Freeze constraints when checking parameter matching and subsumption.

Checking whether two alternatives are the same should not unify
them by instantiating type variables.

Author: odersky

src/dotty/tools/dotc/core/ConstraintHandling.scala

@@ -23,6 +23,7 @@ trait ConstraintHandling {
   implicit val ctx: Context
 
   protected def isSubType(tp1: Type, tp2: Type): Boolean
+  protected def isSameType(tp1: Type, tp2: Type): Boolean
 
   val state: TyperState
   import state.constraint
@@ -111,6 +112,13 @@ trait ConstraintHandling {
     finally frozenConstraint = saved
   }
 
+  final def isSameTypeWhenFrozen(tp1: Type, tp2: Type): Boolean = {
+    val saved = frozenConstraint
+    frozenConstraint = true
+    try isSameType(tp1, tp2)
+    finally frozenConstraint = saved
+  }
+
   /** Test whether the lower bounds of all parameters in this
    *  constraint are a solution to the constraint.
    */

src/dotty/tools/dotc/core/TypeComparer.scala

@@ -685,7 +685,7 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
     case formal1 :: rest1 =>
       formals2 match {
         case formal2 :: rest2 =>
-          (isSameType(formal1, formal2)
+          (isSameTypeWhenFrozen(formal1, formal2)
             || isJava1 && (formal2 isRef ObjectClass) && (formal1 isRef AnyClass)
             || isJava2 && (formal1 isRef ObjectClass) && (formal2 isRef AnyClass)) &&
           matchingParams(rest1, rest2, isJava1, isJava2)
@@ -1059,7 +1059,7 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
     case tp1: ClassInfo =>
       tp2 match {
         case tp2: ClassInfo =>
-          isSubType(tp1.prefix, tp2.prefix) || (tp1.cls.owner derivesFrom tp2.cls.owner)
+          isSubTypeWhenFrozen(tp1.prefix, tp2.prefix) || (tp1.cls.owner derivesFrom tp2.cls.owner)
         case _ =>
           false
       }
@@ -1075,7 +1075,7 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
       tp2 match {
         case tp2: MethodType =>
           def asGoodParams(formals1: List[Type], formals2: List[Type]) =
-            (formals2 corresponds formals1)(isSubType)
+            (formals2 corresponds formals1)(isSubTypeWhenFrozen)
           asGoodParams(tp1.paramTypes, tp2.paramTypes) &&
           (!asGoodParams(tp2.paramTypes, tp1.paramTypes) ||
            isAsGood(tp1.resultType, tp2.resultType))