Types::Const fails due incomplete readonly reference/hash semantics

[fglock]

Summary

The CPAN tester regression for Types::Const v0.5.0 fails because PerlOnJava does not correctly implement the read-only reference/hash semantics required by Const::Fast and Types::Const.

Archived CPAN tester run: 20260824-171428-41866 (Types-Const regression record)

Reproduction

The bounded CPAN run completed in 19 seconds:

! REGRESS Types::Const PASS -> FAIL (7/12)

Failures:

• t/10-basic.t: 3/3 subtests failed
• t/11-parameterized.t: 2/3 subtests failed

The failures are the checks that writable array/hash references are rejected by Const, and that coercion makes nested references read-only while leaving the original value unchanged.

Evidence and suspected cause

The target and its Const::Fast dependency configured and built successfully. The failure is not a timeout or missing-prerequisite failure.

Types::Const calls Const::Fast::_make_readonly, which relies on Internals::SvREADONLY to recursively mark reference values read-only. The relevant runtime gaps are:

1. The SvREADONLY setter for a scalar reference marks the referent/target instead of the reference scalar itself, preventing correct recursive traversal.
2. Hash references have no read-only state: the HASHREFERENCE setter is still a TODO and RuntimeHash has no READONLY_HASH type or mutation guards.

This explains both the failed read-only checks and the failed original-unchanged checks. The regression was observed on the JVM backend; interpreter parity is still unverified.

System Perl passes the complete upstream suite after installing the distribution and prerequisites into an isolated temporary library: 17 files, 12 tests, all successful. The internal cause is therefore attributable to PerlOnJava.

Suggested follow-up

• Add focused project-owned tests for scalar-reference and hash-reference SvREADONLY semantics, including recursive/cyclic structures.
• Implement correct reference-slot readonly marking.
• Add readonly hash state and mutation enforcement.
• Run the Types::Const focused suite on both JVM and interpreter backends.