Type match with a match type when a match type is expected (#15599)

Author: mbovel

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -1609,6 +1609,8 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
             }
 
         val result = pt match {
+          case mt: MatchType if isMatchTypeShaped(mt) =>
+            typedDependentMatchFinish(tree, sel1, selType, tree.cases, mt)
           case MatchType.InDisguise(mt) if isMatchTypeShaped(mt) =>
             typedDependentMatchFinish(tree, sel1, selType, tree.cases, mt)
           case _ =>

tests/pos/match-type-inference.scala

@@ -0,0 +1,22 @@
+type IsString[T <: Any] = T match {
+  case String => true
+  case _ => false
+}
+
+def isString(x: Any): IsString[x.type] = x match
+  case _: String => true
+  case _ => false
+
+def isString2(x: Any): x.type match {
+  case String => true
+  case _ => false
+} = x match
+  case _: String => true
+  case _ => false
+
+def isString3[T](x: T): T match {
+  case String => true
+  case _ => false
+} = x match
+  case _: String => true
+  case _ => false