Cutdown Compatibility Fallback Policy

Principle

Unknown or unsupported constructs must degrade deterministically with diagnostics.

Parsers must not silently reinterpret unknown constructs into different known semantics.

Required Behavior

When input contains syntax or extension behavior not supported by the active profile/version:

• Continue parsing and return AST.
• Apply documented fallback (typically literal-text or core fallback path).
• Emit structured diagnostics with stable code and span.

Forbidden Behavior

• Silent reinterpretation into a different construct.
• Non-deterministic fallback choices across runs/implementations.
• Fail-fast rejection as the default mode.

Conformance Requirements

Conformance tests must include:

• unknown-extension under disabled profile,
• unknown future syntax under current version,
• expected fallback AST and diagnostics for both.

Related

• ./parser-profile-policy.md
• ./diagnostics-policy.md