diff --git a/compiler/src/dotty/tools/backend/jvm/GenBCode.scala b/compiler/src/dotty/tools/backend/jvm/GenBCode.scala
index c57929e4d363..65bd5ca27d7d 100644
--- a/compiler/src/dotty/tools/backend/jvm/GenBCode.scala
+++ b/compiler/src/dotty/tools/backend/jvm/GenBCode.scala
@@ -159,7 +159,7 @@ class GenBCodePipeline(val entryPoints: List[Symbol], val int: DottyBackendInter
      */
     class Worker1(needsOutFolder: Boolean) {
 
-      val caseInsensitively = scala.collection.mutable.Map.empty[String, Symbol]
+      val caseInsensitively = scala.collection.mutable.HashMap.empty[String, Symbol]
 
       def run(): Unit = {
         while (true) {
@@ -190,19 +190,25 @@ class GenBCodePipeline(val entryPoints: List[Symbol], val int: DottyBackendInter
         val claszSymbol = cd.symbol
 
         // GenASM checks this before classfiles are emitted, https://github.com/scala/scala/commit/e4d1d930693ac75d8eb64c2c3c69f2fc22bec739
-        val lowercaseJavaClassName = claszSymbol.name.toString.toLowerCase
-        caseInsensitively.get(lowercaseJavaClassName) match {
-          case None =>
-            caseInsensitively.put(lowercaseJavaClassName, claszSymbol)
-          case Some(dupClassSym) =>
-            // Order is not deterministic so we enforce lexicographic order between the duplicates for error-reporting
-            if (claszSymbol.name.toString < dupClassSym.name.toString)
-              ctx.warning(s"Class ${claszSymbol.name} differs only in case from ${dupClassSym.name}. " +
-                          "Such classes will overwrite one another on case-insensitive filesystems.", claszSymbol.pos)
-            else
-              ctx.warning(s"Class ${dupClassSym.name} differs only in case from ${claszSymbol.name}. " +
-                          "Such classes will overwrite one another on case-insensitive filesystems.", dupClassSym.pos)
+        def checkName(claszSymbol: Symbol): Unit = {
+          val lowercaseJavaClassName = claszSymbol.effectiveName.toString.toLowerCase
+          caseInsensitively.get(lowercaseJavaClassName) match {
+            case None =>
+              caseInsensitively.put(lowercaseJavaClassName, claszSymbol)
+            case Some(dupClassSym) =>
+              if (claszSymbol.effectiveName.toString != dupClassSym.effectiveName.toString) {
+                // Order is not deterministic so we enforce lexicographic order between the duplicates for error-reporting
+                val (cl1, cl2) =
+                  if (claszSymbol.effectiveName.toString < dupClassSym.effectiveName.toString) (claszSymbol, dupClassSym)
+                  else (dupClassSym, claszSymbol)
+                ctx.warning(s"Class ${cl1.effectiveName} differs only in case from ${cl2.effectiveName}. " +
+                  "Such classes will overwrite one another on case-insensitive filesystems.", cl1.pos)
+              }
+          }
         }
+        checkName(claszSymbol)
+        if (int.symHelper(claszSymbol).isModuleClass)
+          checkName(claszSymbol.companionModule)
 
         // -------------- mirror class, if needed --------------
         val mirrorC =
diff --git a/compiler/test/dotty/tools/dotc/CompilationTests.scala b/compiler/test/dotty/tools/dotc/CompilationTests.scala
index 1c5efcd57de7..c85678626d95 100644
--- a/compiler/test/dotty/tools/dotc/CompilationTests.scala
+++ b/compiler/test/dotty/tools/dotc/CompilationTests.scala
@@ -58,6 +58,7 @@ class CompilationTests extends ParallelTesting {
     compileFile("tests/pos-special/i3323.scala", defaultOptions.and("-Xfatal-warnings")) +
     compileFile("tests/pos-special/i3323b.scala", defaultOptions.and("-Xfatal-warnings")) +
     compileFile("tests/pos-special/i3589-b.scala", defaultOptions.and("-Xfatal-warnings")) +
+    compileFile("tests/pos-special/i4166.scala", defaultOptions.and("-Xfatal-warnings")) +
     compileFile("tests/pos-special/completeFromSource/Test.scala", defaultOptions.and("-sourcepath", "tests/pos-special")) +
     compileFile("tests/pos-special/completeFromSource/Test2.scala", defaultOptions.and("-sourcepath", "tests/pos-special")) +
     compileFile("tests/pos-special/completeFromSource/Test3.scala", defaultOptions.and("-sourcepath", "tests/pos-special", "-scansource")) +
diff --git a/tests/neg-custom-args/fatal-warnings/i2673b.scala b/tests/neg-custom-args/fatal-warnings/i2673b.scala
new file mode 100644
index 000000000000..e3c29d6bc8d0
--- /dev/null
+++ b/tests/neg-custom-args/fatal-warnings/i2673b.scala
@@ -0,0 +1,4 @@
+package Foos
+
+class Bar // error
+object bar
diff --git a/tests/neg-custom-args/fatal-warnings/i2673c.scala b/tests/neg-custom-args/fatal-warnings/i2673c.scala
new file mode 100644
index 000000000000..3e9cff320ea2
--- /dev/null
+++ b/tests/neg-custom-args/fatal-warnings/i2673c.scala
@@ -0,0 +1,6 @@
+package Foos
+
+object Outer {
+  class X // error
+  object x
+}
diff --git a/tests/pos-special/i4166.scala b/tests/pos-special/i4166.scala
new file mode 100644
index 000000000000..44684bc68855
--- /dev/null
+++ b/tests/pos-special/i4166.scala
@@ -0,0 +1,7 @@
+package foo {
+  class Hello
+}
+
+package bar {
+  class Hello
+}
diff --git a/tests/pos/test4a.scala b/tests/pos/test4a.scala
index ada0ba4e5f49..3efa059b73c2 100644
--- a/tests/pos/test4a.scala
+++ b/tests/pos/test4a.scala
@@ -7,10 +7,10 @@ class O[X]() {
     val j:I[X] = null;
 }
 
-object o extends O[C]() {
+object p extends O[C]() {
   def c: C = c;
   def main = {
-    o.j.foo(c);
+    p.j.foo(c);
   }
 }
 
diff --git a/tests/pos/test4refine.scala b/tests/pos/test4refine.scala
index abf078193568..5e76d10be3a9 100644
--- a/tests/pos/test4refine.scala
+++ b/tests/pos/test4refine.scala
@@ -10,7 +10,7 @@ object test {
 
 import test._;
 
-trait S extends o.I {
+trait S extends p.I {
     type Y = D;
     def bar: E = foo(c,d);
 }
@@ -25,15 +25,15 @@ abstract class O() {
     val j:I { type Y = X } = null;
 }
 
-object o extends O() {
+object p extends O() {
   type X = C;
 
   def main = {
     val s: S = null;
     import s._;
     foo(c,d);
-    o.i.foo(c,e);
-    o.j.foo(c,c);
+    p.i.foo(c,e);
+    p.j.foo(c,c);
     bar
   }
 }
@@ -42,8 +42,8 @@ class Main() {
   val s: S = null;
   import s._;
   foo(c,d);
-  o.i.foo(c,e);
-  o.j.foo(c,c);
+  p.i.foo(c,e);
+  p.j.foo(c,c);
   bar;
 }