From 403eadd0f10cf6e493b81913148b0b9065e80699 Mon Sep 17 00:00:00 2001
From: Jason Zaugg <jzaugg@gmail.com>
Date: Wed, 29 May 2013 23:44:21 +0200
Subject: 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
```
---
 src/reflect/scala/reflect/internal/Types.scala | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

(limited to 'src')

diff --git a/src/reflect/scala/reflect/internal/Types.scala b/src/reflect/scala/reflect/internal/Types.scala
index ee584bed2c..0cc2b91a44 100644
--- a/src/reflect/scala/reflect/internal/Types.scala
+++ b/src/reflect/scala/reflect/internal/Types.scala
@@ -3218,10 +3218,9 @@ trait Types extends api.Types { self: SymbolTable =>
           sameLength(typeArgs, tp.typeArgs) && {
             val lhs = if (isLowerBound) tp.typeArgs else typeArgs
             val rhs = if (isLowerBound) typeArgs else tp.typeArgs
-            // this is a higher-kinded type var with same arity as tp.
-            // side effect: adds the type constructor itself as a bound
-            addBound(tp.typeConstructor)
-            isSubArgs(lhs, rhs, params, AnyDepth)
+            // This is a higher-kinded type var with same arity as tp.
+            // If so (see SI-7517), side effect: adds the type constructor itself as a bound.
+            isSubArgs(lhs, rhs, params, AnyDepth) && { addBound(tp.typeConstructor); true }
           }
         }
         // The type with which we can successfully unify can be hidden
-- 
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