Do not widen selector type in inline matches

Fixes #12715

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/full.scala

@@ -0,0 +1,41 @@
+package repro
+
+import compiletime.{constValue, erasedValue}
+
+sealed trait ValidateExprInt
+
+class And[A <: ValidateExprInt, B <: ValidateExprInt] extends ValidateExprInt
+class GreaterThan[T <: Int]                           extends ValidateExprInt
+
+object Repro:
+  inline def validate[E <: ValidateExprInt](v: Int): String =
+    inline val failMsg = validateV[E](v)
+    inline if failMsg == "neverPass" then "neverPass"
+    else "something else"
+
+  transparent inline def validateV[E <: ValidateExprInt](v: Int): String =
+    inline erasedValue[E] match
+      case _: GreaterThan[t] =>
+        "GreaterThan"
+      case _: And[a, b] =>
+        inline validateV[a](v) match
+          case "" =>
+            validateV[b](v)
+          case other =>
+            other
+
+  // This one works fine:
+  transparent inline def validateV_fixed[E <: ValidateExprInt](v: Int): String =
+    inline erasedValue[E] match
+      case _: GreaterThan[t] =>
+        "GreaterThan"
+      case _: And[a, b] =>
+        inline val res = validateV[a](v)
+        inline res match
+          case "" =>
+            validateV[b](v)
+          case _ =>
+            res
+
+  @main def test(): Unit =
+    println(validate[And[GreaterThan[10], GreaterThan[12]]](5))

tests/pos/i12715/minimized.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