OneOf inhabitability by jbellenger · Pull Request #1211 · graphql/graphql-spec

jbellenger · 2026-02-17T00:16:11Z

OneOf types can form graphs for which finite values cannot be created.

The simplest example of this is a self-recursing OneOf type:

input A @oneOf { a:A }

While this is the simplest form of the issue, we can combine this with other kinds of input object types to create other uninhabited types:

input A @oneOf { b:B }
input B { a:A! }

These kinds of types are considered valid on two fronts today:

They are valid as OneOf types because all field types are nullable and there are no default values
They are valid as a circular input object because there are no unbreakable chains of non-null non-list fields

While this follows the letter of the Circular References part of the spec, it violates the spirit of the spec in that there is no way to create a finite, legal value for these types. I propose that these topologies be declared invalid.

I took a stab at reworking the Circular References section to consider OneOfs and added a more formal specification framed around finite values.

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benjie · 2026-02-17T08:39:52Z

Excellent catch! I think we can and should fast track this. Once you have the GraphQL.js implementation in place either add it to the next GraphQL WG agenda or let me know if you’d rather I represent it on your behalf.

spec/Section 3 -- Type System.md

benjie

Let's change the named type to an assertion for clarity

benjie · 2026-02-17T10:36:54Z

spec/Section 3 -- Type System.md

+  - Let {namedFieldType} be the underlying named type of {fieldType}.
+  - If {namedFieldType} is not an Input Object type:
+    - Return {true}.
+  - If {namedFieldType} is not a _OneOf Input Object_:
+    - Return {true}.
+  - If {OneOfInputObjectCanBeProvidedAFiniteValue(namedFieldType, nextVisited)}:
+    - Return {true}.


Suggested change

- Let {namedFieldType} be the underlying named type of {fieldType}.

- If {namedFieldType} is not an Input Object type:

- Return {true}.

- If {namedFieldType} is not a _OneOf Input Object_:

- Return {true}.

- If {OneOfInputObjectCanBeProvidedAFiniteValue(namedFieldType, nextVisited)}:

- Return {true}.

- Assert: {fieldType} is a named type.

- If {fieldType} is not an Input Object type:

- Return {true}.

- If {fieldType} is not a _OneOf Input Object_ type:

- Return {true}.

- If {OneOfInputObjectCanBeProvidedAFiniteValue(fieldType, nextVisited)}:

- Return {true}.

viaduct.arbitrary.graphql is a suite of generators used to property test Viaduct and other internal airbnb systems. The generators in this package use a model for GraphQL values, which can be applied to GraphQL objects that require them. Examples of this include the default values for a graphql argument definition, the arguments/variables used in a GraphQL document, or the result of executing a selection set. This value model has evolved over time -- the initial `RawValue` family of types has been gradually replaced with an `IR.Value` model. Currently, both models exist in the code base and are used for slightly different jobs. This PR completely removes the `RawValue` model and updates all systems to use `IR.Value`. Along the way it also updates most components to operate on a ViaductSchema rather than graphql-java's GraphQLSchema. **Better Edgecase Coverage** Before this change, the schema generator would assign default values and applied directives as it generated each type. This created complexity in other parts of the system (eg value generators had to know how to not generate a value for a type that hadn't been defined yet), and made it impossible for the system to generate some kinds of edge cases. This PR changes this behavior to insert default values and applied directives as additional passes, after all types have been generated. This simplifies value generation and allows for more kinds of edge cases to be generated. This ability to generate more edge cases has surfaced some novel issues, such as this potential spec issue with OneOf types: graphql/graphql-spec#1211 Github-Change-Id: 998226 GitOrigin-RevId: 6ae10823b358faf51f883d593d33d6767933ee8b

@OneOf

reframe the issue as as general class of input object habitation, not just a special edge case around pure OneOf trees. This allows handling the case of mixed OneOf/non-OneOf types, like: ```graphql input A @OneOf { b: B } input B { a: A! } ``` To support this, I've also rewritten the previously-proposed `OneOfInputObjectCanBeProvidedAFiniteValue` function as `InputObjectCanBeProvidedAFiniteValue`, which handles both pure inputs, pure OneOfs, and mixed input/OneOfs validation.

jbellenger · 2026-02-26T20:31:38Z

@benjie after looking at this more, I realized that my previous proposal only dealt with closed OneOf trees, but that one can also define uninhabited types by mixing OneOf and non-OneOfs.

