Find & fix a LB/contra case

Author: dwijnand

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

@@ -3136,6 +3136,14 @@ class TrackingTypeComparer(initctx: Context) extends TypeComparer(initctx) {
 
     def instantiateParams(insts: Array[Type]) = new ApproximatingTypeMap {
       variance = 0
+
+      override def range(lo: Type, hi: Type): Type =
+        if variance == 0 && (lo eq hi) then
+          // override the default `lo eq hi` test, which removes the Range
+          // which leads to a Reduced result, instead of NoInstance
+          Range(lower(lo), upper(hi))
+        else super.range(lo, hi)
+
       def apply(t: Type) = t match {
         case t @ TypeParamRef(b, n) if b `eq` caseLambda => insts(n)
         case t: LazyRef => apply(t.ref)

tests/pos/i15926.contra.scala

@@ -0,0 +1,7 @@
+trait Show[-A >: Nothing]
+
+type MT1[I <: Show[Nothing], N] = I match
+  case Show[a] => N match
+    case Int => a
+
+val a = summon[MT1[Show[String], Int] =:= String]