Error on the coexistence of QMJ and QMJ5

src/main/java/org/quiltmc/loader/impl/plugin/quilt/StandardQuiltPlugin.java

@@ -247,6 +247,28 @@ private ModLoadOption[] scan0(Path root, QuiltLoaderIcon fileIcon, ModLocation l
 
 		if (FasterFiles.exists(qmj5)) {
 			if (QuiltLoader.isDevelopmentEnvironment()) {
+				if (FasterFiles.exists(qmj)) {
+					QuiltLoaderText title = QuiltLoaderText.translate("gui.text.qmj_and_qmj5_coexistence.title");
+					QuiltDisplayedError error = context().reportError(title);
+					String describedPath = context().manager().describePath(usedQmj);
+					error.appendReportText(
+							"A coexistence of 'quilt.mod.json5' and 'quilt.mod.json' has been detected at " + describedPath,
+							"These metadata files cannot coexist with each other due to their conflicting purposes!"
+					);
+					error.appendDescription(
+							QuiltLoaderText.translate("gui.text.qmj_and_qmj5_coexistence.desc.0"),
+							QuiltLoaderText.translate("gui.text.qmj_and_qmj5_coexistence.desc.1"),
+							QuiltLoaderText.translate("gui.text.qmj_and_qmj5_coexistence.desc.2", describedPath)
+					);
+					context().manager().getRealContainingFile(root).ifPresent(real ->
+							error.addFileViewButton(real)
+									.icon(QuiltLoaderGui.iconJarFile().withDecoration(QuiltLoaderGui.iconQuilt()))
+					);
+
+					guiNode.addChild(QuiltLoaderText.translate("gui.text.qmj_and_qmj5_coexistence"));
+					return null;
+				}
+
 				usedQmj = qmj5;
 			} else {
 				QuiltLoaderText title = QuiltLoaderText.translate("gui.text.qmj5_on_production.title");

src/main/resources/lang/quilt_loader.properties

@@ -71,6 +71,11 @@ gui.text.qmj5_on_production.title=Attempted to read an unconverted 'quilt.mod.js
 gui.text.qmj5_on_production.desc.0=An attempt to read a 'quilt.mod.json5' file was detected!
 gui.text.qmj5_on_production.desc.1='quilt.mod.json5' files can't be used outside a development environment without converting to a 'quilt.mod.json' file!
 gui.text.qmj5_on_production.desc.2=From %s
+gui.text.qmj_and_qmj5_coexistence=Coexistence of conflicting Quilt metadata found
+gui.text.qmj_and_qmj5_coexistence.title=Detected the coexistence of conflicting metadata!
+gui.text.qmj_and_qmj5_coexistence.desc.0=A coexistence of 'quilt.mod.json5' and 'quilt.mod.json' has been detected.
+gui.text.qmj_and_qmj5_coexistence.desc.1=These metadata files cannot coexist with each other due to their conflicting purposes!
+gui.text.qmj_and_qmj5_coexistence.desc.2=From %s
 error.unhandled_solver=Complex solver error; see the crash report for more information
 error.unhandled_solver.desc=Please try updating quilt-loader to see if an update can describe this.\nOr report this to the quilt-loader project so this can be fixed.
 error.unhandled_solver.desc.rule_n= \n \nRule %s (%s):