Fix scala#6255: Don't check exhaustivity for Expr[T]

This seems to be the best strategy for now, given that
quotes are the implicit args trick is only used for
matching Expr[T], and `case _ =>` is always used in such
cases.

Otherwise, we will need to reason equality of Terms, which is subtle.

Author: liufengyun

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -669,15 +669,14 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
       !tp.hasAnnotation(defn.UncheckedAnnot) && {
         val tpw = tp.widen.dealias
         ctx.settings.YcheckAllPatmat.value ||
-        tpw.typeSymbol.is(Sealed) ||
+        tpw.typeSymbol.is(Sealed) && !tpw.isRef(defn.QuotedExprClass) ||
         tpw.isInstanceOf[OrType] ||
         (tpw.isInstanceOf[AndType] && {
           val and = tpw.asInstanceOf[AndType]
           isCheckable(and.tp1) || isCheckable(and.tp2)
         }) ||
         tpw.isRef(defn.BooleanClass) ||
         tpw.typeSymbol.is(JavaEnum) ||
-        canDecompose(tpw) ||
         (defn.isTupleType(tpw) && tpw.argInfos.exists(isCheckable(_)))
       }
 
@@ -719,7 +718,7 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
   }
 
   private def redundancyCheckable(sel: Tree): Boolean =
-    !sel.tpe.hasAnnotation(defn.UncheckedAnnot)
+    !sel.tpe.hasAnnotation(defn.UncheckedAnnot) && !sel.tpe.widen.isRef(defn.QuotedExprClass)
 
   def checkRedundancy(_match: Match): Unit = {
     val Match(sel, cases) = _match

tests/pos/i6255.scala

@@ -0,0 +1,7 @@
+class Foo {
+  def foo(x: quoted.Expr[Int]) given scala.tasty.Reflection: Unit = x match {
+    case '{ 1 } =>
+    case '{ 2 } =>
+    case _ =>
+  }
+}