Throw if Policies#fragmentMatches receives a falsy __typename.

src/cache/inmemory/__tests__/entityStore.ts

@@ -1199,10 +1199,16 @@ describe('EntityStore', () => {
 
     expect(() => cache.readQuery({
       query: queryWithAliases,
-    })).toThrow(/Can't find field a on object/);
+    })).toThrow(
+      "Attempted to match fragment Rest with type condition ABCs against " +
+        "object with unknown __typename"
+    );
 
     expect(() => cache.readQuery({
       query: queryWithoutAliases,
-    })).toThrow(/Can't find field a on object/);
+    })).toThrow(
+      "Attempted to match fragment Rest with type condition ABCs against " +
+        "object with unknown __typename"
+    );
   });
 });

src/cache/inmemory/__tests__/roundtrip.ts

@@ -308,8 +308,8 @@ describe('roundtrip', () => {
       );
     });
 
-    it('should resolve on union types with inline fragments without typenames with warning', () => {
-      return withWarning(() => {
+    it('should error on union types with inline fragments without typenames', () => {
+      return expect(() => {
         storeRoundtrip(
           gql`
             query {
@@ -337,7 +337,10 @@ describe('roundtrip', () => {
             ],
           },
         );
-      });
+      }).toThrowError(
+        'Attempted to match fragment with type condition Jedi against ' +
+          'object with unknown __typename'
+      );
     });
 
     // XXX this test is weird because it assumes the server returned an incorrect result

src/cache/inmemory/policies.ts

@@ -381,6 +381,14 @@ export class Policies {
     if (!fragment.typeCondition) return true;
 
     const supertype = fragment.typeCondition.name.value;
+
+    invariant(
+      typename,
+      `Attempted to match fragment ${
+        fragment.kind === "InlineFragment" ? "" : fragment.name.value + " "
+      }with type condition ${supertype} against object with unknown __typename`,
+    );
+
     if (typename === supertype) return true;
 
     if (this.usingPossibleTypes) {