For example, this cycle is considered valid by both the current spec and my original proposal:

input A @oneOf { b:B }
input B { a:A! }

I've updated this PR to fold the overly-specific OneOf rules I'd originally written into the Cyclic References section, capturing the general shape of the problem for both flavors of input objects.

This is implemented in graphql-js PR graphql/graphql-js#4564

I was going to plan on being at the graphql-wg call on March 5th, though with the slightly wider scope of this PR I'd be fine to catch a different meeting.

benjie

Great work! For Non-Null type we normally say "Let {innerType} be the inner type of {...}" or "Let {nullableType} be the unwrapped nullable type of", but these are just editorial nits really - the structure looks correct to me.

benjie · 2026-02-26T21:54:12Z

spec/Section 3 -- Type System.md

-3. If an Input Object references itself either directly or through referenced
-   Input Objects, at least one of the fields in the chain of references must be
-   either a nullable or a List type.
+3. {InputObjectCanBeProvidedAFiniteValue(inputObject)} must be {true}.


Nit... Maybe:

Suggested change

3. {InputObjectCanBeProvidedAFiniteValue(inputObject)} must be {true}.

3. {InputObjectHasUnbreakableCycle(inputObject)} must be {false}.

benjie · 2026-02-26T21:59:35Z

spec/Section 3 -- Type System.md

+      - Let {innerType} be the type wrapped by {fieldType}.
+      - If not {FieldTypeCanBeProvidedAFiniteValue(innerType, nextVisited)}:


Suggested change

- Let {innerType} be the type wrapped by {fieldType}.

- If not {FieldTypeCanBeProvidedAFiniteValue(innerType, nextVisited)}:

- Let {nullableType} be the unwrapped nullable type of {fieldType}.

- If not {FieldTypeCanBeProvidedAFiniteValue(nullableType, nextVisited)}:

benjie · 2026-02-26T22:01:08Z

spec/Section 3 -- Type System.md

+  - Let {innerType} be the type wrapped by {fieldType}.
+  - Return {FieldTypeCanBeProvidedAFiniteValue(innerType, visited)}.


Suggested change

- Let {innerType} be the type wrapped by {fieldType}.

- Return {FieldTypeCanBeProvidedAFiniteValue(innerType, visited)}.

- Let {nullableType} be the unwrapped nullable type of {fieldType}.

- Return {FieldTypeCanBeProvidedAFiniteValue(nullableType, visited)}.

benjie · 2026-02-26T22:03:03Z

spec/Section 3 -- Type System.md

+  - Let {innerType} be the type wrapped by {fieldType}.
+  - Return {FieldTypeCanBeProvidedAFiniteValue(innerType, visited)}.
+- Assert: {fieldType} is a named type.
+- If {fieldType} is not an Input Object type:


Not sure if we need this added for clarity... To make it clear that OneOf Input Objects are included?

Suggested change

- If {fieldType} is not an Input Object type:

- If {fieldType} is not an Input Object type (including variants):

spec edits for OneOf inhabitability

1e86f0a

jbellenger marked this pull request as ready for review February 17, 2026 00:17

benjie added the 💭 Strawman (RFC 0) RFC Stage 0 (See CONTRIBUTING.md) label Feb 17, 2026

martinbonnin mentioned this pull request Feb 17, 2026

MegaIssue: GraphQL spec tracking apollographql/apollo-kotlin#6474

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This was referenced Feb 17, 2026

OneOf Inhabitability graphql/graphql-js#4564

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OneOf Inhabitability graphql-java/graphql-java#4248

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Add OneOf Inhabitability validation graphql-java/graphql-java#4250

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jbellenger mentioned this pull request Feb 27, 2026

Add OneOf Inhabitability to March WG agenda graphql/graphql-wg#1937

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OneOf inhabitability#1211

OneOf inhabitability#1211
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	3. {InputObjectCanBeProvidedAFiniteValue(inputObject)} must be {true}.
	3. {InputObjectHasUnbreakableCycle(inputObject)} must be {false}.

		- Let {innerType} be the type wrapped by {fieldType}.
		- If not {FieldTypeCanBeProvidedAFiniteValue(innerType, nextVisited)}:

		- Let {innerType} be the type wrapped by {fieldType}.
		- Return {FieldTypeCanBeProvidedAFiniteValue(innerType, visited)}.

	- If {fieldType} is not an Input Object type:
	- If {fieldType} is not an Input Object type (including variants):

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