Avoid null types when reducing match types

odersky · odersky · commit d6dd904a1aec · 2022-07-25T18:52:01.000+02:00
Fixes #15687
diff --git a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
@@ -2988,7 +2988,7 @@ class TrackingTypeComparer(initctx: Context) extends TypeComparer(initctx) {
       def redux(canApprox: Boolean): MatchResult =
         caseLambda match
           case caseLambda: HKTypeLambda =>
-            val instances = paramInstances(canApprox)(new Array(caseLambda.paramNames.length), pat)
+            val instances = paramInstances(canApprox)(Array.fill(caseLambda.paramNames.length)(NoType), pat)
             instantiateParams(instances)(body) match
               case Range(lo, hi) =>
                 MatchResult.NoInstance {
diff --git a/tests/neg/i15687.scala b/tests/neg/i15687.scala
@@ -0,0 +1,2 @@
+val _ = summon[Tuple.IsMappedBy[[X] =>> Int][(4, 2)]] // error
+

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,2 @@`
	`1`	`+val _ = summon[Tuple.IsMappedBy[[X] =>> Int][(4, 2)]] // error`
	`2`	`+`