Perform Matchable check only if type test is needed

Fixes #16808

Author: odersky

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

@@ -1258,8 +1258,6 @@ trait Applications extends Compatibility {
   def typedUnApply(tree: untpd.Apply, selType: Type)(using Context): Tree = {
     record("typedUnApply")
     val Apply(qual, args) = tree
-    if !ctx.mode.is(Mode.InTypeTest) then
-      checkMatchable(selType, tree.srcPos, pattern = true)
 
     def notAnExtractor(tree: Tree): Tree =
       // prefer inner errors
@@ -1398,12 +1396,13 @@ trait Applications extends Compatibility {
         val unapplyArgType = mt.paramInfos.head
         unapp.println(i"unapp arg tpe = $unapplyArgType, pt = $selType")
         val ownType =
-          if (selType <:< unapplyArgType) {
+          if selType <:< unapplyArgType then
             unapp.println(i"case 1 $unapplyArgType ${ctx.typerState.constraint}")
             fullyDefinedType(unapplyArgType, "pattern selector", tree.srcPos)
             selType.dropAnnot(defn.UncheckedAnnot) // need to drop @unchecked. Just because the selector is @unchecked, the pattern isn't.
-          }
-          else {
+          else
+            if !ctx.mode.is(Mode.InTypeTest) then
+              checkMatchable(selType, tree.srcPos, pattern = true)
             // We ignore whether constraining the pattern succeeded.
             // Constraining only fails if the pattern cannot possibly match,
             // but useless pattern checks detect more such cases, so we simply rely on them instead.
@@ -1412,7 +1411,7 @@ trait Applications extends Compatibility {
             if (patternBound.nonEmpty) unapplyFn = addBinders(unapplyFn, patternBound)
             unapp.println(i"case 2 $unapplyArgType ${ctx.typerState.constraint}")
             unapplyArgType
-          }
+
         val dummyArg = dummyTreeOfType(ownType)
         val unapplyApp = typedExpr(untpd.TypedSplice(Apply(unapplyFn, dummyArg :: Nil)))
         def unapplyImplicits(unapp: Tree): List[Tree] = {

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

@@ -1461,7 +1461,6 @@ trait Checking {
 
   def checkMatchable(tp: Type, pos: SrcPos, pattern: Boolean)(using Context): Unit =
     if !tp.derivesFrom(defn.MatchableClass) && sourceVersion.isAtLeast(`future-migration`) then
-      val kind = if pattern then "pattern selector" else "value"
       report.warning(MatchableWarning(tp, pattern), pos)
 
   /** Check that there is an implicit capability to throw a checked exception

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -48,15 +48,14 @@ class CompilationTests {
       compileFilesInDir("tests/pos", defaultOptions.and("-Ysafe-init", "-Ylegacy-lazy-vals", "-Ycheck-constraint-deps"), FileFilter.include(TestSources.posLazyValsAllowlist)),
       compileFilesInDir("tests/pos-deep-subtype", allowDeepSubtypes),
       compileFilesInDir("tests/pos-custom-args/no-experimental", defaultOptions.and("-Yno-experimental")),
+      compileFilesInDir("tests/pos-custom-args/strict", defaultOptions.and("-source", "future", "-deprecation", "-Xfatal-warnings")),
       compileDir("tests/pos-special/java-param-names", defaultOptions.withJavacOnlyOptions("-parameters")),
       compileFile(
         // succeeds despite -Xfatal-warnings because of -nowarn
         "tests/neg-custom-args/fatal-warnings/xfatalWarnings.scala",
         defaultOptions.and("-nowarn", "-Xfatal-warnings")
       ),
       compileFile("tests/pos-special/typeclass-scaling.scala", defaultOptions.and("-Xmax-inlines", "40")),
-      compileFile("tests/pos-special/i7296.scala", defaultOptions.and("-source", "future", "-deprecation", "-Xfatal-warnings")),
-      compileDir("tests/pos-special/adhoc-extension", defaultOptions.and("-source", "future", "-feature", "-Xfatal-warnings")),
       compileFile("tests/pos-special/i7575.scala", defaultOptions.andLanguageFeature("dynamics")),
       compileFile("tests/pos-special/kind-projector.scala", defaultOptions.and("-Ykind-projector")),
       compileFile("tests/pos-special/kind-projector-underscores.scala", defaultOptions.and("-Ykind-projector:underscores")),
@@ -65,7 +64,6 @@ class CompilationTests {
       compileFile("tests/pos-custom-args/i9267.scala", defaultOptions.and("-Ystop-after:erasure")),
       compileFile("tests/pos-special/extend-java-enum.scala", defaultOptions.and("-source", "3.0-migration")),
       compileFile("tests/pos-custom-args/help.scala", defaultOptions.and("-help", "-V", "-W", "-X", "-Y")),
-      compileFile("tests/pos-custom-args/i10383.scala", defaultOptions.and("-source", "future", "-deprecation", "-Xfatal-warnings")),
       compileFile("tests/pos-custom-args/i13044.scala", defaultOptions.and("-Xmax-inlines:33")),
       compileFile("tests/pos-custom-args/jdk-8-app.scala", defaultOptions.and("-release:8")),
     ).checkCompile()

tests/pos-special/adhoc-extension/A.scala renamed to tests/pos-custom-args/strict/adhoc-extension/A.scala

@@ -0,0 +1,2 @@
+def collectKeys[A, B, C](xs: Map[A, B])(f: PartialFunction[A, C]): Map[C, B] =
+	xs.collect{ case (f(c) , b) => (c, b) }