C++20 Modules: Prebuilt Binary Module Interface Reuse Across Compiler Patch Releases

Compatibility Boundary for Prebuilt Module Interfaces

The C++20 standard specifies module interface units and import declarations but does not define a binary module interface format. Compiler vendors implement proprietary binary formats for prebuilt module interfaces, and these formats are sensitive to compiler version, standard library version, target triple, and compilation flags.

A practical goal is to determine whether a prebuilt binary module interface generated by one compiler patch release can be safely imported by a translation unit compiled with a subsequent patch release of the same compiler series without regeneration. Implementations vary: some vendors treat minor patches as ABI-compatible for module interfaces, while others require regeneration for any version change.

Constraints include the absence of a standardized versioning scheme for binary module interfaces, the dependence on compiler-internal data structures, and the interaction with build-system dependency scanning that may cache or reuse scanned dependency graphs across compiler invocations.

Open Questions

1. Which major compiler implementations document a compatibility policy for prebuilt module interfaces across minor or patch releases?
2. What specific compiler flags, standard library versions, or target triples are known to invalidate a prebuilt module interface without a diagnostic?
3. How do build systems that cache dependency scanning results handle a compiler upgrade that changes the binary module interface format?