diff --git a/compiler/src/dotty/tools/backend/jvm/PostProcessor.scala b/compiler/src/dotty/tools/backend/jvm/PostProcessor.scala
index 06c3c7f1cb4f..9f172806a3b5 100644
--- a/compiler/src/dotty/tools/backend/jvm/PostProcessor.scala
+++ b/compiler/src/dotty/tools/backend/jvm/PostProcessor.scala
@@ -73,7 +73,7 @@ class PostProcessor(val frontendAccess: PostProcessorFrontendAccess, val bTypes:
           else s" (defined in ${pos2.source.file.name})"
         def nicify(name: String): String = name.replace('/', '.').nn
         if name1 == name2 then
-          backendReporting.warning(
+          backendReporting.error(
             em"${nicify(name1)} and ${nicify(name2)} produce classes that overwrite one another", pos1)
         else
           backendReporting.warning(
diff --git a/tests/neg/i19248/Foo.scala b/tests/neg/i19248/Foo.scala
new file mode 100644
index 000000000000..f24651234eb9
--- /dev/null
+++ b/tests/neg/i19248/Foo.scala
@@ -0,0 +1,7 @@
+trait Foo { // error
+  class Bar
+
+  type T = Foo.this.Bar
+
+  inline def f: Int = ???
+}
diff --git a/tests/neg/i19248/Main.scala b/tests/neg/i19248/Main.scala
new file mode 100644
index 000000000000..bf4e3a48b279
--- /dev/null
+++ b/tests/neg/i19248/Main.scala
@@ -0,0 +1,3 @@
+@main
+def Main(args: String*): Unit =
+  ()
diff --git a/tests/neg/i19248/Scope.scala b/tests/neg/i19248/Scope.scala
new file mode 100644
index 000000000000..a3135d93084f
--- /dev/null
+++ b/tests/neg/i19248/Scope.scala
@@ -0,0 +1,4 @@
+object Scope {
+}
+object Foo {
+}
diff --git a/tests/neg/i19248/empty.scala b/tests/neg/i19248/empty.scala
new file mode 100644
index 000000000000..8b137891791f
--- /dev/null
+++ b/tests/neg/i19248/empty.scala
@@ -0,0 +1 @@
+
diff --git a/tests/neg/main-functions-nameclash.scala b/tests/neg/main-functions-nameclash.scala
new file mode 100644
index 000000000000..23a530e28271
--- /dev/null
+++ b/tests/neg/main-functions-nameclash.scala
@@ -0,0 +1,3 @@
+object foo {
+  @main def foo(x: Int) = () // error: class foo and object foo produce classes that overwrite one another
+}
diff --git a/tests/pos-with-compiler-cc/backend/jvm/GenBCode.scala b/tests/pos-with-compiler-cc/backend/jvm/GenBCode.scala
index 1af7e5dd705a..3bc9fb5592ee 100644
--- a/tests/pos-with-compiler-cc/backend/jvm/GenBCode.scala
+++ b/tests/pos-with-compiler-cc/backend/jvm/GenBCode.scala
@@ -214,7 +214,7 @@ class GenBCodePipeline(val int: DottyBackendInterface, val primitives: DottyPrim
           val same = classSymbol.effectiveName.toString == dupClassSym.effectiveName.toString
           atPhase(typerPhase) {
             if (same)
-              report.warning( // FIXME: This should really be an error, but then FromTasty tests fail
+              report.error(
                 em"$cl1 and ${cl2.showLocated} produce classes that overwrite one another", cl1.sourcePos)
             else
               report.warning(
diff --git a/tests/warn/main-functions-nameclash.scala b/tests/warn/main-functions-nameclash.scala
deleted file mode 100644
index bc0fe64379d4..000000000000
--- a/tests/warn/main-functions-nameclash.scala
+++ /dev/null
@@ -1,5 +0,0 @@
-
-
-object foo {
-  @main def foo(x: Int) = () // warn: class foo and object foo produce classes that overwrite one another
-}