mergeSchemas leads to swallowed/different errors

[electerious]

Using mergeSchemas together with makeExecutableSchema leads to different errors, even when just merging one schema.

Example

https://codesandbox.io/s/immutable-tdd-wlwrz?file=/index.js

// This throws "Cannot return null for non-nullable field Definition.a."
const server = new ApolloServer({
  schema: mergeSchemas({
    schemas: [createSchema()],
    resolvers: {}
  })
});

// This throws "This is an error" from the resolver
const server = new ApolloServer({
   schema: createSchema()
});

Expected behaviour

I've expected to see a "This is an error" error in the console in both cases as the typeDefs and schema is the same.

Actual behaviour

Apollo throws "Cannot return null for non-nullable field Definition.a." when using mergeSchemas and "This is an error" without mergeSchemas.

More details

• "This is an error" is not in the console output
• "This is an error" is not in formatError. Neither as err nor as err.originalError
• There seems to be a relation between makeExecutableSchema, mergeTypeDefs and mergeSchemas, because it only happens when using this combination
• Throwing the error inside the mutation resolver works and shows the error
• I'm using apollo-server in version 2.19.0

Let me know if there's anything I can help with. It took me several hours to reproduce this bug. The code example is reduced and I'm obviously not merging one schema in the code I've actually written ;)

[glasser]

mergeSchemas is part of the graphql-tools project. This was previously maintained by the Apollo Server team but it is now maintained by its own community at https://www.graphql-tools.com/

While AS2 does re-export the symbols from graphql-tools v4 for "convenience", graphql-tools v4 is not being maintained. It's quite possible that your concern has already been fixed in newer versions of graphql-tools (I believe mergeSchemas is part of @graphql-tools/merge.) If the bug still exists, report it in https://github.com/ardatan/graphql-tools !