Reject enum values, interface fields and union members that a type declares twice in 15.0.0 #863

Since #855, extensions are merged into the type they extend. When a type ends up declaring an enum value, interface field or union member twice, e.g. in the type and again in an extension, tools keeps the first one and logs a warning:

enum Color { RED }
extend enum Color { RED GREEN }

Type 'Color' declares value 'RED' more than once, only the first one is kept.

graphql-java's SchemaGenerator rejects these schemas, and the spec doesn't allow an extension to redeclare what the type already has. But they built before #855, since the extension used to replace the type, so failing on them has to wait for a major release.

Keeping the first one hides mistakes though. interface Node { id: ID } plus extend interface Node { id: String } builds with id: ID, and only a log line says the extension's id was dropped.

What to change

• replace the warning in SchemaParser#warnRepeated with a SchemaError that names the type and the repeated value, field or member
• drop the distinctBy { it.name } on the enum values in createEnumObject and the interface fields in createInterfaceObject
• move the repeats out of TypeExtensionTest.repeats and extensions without a type keep building into a test for the error
• mention it in the 15.0.0 release notes