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

This seems to be the best strategy for now, given that
the implicit args trick is only used for matching Expr[T].

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

Author: liufengyun

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

@@ -545,7 +545,7 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
    *          C            -->  C     if current owner is C !!!
    *
    */
-  def showType(tp: Type): String = {
+  def showType(tp: Type, showTypeArgs: Boolean = false): String = {
     val enclosingCls = ctx.owner.enclosingClass
 
     def isOmittable(sym: Symbol) =
@@ -564,25 +564,25 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
       case _ => tp.show
     }
 
-    def refine(tp: Type): String = tp match {
+    def refine(tp: Type): String = tp.stripAnnots match {
       case tp: RefinedType => refine(tp.parent)
-      case tp: AppliedType => refine(tp.typeConstructor)
+      case tp: AppliedType =>
+        refine(tp.typeConstructor) + (
+          if (showTypeArgs) tp.argInfos.map(refine).mkString("[", ",", "]")
+          else ""
+        )
       case tp: ThisType => refine(tp.tref)
       case tp: NamedType =>
         val pre = refinePrefix(tp.prefix)
         if (tp.name == tpnme.higherKinds) pre
         else if (pre.isEmpty) tp.name.show.stripSuffix("$")
         else pre + "." + tp.name.show.stripSuffix("$")
+      case tp: OrType => refine(tp.tp1) + " | " + refine(tp.tp2)
+      case _: TypeBounds => "_"
       case _ => tp.show.stripSuffix("$")
     }
 
-    val text = tp.stripAnnots match {
-      case tp: OrType => showType(tp.tp1) + " | " + showType(tp.tp2)
-      case tp => refine(tp)
-    }
-
-    if (text.isEmpty) enclosingCls.show.stripSuffix("$")
-    else text
+    refine(tp)
   }
 
   /** Whether the counterexample is satisfiable. The space is flattened and non-empty. */
@@ -646,7 +646,7 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
         else if (tp.classSymbol.is(CaseClass) && !hasCustomUnapply(tp.classSymbol))
         // use constructor syntax for case class
           showType(tp) + params(tp).map(_ => "_").mkString("(", ", ", ")")
-        else if (decomposed) "_: " + showType(tp)
+        else if (decomposed) "_: " + showType(tp, showTypeArgs = true)
         else "_"
       case Prod(tp, fun, sym, params, _) =>
         if (ctx.definitions.isTupleType(tp))
@@ -677,7 +677,6 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
         }) ||
         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/patmat/3144.check

@@ -1 +1 @@
-6: Pattern Match Exhaustivity: _: Foo
+6: Pattern Match Exhaustivity: _: Foo[T]

tests/patmat/i4226b.check

@@ -1 +1 @@
-11: Pattern Match Exhaustivity: Just(_), _: Empty
+11: Pattern Match Exhaustivity: Just(_), _: Empty[Int]

tests/patmat/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 _ =>
+  }
+}

tests/patmat/i6255b.check

@@ -0,0 +1 @@
+2: Pattern Match Exhaustivity: _: Expr[Int]

tests/patmat/i6255b.scala

@@ -0,0 +1,6 @@
+class Foo {
+  def foo(x: quoted.Expr[Int]) given scala.tasty.Reflection: Unit = x match {
+    case '{ 1 } =>
+    case '{ 2 } =>
+  }
+}

tests/patmat/t9779.check

@@ -1 +1 @@
-10: Pattern Match Exhaustivity: _: a.Elem
+10: Pattern Match Exhaustivity: _: a.Elem[_]