mergeSchemas cannot resolve scalar type in Query, Mutation if type is present by extend

[NattapongSiri]

• has-reproduction
• feature
• docs
• blocking
• good first issue

define type by

type Item {
  name: String
}

then makeExectuableSchema out of it.
define another type

extend type Item {
  cost: Float
}
type Query: {
  findItemCostMoreThan(giveCost: Float): [Item]
}

them mergeSchemas of both. Now it complain that it doesn't know Float.
workaround is to split second definition into extend and query then call mergeSchemas twice.
First with extend type definition. Second with Query.
Based on some of my investigation, mergeSchemas seem to detect type out of compile schema but not on string based schema. So split mergeSchemas into two phrase can workaround this.

This is possibly the same issue with #825 and #815

[NattapongSiri]

After spend some more time investigate mergeSchemas function...
I found that root cause is due to this part inside mergeSchemas.

https://github.com/apollographql/graphql-tools/blob/248527d497aa1fe5ff5648e94bf38366121d9d92/src/stitching/mergeSchemas.ts#L243-L254

Query, Mutation, and Subscription are being used prior to extensions being apply.
Some interesting thing is..
Will this enhancement fixs this issue ?

[grantwwu]

@NattapongSiri graphql-js isn't a dependency of this project, so I don't think that would fix it.

EDIT: Oops, didn't realize that the package name was graphql.

This issue is blocking my org's forward progress, so I'm going to be working on this until it gets fixed...

[grantwwu]

https://github.com/grantwwu/graphql-tools-847-repro/ I have uploaded a minimal reproducing repository.

[yaacovCR]

mergeSchemas does not merge typedefs (!), it stitches them together by proxying. Merging can be done within makeExecutableSchema by just using an array of strings for typeDefs.

See #443 (comment)