[Compose] Enable composition of unknown json

Bug: b/241763080
Change-Id: I4a85054b58a864d5d2967178d759125bc4bdb504
diff --git a/src/main/java/com/android/tools/r8/naming/mappinginformation/UnknownJsonMappingInformation.java b/src/main/java/com/android/tools/r8/naming/mappinginformation/UnknownJsonMappingInformation.java
index 98699b2..3e061db 100644
--- a/src/main/java/com/android/tools/r8/naming/mappinginformation/UnknownJsonMappingInformation.java
+++ b/src/main/java/com/android/tools/r8/naming/mappinginformation/UnknownJsonMappingInformation.java
@@ -4,7 +4,6 @@
 
 package com.android.tools.r8.naming.mappinginformation;
 
-import com.android.tools.r8.errors.Unreachable;
 import com.android.tools.r8.naming.MappingComposeException;
 import com.android.tools.r8.naming.mappinginformation.MappingInformation.PositionalMappingInformation;
 import com.google.gson.JsonObject;
@@ -31,7 +30,7 @@
 
   @Override
   public String serialize() {
-    throw new Unreachable("We should not at this point serialize unknown information");
+    return payload;
   }
 
   @Override
diff --git a/src/test/java/com/android/tools/r8/mappingcompose/ComposeUnknownJsonTest.java b/src/test/java/com/android/tools/r8/mappingcompose/ComposeUnknownJsonTest.java
index ddc03a8..022f42f 100644
--- a/src/test/java/com/android/tools/r8/mappingcompose/ComposeUnknownJsonTest.java
+++ b/src/test/java/com/android/tools/r8/mappingcompose/ComposeUnknownJsonTest.java
@@ -4,12 +4,12 @@
 
 package com.android.tools.r8.mappingcompose;
 
-import static org.junit.Assert.assertThrows;
+import static com.android.tools.r8.mappingcompose.ComposeTestHelpers.doubleToSingleQuote;
+import static org.junit.Assert.assertEquals;
 
 import com.android.tools.r8.TestBase;
 import com.android.tools.r8.TestParameters;
 import com.android.tools.r8.TestParametersCollection;
-import com.android.tools.r8.errors.Unreachable;
 import com.android.tools.r8.naming.ClassNameMapper;
 import com.android.tools.r8.naming.MappingComposer;
 import com.android.tools.r8.utils.StringUtils;
@@ -41,12 +41,20 @@
           "# { id: 'some.third.unknown.identifier', bar: 'message3' }",
           "a -> b:",
           "# { id: 'some.fourth.unknown.identifier', baz: 'message4' }");
+  private static final String mappingResult =
+      StringUtils.unixLines(
+          "# { id: 'some.unknown.identifier', settings: 'message1' }",
+          "# { id: 'some.third.unknown.identifier', bar: 'message3' }",
+          "# {'id':'com.android.tools.r8.mapping','version':'2.2'}",
+          "com.foo -> b:",
+          "# {'id':'some.other.unknown.identifier','foo':'message2'}",
+          "# {'id':'some.fourth.unknown.identifier','baz':'message4'}");
 
   @Test
   public void testCompose() throws Exception {
     ClassNameMapper mappingForFoo = ClassNameMapper.mapperFromStringWithPreamble(mappingFoo);
     ClassNameMapper mappingForBar = ClassNameMapper.mapperFromStringWithPreamble(mappingBar);
-    // TODO(b/241763080): We should not throw when composing mapping information.
-    assertThrows(Unreachable.class, () -> MappingComposer.compose(mappingForFoo, mappingForBar));
+    String composed = MappingComposer.compose(mappingForFoo, mappingForBar);
+    assertEquals(mappingResult, doubleToSingleQuote(composed));
   }
 }