mergeSchemas throws when typeDefs include default values

[DrewML]

Minimal Repro

const exampleTypeDefs = gql`
    type Query {
        foo(arg: String = "1"): String
    }
`;
const exampleSchema = mergeSchemas({ typeDefs: exampleTypeDefs });

//TypeError: String cannot represent value: { kind: "StringValue", value: "1", block: false }
//           at GraphQLScalarType.serializeString [as serialize]

Believe this relates to the fix done for #1121

Looks like the whole AST node for defaultValue is being passed to serializeString, serializeInt, etc in the graphql package, but those methods only expect the raw value from the value prop, not the node wrapper.

[DrewML]

Looks like this is the rough call path:

1. mergeSchemas calls addResolversToSchema
2. addResolversToSchema runs this code:

   // serialize all default values prior to healing fields with new 	scalar/enum types.
   forEachDefaultValue(schema, serializeInputValue);

3. serializeInputValue above is a wrapper that invokes the serializeX methods from graphql types. It is expecting a type def and a value
4. forEachDefaultValue runs this code:

   // arg.defaultValue as the second arg to `fn` is an AST node, but fn is expecting a simple value
   arg.defaultValue = fn(arg.type, arg.defaultValue);

[yaacovCR]

Thanks for pointing this out, see PR for canary build.

[DrewML]

Thanks for the quick fix @yaacovCR! Confirmed 5.0.1-2def1b9.0 fixes it 🥂

[yaacovCR]

This change should also preserve directives in the AST for the gateway types -- this missing behavior likely led to bugs with directives on new types using mergeSchemas.

Directives were probably working on type extensions, as those are separated out and handled by extendSchema.

@DrewML -- thank you for bringing this to our attention!!!!!!!

[yaacovCR]

Closed by #1419.