[tests] add failing test for #11904

tests/server/src/cases/issues/Issue11904.hx

@@ -0,0 +1,30 @@
+package cases.issues;
+
+import haxe.display.Diagnostic;
+
+class Issue11904 extends TestCase {
+	function test(_) {
+		vfs.putContent("Issue11904.hx", getTemplate("issues/Issue11904.hx"));
+		var args = ["-main", "Issue11904", "--js", "no.js", "--no-output"];
+		runHaxe(args);
+		runHaxeJson([], ServerMethods.Invalidate, {file: new FsPath("Issue11904.hx")});
+		runHaxeJsonCb(args, DisplayMethods.Diagnostics, {file: new FsPath("Issue11904.hx")}, res -> {
+			Assert.equals(1, res.length);
+			Assert.equals(2, res[0].diagnostics.length);
+
+			function check<T>(d:Diagnostic<T>) {
+				switch (d.kind) {
+					case ReplaceableCode:
+						Assert.equals("Unused variable", d.args.description);
+
+					case _:
+						// trace(d);
+						Assert.fail("Unexpected diagnostics kind: " + d.kind);
+				}
+			}
+
+			var diag = res[0].diagnostics;
+			for (d in diag) check(d);
+		});
+	}
+}

tests/server/test/templates/issues/Issue11904.hx

@@ -0,0 +1,9 @@
+using StringTools;
+
+@:nullSafety
+class Issue11904 {
+	static function main() {}
+	static function extractReturnType(hint:String):Void {
+		for (i => code in hint) {}
+	}
+}