From c6deecd556d73dc0c80e84cf3eeefdabfeb92558 Mon Sep 17 00:00:00 2001
From: Martin <teknocide@gmail.com>
Date: Thu, 1 Apr 2021 22:58:49 +0200
Subject: [PATCH] Warn for usage of open modifier on traits or classes where
 redundant Fixes #11963

---
 compiler/src/dotty/tools/dotc/typer/Checking.scala | 12 ++++++++++--
 tests/neg-custom-args/fatal-warnings/i11963a.scala |  1 +
 tests/neg-custom-args/fatal-warnings/i11963b.scala |  1 +
 tests/neg-custom-args/fatal-warnings/i11963c.scala |  6 ++++++
 4 files changed, 18 insertions(+), 2 deletions(-)
 create mode 100644 tests/neg-custom-args/fatal-warnings/i11963a.scala
 create mode 100644 tests/neg-custom-args/fatal-warnings/i11963b.scala
 create mode 100644 tests/neg-custom-args/fatal-warnings/i11963c.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/Checking.scala b/compiler/src/dotty/tools/dotc/typer/Checking.scala
index 9f48913d1f67..471353d6fadf 100644
--- a/compiler/src/dotty/tools/dotc/typer/Checking.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Checking.scala
@@ -425,6 +425,7 @@ object Checking {
   /** Check that symbol's definition is well-formed. */
   def checkWellFormed(sym: Symbol)(using Context): Unit = {
     def fail(msg: Message) = report.error(msg, sym.srcPos)
+    def warn(msg: Message) = report.warning(msg, sym.srcPos)
 
     def checkWithDeferred(flag: FlagSet) =
       if (sym.isOneOf(flag))
@@ -464,8 +465,15 @@ object Checking {
       fail(em"only classes can be ${(sym.flags & ClassOnlyFlags).flagsString}")
     if (sym.is(AbsOverride) && !sym.owner.is(Trait))
       fail(AbstractOverrideOnlyInTraits(sym))
-    if (sym.is(Trait) && sym.is(Final))
-      fail(TraitsMayNotBeFinal(sym))
+    if sym.is(Trait) then
+      if sym.is(Final) then
+        fail(TraitsMayNotBeFinal(sym))
+      else if sym.is(Open) then
+        warn(RedundantModifier(Open))
+    if sym.isAllOf(Abstract | Open) then
+      warn(RedundantModifier(Open))
+    if sym.is(Open) && sym.isLocal then
+      warn(RedundantModifier(Open))
     // Skip ModuleVal since the annotation will also be on the ModuleClass
     if (sym.hasAnnotation(defn.TailrecAnnot) && !sym.isOneOf(Method | ModuleVal))
       fail(TailrecNotApplicable(sym))
diff --git a/tests/neg-custom-args/fatal-warnings/i11963a.scala b/tests/neg-custom-args/fatal-warnings/i11963a.scala
new file mode 100644
index 000000000000..58d64d061162
--- /dev/null
+++ b/tests/neg-custom-args/fatal-warnings/i11963a.scala
@@ -0,0 +1 @@
+open trait Foo // error
diff --git a/tests/neg-custom-args/fatal-warnings/i11963b.scala b/tests/neg-custom-args/fatal-warnings/i11963b.scala
new file mode 100644
index 000000000000..9fae92747d53
--- /dev/null
+++ b/tests/neg-custom-args/fatal-warnings/i11963b.scala
@@ -0,0 +1 @@
+open abstract class Foo // error
diff --git a/tests/neg-custom-args/fatal-warnings/i11963c.scala b/tests/neg-custom-args/fatal-warnings/i11963c.scala
new file mode 100644
index 000000000000..ebd56e1127c8
--- /dev/null
+++ b/tests/neg-custom-args/fatal-warnings/i11963c.scala
@@ -0,0 +1,6 @@
+object Test {
+  def foo: Any = {
+    open class Bar // error
+    new Bar
+  }
+}