SI-7517 Fix higher kinded type inference regression

 - Discovered in 2.10.2-RC1
 - Ostensibly regressed in 7e52fb910b, which conceptually reverted
   part of 0cde930b so that (mutable) TypeVars don't use structural equality.
 - But, does *not* fail if 7e52fb910b is cherry-picked directly after 0cde930b,
   suggesting that it shone a light on a behaviour change in some other commit
   in between the two.
 - Indeed, the true regression came in https://github.com/scala/scala/commit/e5da30b843#L5L3176
 - A targeted revert of e5da30b843 is undesirable, as we'd like SI-6846 to stay fixed

What's happening here? In the enclosed test case, higher kinded type
inference explores two possibilities:

    Composed.this.Split[A]
    K[[T]A[B[T]]]           // `Split[A]` dealiased

The difference in the flow of type inference can be seen from the diff
below. Notice how now we no longer register `?K.addBound(Composed.this.Split)`,
we instead only register `?K.addBound(K)`

```patch
--- sandbox/old.log 2013-05-30 00:27:34.000000000 +0200
+++ sandbox/new.log 2013-05-30 00:28:28.000000000 +0200
@@ -1,55 +1,114 @@
 ?K.unifyFull(Composed.this.Split[A])
   ?K.unifySpecific(Composed.this.Split[A])
-  ?K.addBound(Composed.this.Split)
     ?B.unifyFull(T)
       ?B.unifySpecific(T)
       `-> false
     ?B.unifyFull(Any)
       ?B.unifySpecific(Any)
       `-> false
   `-> false
   ?K.unifySpecific(L[[T]A[B[T]]])
-  ?K.addBound(L)
     ?B.unifyFull(B[T])
       ?B.unifySpecific(B[T])
       ?B.addBound(B)
       `-> true
     ?B.unifyFull(B[T])
       ?B.unifySpecific(B[T])
       ?B.addBound(B)
       `-> true
     ?B.unifyFull(B[T])
       ?B.unifySpecific(B[T])
       ?B.addBound(B)
       `-> true
     ?B.unifyFull(B[T])
       ?B.unifySpecific(B[T])
       ?B.addBound(B)
       `-> true
+  ?K.addBound(L)
   `-> true
 ?K.unifyFull(Composed.this.Split[A])
   ?K.unifySpecific(Composed.this.Split[A])
-  ?K.addBound(Composed.this.Split)
     ?B.unifyFull(x)
       ?B.unifySpecific(x)
       `-> false
   `-> false
   ?K.unifySpecific(L[[T]A[B[T]]])
+    ?B.unifyFull(B[T])
+      ?B.unifySpecific(B[T])
+      ?B.addBound(B)
+      `-> true
+    ?B.unifyFull(B[T])
+      ?B.unifySpecific(B[T])
+      ?B.addBound(B)
+      `-> true
+    ?B.unifyFull(B[T])
+      ?B.unifySpecific(B[T])
+      ?B.addBound(B)
+      `-> true
+    ?B.unifyFull(B[T])
+      ?B.unifySpecific(B[T])
+      ?B.addBound(B)
+      `-> true
   ?K.addBound(L)
+  `-> true
+?K.unifyFull(Composed.this.Split[A])
+  ?K.unifySpecific(Composed.this.Split[A])
+    ?B.unifyFull(T)
+      ?B.unifySpecific(T)
+      `-> false
+    ?B.unifyFull(Any)
+      ?B.unifySpecific(Any)
+      `-> false
+  `-> false
+  ?K.unifySpecific(L[[T]A[B[T]]])
     ?B.unifyFull(B[T])
       ?B.unifySpecific(B[T])
       ?B.addBound(B)
       `-> true
     ?B.unifyFull(B[T])
       ?B.unifySpecific(B[T])
       ?B.addBound(B)
       `-> true
     ?B.unifyFull(B[T])
       ?B.unifySpecific(B[T])
       ?B.addBound(B)
       `-> true
     ?B.unifyFull(B[T])
       ?B.unifySpecific(B[T])
       ?B.addBound(B)
       `-> true
+  ?K.addBound(L)
+  `-> true
+?K.unifyFull(Composed.this.Split[A])
+  ?K.unifySpecific(Composed.this.Split[A])
+    ?B.unifyFull(x)
+      ?B.unifySpecific(x)
+      `-> false
+  `-> false
+  ?K.unifySpecific(L[[T]A[B[T]]])
+    ?B.unifyFull(B[T])
+      ?B.unifySpecific(B[T])
+      ?B.addBound(B)
+      `-> true
+    ?B.unifyFull(B[T])
+      ?B.unifySpecific(B[T])
+      ?B.addBound(B)
+      `-> true
+    ?B.unifyFull(B[T])
+      ?B.unifySpecific(B[T])
+      ?B.addBound(B)
+      `-> true
+    ?B.unifyFull(B[T])
+      ?B.unifySpecific(B[T])
+      ?B.addBound(B)
+      `-> true
+  ?K.addBound(L)
+  `-> true
+?K.unifyFull(L[A])
+  ?K.unifySpecific(L[A])
+  ?K.addBound(L)
+  `-> true
+?K.unifyFull(L[A])
+  ?K.unifySpecific(L[A])
+  ?K.addBound(L)
   `-> true
```

1 files changed, 22 insertions, 0 deletions

diff --git a/test/files/pos/t7517.scala b/test/files/pos/t7517.scala
new file mode 100644
index 0000000000..7ce4c6b13e
--- /dev/null
+++ b/test/files/pos/t7517.scala
@@ -0,0 +1,22 @@
+trait Box[ K[A[x]] ]
+
+object Box {
+   // type constructor composition
+   sealed trait ∙[A[_], B[_]] { type l[T] = A[B[T]] }
+
+   // composes type constructors inside K
+   type SplitBox[K[A[x]], B[x]] = Box[ ({ type l[A[x]] = K[ (A ∙ B)#l] })#l ]
+
+   def split[ K[A[x]], B[x] ](base: Box[K]): SplitBox[K,B] = ???
+
+   class Composed[B[_], L[A[x]] ] {
+      val box: Box[L] = ???
+
+      type Split[ A[x] ] = L[ (A ∙ B)#l ]
+      val a: Box[Split] = Box.split(box)
+
+      //Either of these work:
+      val a1: Box[Split] = Box.split[L,B](box)
+      val a2: Box[ ({ type l[A[x]] = L[ (A ∙ B)#l ] })#l ] = Box.split(box)
+   }
+}
\ No newline at end of file