Merge pull request #8413 from dotty-staging/fix-#8404

Fix #8404: Use both scrutinee type and given type in pattern ascriptions

Author: odersky

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

@@ -1573,8 +1573,21 @@ class Typer extends Namer
           // for a singleton pattern like `x @ Nil`, `x` should get the type from the scrutinee
           // see tests/neg/i3200b.scala and SI-1503
           val symTp =
-            if (body1.tpe.isInstanceOf[TermRef]) pt1
-            else body1.tpe.underlyingIfRepeated(isJava = false)
+            if body1.tpe.isInstanceOf[TermRef] then pt1
+            else if isWildcardStarArg(body1)
+                    || pt1 == defn.ImplicitScrutineeTypeRef
+                    || body1.tpe <:< pt  // There is some strange interaction with gadt matching.
+                                         // and implicit scopes.
+                                         // run/t2755.scala fails to compile if this subtype test is omitted
+                                         // and the else clause is changed to `body1.tpe & pt1`. What
+                                         // happens is that we get either an Array[Float] or an Array[T]
+                                         // where T is GADT constrained to := Float. But the case body
+                                         // compiles only if the bound variable is Array[Float]. If
+                                         // it is Array[T] we get an implicit not found. To avoid fragility
+                                         // wrt to operand order for `&`, we include the explicit subtype test here.
+                                         // See also #5649.
+            then body1.tpe
+            else pt1 & body1.tpe
           val sym = ctx.newPatternBoundSymbol(name, symTp, tree.span)
           if (pt == defn.ImplicitScrutineeTypeRef || tree.mods.is(Given)) sym.setFlag(Given)
           if (ctx.mode.is(Mode.InPatternAlternative))

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

@@ -44,7 +44,7 @@ class CompilationTests extends ParallelTesting {
       compileFile("tests/pos-special/sourcepath/outer/nested/Test4.scala", defaultOptions.and("-sourcepath", "tests/pos-special/sourcepath")),
       compileFilesInDir("tests/pos-special/fatal-warnings", defaultOptions.and("-Xfatal-warnings", "-feature")),
       compileFilesInDir("tests/pos-special/spec-t5545", defaultOptions),
-      compileFilesInDir("tests/pos-special/strawman-collections", defaultOptions),
+      compileFilesInDir("tests/pos-special/strawman-collections", allowDeepSubtypes),
       compileFilesInDir("tests/pos-special/isInstanceOf", allowDeepSubtypes.and("-Xfatal-warnings")),
       compileFilesInDir("tests/new", defaultOptions),
       compileFilesInDir("tests/pos-scala2", scala2CompatMode),

tests/neg/i8338.scala

@@ -0,0 +1,8 @@
+object test {
+  trait B {
+    type X; def mkX(i: Int): B#X
+  }
+  val b: B = new B {
+    type X = Int; def mkX(i: Int): X = i  // error
+  }
+}

tests/pos/i3208.scala

@@ -0,0 +1,6 @@
+class Test {
+  trait E
+  trait Marker
+
+  def test(es: List[E]): List[E] = es.collect { case e: Marker => e }
+}

tests/neg/i5418.scala renamed to tests/pos/i5418.scala

@@ -0,0 +1,10 @@
+trait ActorRefScope
+class ActorRef
+case class DeathWatchNotification(actor: ActorRef)
+object Main {
+  val ref: ActorRef = ???
+  ref match {
+    case r: ActorRefScope => DeathWatchNotification(r)
+    case _ =>
+  }
+}