Patmat: Use less type variables in prefix inference

[dwijnand]

In code like:

class Outer:
  sealed trait Foo
  case class Bar() extends Foo

  def mat(foo: Foo) = foo match
    case Bar() =>

When in the course of decomposing the scrutinee's type, which is
Outer.this.Foo, we're trying to instantiate subclass Outer.this.Bar,
the Outer.this is fixed - it needn't be inferred, via type variables
and type bounds. Cutting down on type variables, particularly when GADT
symbols are also present, can really speed up the operation, including
making code that used to hang forever compile speedily.

Fixes #16785

[liufengyun]

LGTM

[TomasMikula]

Hi, will this not be backported to release 3.3.0?

My project now

• compiles with 3.2.2
• compilation hangs with 3.3.0-RC4 (i.e. a regression)
• compiles with 3.3.1-RC1-bin-20230425-c9ca7ce-NIGHTLY (I assume thanks to this fix, but can't be sure).

Note that the example in #16785 also hanged in 3.2.2, so it was not classified as a regression, whereas now I'm hitting a regression.

[TomasMikula]

Here's proof of regression:

git clone https://github.com/TomasMikula/libretto.git
cd libretto
git checkout b042b14bc4162488f2ad36c3727272212380b295

# compiles with the used Scala version 3.2.2
sbt lambda/compile

# Edit build.sbt, set
# ThisBuild / scalaVersion := "3.3.0-RC4"

# compilation hangs
sbt lambda/compile