Do not widen selector type in inline matches

Fixes #12715

Author: tgodzik

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

@@ -1465,8 +1465,8 @@ class Typer extends Namer
       case _ =>
         if tree.isInline then checkInInlineContext("inline match", tree.srcPos)
         val sel1 = typedExpr(tree.selector)
-        val selType = fullyDefinedType(sel1.tpe, "pattern selector", tree.span).widen
-
+        val rawSelectorTpe = fullyDefinedType(sel1.tpe, "pattern selector", tree.span)
+        val selType = if (tree.isInline) rawSelectorTpe else rawSelectorTpe.widen
         /** Extractor for match types hidden behind an AppliedType/MatchAlias */
         object MatchTypeInDisguise {
           def unapply(tp: AppliedType): Option[MatchType] = tp match {

tests/pos/i12715.scala

@@ -0,0 +1,8 @@
+transparent inline def f: String =
+  inline 10 match
+    case _ =>
+      inline "foo" match
+        case x : String => x
+
+def test =
+  inline val failMsg = f