Flag class file collision as error

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(

tests/neg/i19248/Foo.scala

@@ -0,0 +1,7 @@
+trait Foo { // error
+  class Bar
+
+  type T = Foo.this.Bar
+
+  inline def f: Int = ???
+}

tests/neg/i19248/Main.scala

@@ -0,0 +1,3 @@
+@main
+def Main(args: String*): Unit =
+  ()

tests/neg/i19248/Scope.scala

@@ -0,0 +1,4 @@
+object Scope {
+}
+object Foo {
+}

tests/neg/i19248/empty.scala

@@ -0,0 +1 @@
+

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
+}

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(