix,module class <root>)),object scala),class Int) -- false

---

Assume that type A >: AL <: AU, type B >: BL <: BU, where AL, AU, BL, BU are all concrete. (ex. a generic with no bounds, i.e. def foo[A], would have AL = Nothing and AU = Any.) An A <:!< B should be legal iff:

• BU <:< AL and !(AL =:= BU) (widening to the upper bound of B would not enter the lower bound of A), or
• ProvablyDisjoint[A, B]
  Otherwise, A and B are possibly not disjoint, and if one is joint with the other, they share at least one type of overlap

For an A =:!= B to be legal:

• BU <:< AL and !(AL =:= BU) (A <:!< B), or
• AU <:< BL and !(BL =:= AU) (B <:!< A), or
• ProvablyDisjoint[A, B]

I think ProvablyDisjoint is easy once I have AL, AU, BL, BU: I just check that there's no possible subtyping relationships between any of the bounds. However, the difficulty still lies in finding those four types. Is there something in the macro system that will give them to me?