Fix mojmap package names for realms and math

src/main/java/net/fabricmc/loom/configuration/providers/mappings/mojmap/MojangMappingLayer.java

@@ -60,6 +60,8 @@ public record MojangMappingLayer(MinecraftVersionMeta.Download clientDownload,
 
 		printMappingsLicense(clientMappings);
 
+		mappingVisitor = new MojangPackageFixingMappingVisitor(mappingVisitor);
+
 		// Filter out field names matching the pattern
 		DstNameFilterMappingVisitor nameFilter = new DstNameFilterMappingVisitor(mappingVisitor, SYNTHETIC_NAME_PATTERN);
 

src/main/java/net/fabricmc/loom/configuration/providers/mappings/mojmap/MojangPackageFixingMappingVisitor.java

@@ -0,0 +1,30 @@
+package net.fabricmc.loom.configuration.providers.mappings.mojmap;
+
+import net.fabricmc.mappingio.MappedElementKind;
+import net.fabricmc.mappingio.MappingVisitor;
+import net.fabricmc.mappingio.adapter.ForwardingMappingVisitor;
+
+import java.io.IOException;
+
+/*
+ * This is temporary, since currently using the Quilt mappings layered on top of mojmap
+ * crashes the game due to a conflict in mapped packages for anonymous inner classes.
+ * 
+ * The problematic Mojang packages are replaced with their Quilt counterparts and the
+ * remapped game then properly works.
+ */
+public class MojangPackageFixingMappingVisitor extends ForwardingMappingVisitor {
+	public MojangPackageFixingMappingVisitor(MappingVisitor next) {
+		super(next);
+	}
+
+	@Override
+	public void visitDstName(MappedElementKind targetKind, int namespace, String name) throws IOException {
+		if (targetKind == MappedElementKind.CLASS)
+			name = name
+					.replace("com/mojang/realmsclient/", "net/minecraft/client/realms/")
+					.replace("com/mojang/math/", "net/minecraft/util/math/");
+
+		super.visitDstName(targetKind, namespace, name);
+	}
+}