SI-7983 Fix regression in implicit divergence checking

As seen in Slick. Regressed in 039b1cb1a5b8.

In that commit, a call to `normalize` was replaced with `dealiasWiden`
as part of a broad sweeping change. However, while the latter does
less than the former with regards to eta expansion ofhigher kinded
type refs (the motivation of that commit), it also does more when it
comes to singleton types: they are widened to the underlying type.

This was widening away `SingleType(<<package a.b>>, <<package c>>)`
before the special case assiging that type component a complexity of
zero could kick in.

In the test case, extracted from Slick, this meant that the
divergence check would consider that the second type below to
outrank the first in the complexity measure, and abort the implicit
search.

    Shape[?,                        ?  (Coffees, Coffees), ?]
    Shape[DivergenceTest.this.Flat, ?, Coffees,            ?]

After this change, the number of packages enclosing `DivergenceTest`
no longer has a bearing on the complexity score of the type.

Here's how the race was run before hand:

	complexity(<noprefix>): 0
	complexity(<root>): 1
	complexity(foo.type): 2
	complexity(foo.bar.type): 3
	complexity(foo.bar.baz.type): 4
	complexity(DivergenceTest.this.type): 5
	complexity(<noprefix>): 0
	complexity(<root>): 1
	complexity(foo.type): 2
	complexity(foo.bar.type): 3
	complexity(foo.bar.baz.type): 4
	complexity(DivergenceTest.this.type): 5
	complexity(DivergenceTest.this.Flat): 6
	complexity(<noprefix>): 0
	complexity(<root>): 1
	complexity(scala.type): 2
	complexity(Int): 3
	complexity(?): 1
	complexity(DivergenceTest.this.Shape2[DivergenceTest.this.Flat,Int,?]): 16

	complexity(<noprefix>): 0
	complexity(<root>): 1
	complexity(foo.type): 2
	complexity(foo.bar.type): 3
	complexity(foo.bar.baz.type): 4
	complexity(DivergenceTest.this.type): 5
	complexity(?): 1
	complexity(<noprefix>): 0
	complexity(<root>): 1
	complexity(scala.type): 2
	complexity(<noprefix>): 0
	complexity(Coffees): 1
	complexity(<noprefix>): 0
	complexity(<root>): 1
	complexity(scala.type): 2
	complexity(Int): 3
	complexity((Coffees, Int)): 7
	complexity(?): 1
	complexity(DivergenceTest.this.Shape2[?,(Coffees, Int),?]): 15

	dominates(DivergenceTest.this.Shape2[_ <: DivergenceTest.this.Flat, Int, U2], DivergenceTest.this.Shape2[_ <: Level, (Coffees, Int), U1]): true

And afterwards:

	complexity(DivergenceTest.this.type): 1
	complexity(DivergenceTest.this.type): 1
	complexity(DivergenceTest.this.Flat): 2
	complexity(scala.type): 1
	complexity(Int): 2
	complexity(?): 1
	complexity(DivergenceTest.this.Shape2[DivergenceTest.this.Flat,Int,?]): 7

	complexity(DivergenceTest.this.type): 1
	complexity(?): 1
	complexity(scala.type): 1
	complexity(<noprefix>): 0
	complexity(Coffees): 1
	complexity(scala.type): 1
	complexity(Int): 2
	complexity((Coffees, Int)): 5
	complexity(?): 1
	complexity(DivergenceTest.this.Shape2[?,(Coffees, Int),?]): 9

	dominates(DivergenceTest.this.Shape2[_ <: DivergenceTest.this.Flat, Int, U2], DivergenceTest.this.Shape2[_ <: Level, (Coffees, Int), U1]): false

Notice that even in the after shot, `scala.Int` has a complexity of 2.
It should be 1; this will be fixed in a separate commit.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Implicits.scala b/src/compiler/scala/tools/nsc/typechecker/Implicits.scala
index 01acbb8cc2..c3891dc707 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Implicits.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Implicits.scala
@@ -402,7 +402,7 @@ trait Implicits {
         deriveTypeWithWildcards(syms.distinct)(tp)
       }
       def sum(xs: List[Int]) = (0 /: xs)(_ + _)
-      def complexity(tp: Type): Int = tp.dealiasWiden match {
+      def complexity(tp: Type): Int = tp.dealias match {
         case NoPrefix =>
           0
         case SingleType(pre, sym) =>