Handle singlton scrutinees of inline matches

Author: nicolasstucki

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

@@ -1467,8 +1467,8 @@ class Typer extends Namer
         val sel1 = typedExpr(tree.selector)
         val rawSelectorTpe = fullyDefinedType(sel1.tpe, "pattern selector", tree.span)
         val selType = rawSelectorTpe match
-          case c: ConstantType if tree.isInline => c
-          case otherTpe => otherTpe.widen
+          case tpe: SingletonType if tree.isInline => tpe
+          case tpe => tpe.widen
         /** Extractor for match types hidden behind an AppliedType/MatchAlias */
         object MatchTypeInDisguise {
           def unapply(tp: AppliedType): Option[MatchType] = tp match {

tests/pos/i12715/full.scala renamed to tests/pos/i12715.scala

@@ -0,0 +1,8 @@
+transparent inline def f(x: Any): Any =
+  inline 10 match
+    case _ =>
+      inline x match
+        case y => y
+
+def test =
+  def failMsg(x: Int): x.type = f(x)