Make the reason argument in `@deprecated` non-nullable

At a glance

• Identifier: #1040
• Stage: RFC3: Accepted
• Champion: @martinbonnin
• PR: Make the reason argument in `@deprecated` non-nullable

Timeline

• Added to 2024-12-05 WG agenda
• Mentioned in 2024-12 WG notes
• 2 commits pushed on 2024-11-22:
  • update language for deprecationReason by @martinbonnin
  • add missing bang by @martinbonnin
• Added to 2024-11-07 WG agenda
• Spec PR created on 2023-08-23 by martinbonnin
• Commit pushed: Make the reason argument in `@deprecated` non-nullable on 2023-08-23 by @martinbonnin

---

Follow up from https://github.com/graphql/graphql-spec/issues/53#issuecomment-1688335159

Make reason non-nullable:

directive @deprecated(
  reason: String! = "No longer supported"
) on FIELD_DEFINITION | ARGUMENT_DEFINITION | INPUT_FIELD_DEFINITION | ENUM_VALUE

This is technically a breaking change for someone that does this:

type Foo {
  bar: String! @deprecated(reason: null)
}

But feels like this shouldn't be allowed in the first place?

Fixes https://github.com/graphql/graphql-spec/issues/53