Throw when a resolver method is missing a parameter for an argument in 15.0.0 #889

Follow-up to #418 and #890. Tools now warns at build when a resolver method is missing the parameter for an argument and has a DataFetchingEnvironment, GraphQLContext or contextClass parameter instead. The parameter count still matches, so the scanner accepts it and maps the arguments by position:

type Query {
    users(first: Int): UserConnection
}

fun users(env: DataFetchingEnvironment): Connection<User>

Every query of users then fails, with Parameter specified as non-null is null: ... parameter env when first is omitted and Cannot construct instance of graphql.schema.DataFetchingEnvironment when it's sent. The build logs:

Field 'Query.users' has no parameter for its argument 'first: Int' in scenarios.ContextMisboundQuery.users, so the DataFetchingEnvironment parameter gets the argument instead. Add a parameter for each argument, in order, before the DataFetchingEnvironment parameter.

For the contextClass it only warns when the argument is a spec scalar, since the context class can also be a real argument type (e.g. contextClass(Map) with an input object mapped to a Map).

A resolver like this can't work, so it should fail the build. But a schema whose broken field is never queried builds and runs fine today, so throwing has to wait for a major release.

What to change

• replace the warning in SchemaClassScanner#warnIfParameterMissing with a FieldResolverError that keeps the same message
• throw it even with allowUnimplementedResolvers or missingResolverDataFetcher set, since the resolver exists but can't be called
• turn MethodFieldResolverTest."should warn when a resolver method is missing a parameter for an argument" into a test for the error
• mention it in the 15.0.0 release notes