openapi: decide how nullable in an OpenAPI 3.1 document is treated #632

Problem

openapi: 3.1.0
…
    N: {type: string, nullable: true}

OpenAPI 3.1 removed nullable; JSON Schema 2020-12 ignores keywords it doesn't define. The compiler honors it anyway: N's reference is nullable: true, with no diagnostic and no inference marker.

Honoring it may well be the right leniency, since documents migrated from 3.0 keep it. But it asserts something the 3.1 document doesn't say, so invariant 6 applies.

Decide

Keep honoring it, but mark the result Inferred and report it at info (or make it a policy that can be switched off).