Add quoted pattern type splices

[nicolasstucki]

Support type spliced in quoted patterns

New quoted patterns

We introduce type splices $t in type positions. In the following example we want to match a list of any element type

case '{ ($ls: List[$t]).filter($f).filter($g) } =>
  // `t` is of type `Type[T$1]` for some unknown T$1
  // `t` is implicitly available
  '{ $ls.filter(x => ${f('x)} && ${g('x)}) }

If ls is matched against a List[Int] the type t will be Type[Int] or if it was a List[Foo] it would return a Type[Foo].

Note that in $t, t is a pattern and therefore is just a name binding. A fresh type is generated for it and cannot be named.

We can also splice on a type definition type $t. Unlike the previous variant the type will be named $t and can be used by referencing it using back-ticks and could be bounded.

case '{ type $t; ($ls: List[`$t`]).filter($f).filter($g) } =>
  // `t` is of type `Type[`$t`]` for some unknown type `$t`
  // `t` is implicitly available
  '{ $ls.filter(x => ${f('x)} && ${g('x)}) }

Overview of desugaring and tying

case '{ ($ls: List[$t]) } => ...

For each type splice we will create a new type binding in the pattern match ($t @ _ in this case) and a corresponding type in the quoted pattern as a hole (@patternBindHole type $t in this case). All these generated types are inserted at the start of the quoted code.

case scala.internal.quoted.Matcher.unapply[
    Tuple1[$t @ _], // Type binging definition
    Tuple2[Type[$t], Expr[List[$t]]] // Typing the result of the pattern match
  ](
    Tuple2.unapply
      [Type[$t], Expr[List[$t]]] //Propagated from the tuple above
      (implict t @ _, ls @ _: Expr[List[$t]])
  )(
    '{ // Runtime quote Matcher.unapply uses to mach against
        @scala.internal.Quoted.patternBindHole type $t
        scala.internal.Quoted.patternHole[List[$t]]
     }, 
     true, // If there is at least one type splice. Used to instantiate the context with or without GADT constraints
    x$2 // tasty.Reflection instance
  ) => ...

For type definition we do the same as above, except that we just reuse type $t as our @patternBindHole type $t.

case '{ type $t; ($ls: List[`$t`]) } =>

[nicolasstucki]

Rebased

[odersky]

Otherwise LGTM

[odersky]

Add a description of the scheme for translating patterns to doc comment above

[nicolasstucki]

Added

[odersky]

Add a description of the scheme for translating patterns to doc comment above

[nicolasstucki]

Added

[nicolasstucki]

Rebased

[nicolasstucki]

Will need to be rebased

[nicolasstucki]

@odersky Requested changes have been addressed.

[nicolasstucki]

Rebased