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Null-Only-On-Error / Semantically-Non-Null type (asterisk)

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This PR replaces #1046. Here's an updated description:

Inspired by @captbaritone's True Nullability Schema discussion (https://github.com/graphql/graphql-wg/discussions/1394) and following @fotoetienne's excellent talk at GraphQLConf and feedback on my resulting PR #1046, I am proposing that we introduce a new wrapper type, the "Null-Only-On-Error" type, represented via asterisk *. This new type walks the line between the default nullable types and the existing Non-Null type wrapper - it states that the value may only be {null} if a field error has been raised - i.e. it represents the "true nullability" of the field, whilst still acting as an error boundary to avoid null propagation.

Critically, this type would "evaporate" for legacy clients, appearing the same as a nullable field. (This is enabled via the includeNullOnlyOnError argument to the __Field.type field, which defaults to false.)

This is a much more complete RFC that #1046 was, and happily leverages a lot of the existing behavior of the spec.