Add @semanticNonNull draft (#49)

I currently use
https://specs.apollo.dev/nullability/v0.4/#@semanticNonNull as reference
but GAPs feel like a better place.

Also, contrary to `onError`, I don't necessarily expect that
`@semanticNonNull` makes it to the main spec since it's mainly used for
compatibility. Hence me opening that GAP. Let me know what you think!

Author: martinbonnin

gaps/GAP-49/DRAFT.md

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+# @semanticNonNull Directive Specification
+
+## Introduction
+
+:: This document specifies the `@semanticNonNull` directive, a schema directive
+that indicates a field position is _semantically non null_: it is only null when
+there is a matching error in the response `errors` array, and is otherwise
+guaranteed to contain a value.
+
+GraphQL's Non-Null type (`!`) serves two purposes at once: it asserts that a
+position can never logically be null, and it dictates that, when an error occurs
+in that position during execution, the resulting null is _propagated_ to the
+nearest nullable parent. Because error propagation can erase large amounts of
+otherwise-valid data, API authors frequently make fields nullable purely to
+contain the blast radius of field errors — even when those fields are, in the
+absence of an error, always present.
+
+`@semanticNonNull` lets a schema separate these two concerns. A field may remain
+nullable on the wire (so that an error in it does not propagate) while still
+declaring that, absent a matching error, the position always contains a value.
+
+**Example**
+
+```graphql example
+type User {
+  # `name` is nullable on the wire, but is only ever null if `name` itself
+  # produced an error. Tooling may treat it as non-null otherwise.
+  name: String @semanticNonNull
+
+  # The list and each of its elements are semantically non null.
+  friends: [User] @semanticNonNull(levels: [0, 1])
+}
+```
+
+**Use Cases**
+
+- Code generators may emit non-null types for semantically non-null positions
+  when the client handles field errors through an out-of-band mechanism (such as
+  a result type or an exception raised at the edge of the selection set), sparing
+  developers from null checks on positions that are only ever null on error.
+- Schema authors may keep fields nullable for resilience (so that field errors
+  do not propagate and erase sibling data) without sacrificing the ability to
+  communicate that those fields are, in practice, always present.
+
+**Relationship to `onError`**
+
+The [`onError` proposal](https://github.com/graphql/graphql-spec/pull/1163)
+allows a client to opt out of error propagation (`onError: "NULL"`), so that a field error produces
+a localized {null} rather than propagating up to the nearest nullable parent. Once
+error propagation is no longer a concern, the _Non-Null_ type (`!`) can be used
+to mark every position where {null} is not a semantically valid value — the true
+nullability of the schema.
+
+Representing that nullability with `!` is not always possible for existing services, however. Adding
+`!` to an existing field may increase the blast radius of an error for clients that have not opted
+out of error propagation.
+
+`@semanticNonNull` addresses this. Because it is additive metadata that does not
+change the wire type, a nullability-aware client may read the true nullability of
+a field and omit its non-null checks, while clients that are unaware of the
+directive observe the field exactly as before. `onError` and `@semanticNonNull`
+describe the same underlying truth — which positions are truly non-null — but
+`@semanticNonNull` can be adopted incrementally without affecting existing
+clients.
+
+## Definition
+
+```graphql
+directive @semanticNonNull(levels: [Int!]! = [0]) on FIELD_DEFINITION
+```
+
+The `@semanticNonNull` directive indicates that a field's result is
+_semantically non null_ at the indicated _levels_.
+
+A position is _semantically non null_ if it is only {null} when there is a
+matching error in the `errors` array of the response. In all other cases, the
+position contains a value.
+
+`@semanticNonNull` may only be applied to a _FieldDefinition_ whose type is
+nullable at the indicated _levels_. Applying it to a position that is already a
+_Non-Null_ type at that level is meaningless (the position is already
+guaranteed non-null by the type system) and must be considered an error.
+
+## The levels argument
+
+The _levels_ argument selects which _levels_ of a (possibly nested) _List_ type
+are semantically non null. Levels are zero-indexed, where level {0} is the
+outermost position (the field's own value).
+
+- For a non-list type, only level {0} is meaningful, and it refers to the
+  field's value itself.
+- For a _List_ type, level {0} refers to the list itself, level {1} refers to
+  each element of the list, level {2} to each element of each nested list, and
+  so on.
+
+The default value of `[0]` makes only the outermost position semantically non
+null, matching the common case of a non-list field.
+
+A _level_ that is negative, or that exceeds the list dimensionality of the
+annotated field, must be considered an error.
+
+**Examples**
+
+Given the field type {"[[String]]"}:
+
+| Application                           | Semantically non-null positions                    |
+| ------------------------------------- | -------------------------------------------------- |
+| `@semanticNonNull`                    | the outer list                                     |
+| `@semanticNonNull(levels: [1])`       | each inner list                                    |
+| `@semanticNonNull(levels: [2])`       | each {String} element                              |
+| `@semanticNonNull(levels: [0, 1, 2])` | the outer list, each inner list, and each {String} |
+
+```graphql example
+type Query {
+  # The list itself is semantically non null; individual elements may be null.
+  tags: [String] @semanticNonNull
+
+  # Both the list and each element are semantically non null.
+  ids: [ID] @semanticNonNull(levels: [0, 1])
+}
+```

gaps/GAP-49/README.md

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+# GAP-49: @semanticNonNull Directive
+
+## Overview
+
+This proposal defines the `@semanticNonNull` directive, a schema directive that
+marks a field position as _semantically non null_: the position is guaranteed to
+contain a value unless there is a matching error in the response's `errors`
+array.
+
+This separates two concepts that GraphQL's `!` (Non-Null) type conflates:
+
+- **"this position can never logically be null"** (semantic nullability), and
+- **"this position becomes null when the field errors"** (error nullability).
+
+By expressing semantic nullability separately, code generation tools can emit
+non-null types for clients that can opt-out of error propagation with `onError: NULL`, removing the unnecessary null checks that
+clients would otherwise have to write.
+
+## Relationship to prior art
+
+This directive is part of the broader
+[GraphQL Nullability specification](https://specs.apollo.dev/nullability/) being
+developed in the
+[GraphQL Nullability Working Group](https://github.com/graphql/nullability-wg).
+The full nullability specification also defines client-side directives
+(`@catch`, `@catchByDefault`) describing how clients handle field errors. This
+GAP scopes itself to the schema-side `@semanticNonNull` (and the companion
+`@semanticNonNullField`) directive, which is independently useful and can be
+adopted on its own.
+
+Related discussions and prior art:
+
+- [GraphQL Nullability WG](https://github.com/graphql/nullability-wg)
+- [graphql-spec #1452 — "Client Controlled Nullability"](https://github.com/graphql/graphql-spec/pull/1452)
+  and the broader nullability discussions in the GraphQL specification.
+- [`specs.apollo.dev/nullability/v0.4`](https://specs.apollo.dev/nullability/v0.4/#@semanticNonNull).

gaps/GAP-49/metadata.yml

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+id: 49
+title: "@semanticNonNull Directive"
+summary: >
+  A schema directive that marks a field position as "semantically non null" —
+  guaranteed to be non-null unless there is a matching error in the response
+  `errors` array — allowing tooling to generate non-null types for newer clients
+  without breaking older clients that cannot enable `onError: "NULL"`.
+status: proposal
+authors:
+  - name: "Martin Bonnin"
+    email: "martin@apollographql.com"
+    githubUsername: "@martinbonnin"
+sponsor: "@martinbonnin"
+discussion: "https://github.com/graphql/gaps/pull/49"