Refine checking condition

Don't check for matchability if

 - pattern type is a subtype of tested type
 - we are unapplying a typetest or classtag test

Author: odersky

compiler/src/dotty/tools/dotc/core/Mode.scala

@@ -116,4 +116,7 @@ object Mode {
 
   /** Are we typechecking the rhs of an extension method? */
   val InExtensionMethod: Mode = newMode(26, "InExtensionMethod")
+
+  /** Are we resolving a TypeTest node? */
+  val InTypeTest: Mode = newMode(27, "InTypeTest")
 }

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

@@ -1158,7 +1158,8 @@ trait Applications extends Compatibility {
   def typedUnApply(tree: untpd.Apply, selType: Type)(using Context): Tree = {
     record("typedUnApply")
     val Apply(qual, args) = tree
-    checkMatchable(selType, tree.srcPos, pattern = true)
+    if ctx.mode.is(Mode.InTypeTest) then
+      checkMatchable(selType, tree.srcPos, pattern = true)
 
     def notAnExtractor(tree: Tree): Tree =
       // prefer inner errors

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

@@ -778,7 +778,10 @@ class Typer extends Namer
         val matched = ascription(tpt1, isWildcard = true)
         // special case for an abstract type that comes with a class tag
         val result = tryWithTypeTest(matched, pt)
-        if (result eq matched) && pt != defn.ImplicitScrutineeTypeRef then
+        if (result eq matched)
+           && pt != defn.ImplicitScrutineeTypeRef
+           && !(pt <:< tpt1.tpe)
+        then
           // no check for matchability if TestTest was applied
           checkMatchable(pt, tree.srcPos, pattern = true)
         result
@@ -806,9 +809,12 @@ class Typer extends Namer
           case _ =>
             None
       }
-      val tag = withTag(defn.TypeTestClass.typeRef.appliedTo(pt, tref))
-        .orElse(withTag(defn.ClassTagClass.typeRef.appliedTo(tref)))
-        .getOrElse(tree)
+      val tag =
+        withMode(Mode.InTypeTest) {
+          withTag(defn.TypeTestClass.typeRef.appliedTo(pt, tref))
+          .orElse(withTag(defn.ClassTagClass.typeRef.appliedTo(tref)))
+          .getOrElse(tree)
+        }
       if tag.symbol.owner == defn.ClassTagClass && config.Feature.sourceVersion.isAtLeast(config.SourceVersion.`3.1`) then
         report.warning("Use of `scala.reflect.ClassTag` for type testing may be unsound. Consider using `scala.reflect.TypeTest` instead.", tree.srcPos)
       tag