Reject schema properties marked both readOnly and writeOnly

[shibayan]

Summary

OpenAPI allows declaring a property as readOnly: true and writeOnly: true, but the combination has no well-defined serialization direction. The dictionary converter generated code that skipped such properties in both directions (IsRequestSerializerOptions(options) || IsResponseSerializerOptions(options) → reader.Skip()), so a required property in that state silently dropped on every read — data loss without diagnostic.

Detect the combination while building each SchemaDefinition and throw UnsupportedGenerationException, which surfaces as OAW004. Consumers that hit this either fix the schema (the spec-correct outcome) or get a clear failure instead of silent data loss.

Related issue

None.

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation
• Refactoring or maintenance

Validation

• dotnet build OpenApiWeaver.slnx
• dotnet test OpenApiWeaver.slnx

Impact checklist

• Added or updated tests for generator output, diagnostics, or supported OpenAPI shapes when behavior changed
• Updated README.md or documentation when user-facing behavior, configuration, or limitations changed
• Updated AnalyzerReleases.Unshipped.md when diagnostics were introduced or changed
• Called out breaking changes or notable generated output changes in the summary

Notes for reviewers

• No new diagnostic ID: reuses the existing OAW004 "unsupported feature" channel.
• AnalyzerReleases.Unshipped.md is unchanged because no new rule is introduced.
• The dictionary converter retains its { ReadOnly: true, WriteOnly: true } branch in BuildRequiredDictionaryConverterPropertyExpression and the skip block — those paths are now unreachable from any valid input, but removing them is a separate clean-up.
• Breaking change scope: previously this generated client silently lost data; now it fails at compile time. Anyone hitting OAW004 needs to drop one of the flags on their schema.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This pull request prevents silent data loss in generated dictionary converters by rejecting OpenAPI schema properties that are simultaneously marked readOnly: true and writeOnly: true, reporting the existing unsupported-feature diagnostic (OAW004) instead of generating code that skips the property in both directions.

Changes:

• Add validation in schema transformation to throw UnsupportedGenerationException when a property is both readOnly and writeOnly.
• Add a diagnostics test asserting OAW004 is produced and no client source is generated for such schemas.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
tests/OpenApiWeaver.Tests/ClientGeneratorTests.Diagnostics.cs	Adds a regression test ensuring the invalid RO+WO combination reports OAW004 and blocks client generation.
src/OpenApiWeaver/ClientGenerator.Transformer.Schemas.cs	Adds early validation during SchemaDefinition construction to reject properties marked both readOnly and writeOnly.

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