substSym call in matchesQuantified could be avoided

[retronym]

I think we could avoid this by first checking MethodType.isTrivial (which is cached) to avoid this call. We might also be able avoid it under phase.erasedTypes.

• Add an assertion to check that the substSym is an identity when each of those conditions hold, run the test suite and boostrap to test this out
• Check those conditions before calling it.

[diesalbla]

@retronym I have tried changing the code of matchesQuantified in the line you suggested of using isTrivial, by changing the following line:

     def matchesQuantified(tparams1: List[Symbol], tparams2: List[Symbol], res1: Type, res2: Type): Boolean = (
       sameLength(tparams1, tparams2) &&
-      matchesType(res1, res2.substSym(tparams2, tparams1), alwaysMatchSimple)
+      matchesType(res1, if (res2.isTrivial) res2 else res2.substSym(tparams2, tparams1), alwaysMatchSimple)
     )

And I have run partest with the modified code, but it fails at some tests. In particular, at the test/run/t6135, which compiles in the current version 2.13.x, it rejects the program with the following error:

!! 1 - run/t6135.scala                           [compilation failed with 1 errors]
##### Log file '/Users/diesalbla/floss/scalac/test/files/run/t6135-run.log' from failed test #####

t6135.scala:8: error: object creation impossible, since method foo in class B of type (t: String, a: Test.A)(v: a.V)Unit is not defined
(Note that a.V does not match a.V: their prefixes (i.e., enclosing instances) differ)
  val b1 = new B[String] {
               ^
one error found

This example relates to path-dependent types.