Stdlib: str_decrement('a') — must return backtick not out-of-range Error (ext/standard/string.c)

[PurHur]

Summary

str_decrement('a') must return ` (single-byte decrement below 'a'). This compiler throws:

Error: str_decrement(): Argument #1 ($string) "a" is out of decrement range

Multi-character strings like 'aa' → 'z' and 'ba' → 'az' work; only single lowercase 'a' is wrong.

Repro

./script/docker-exec.sh -- php bin/vm.php test/repro/maintainer_gap_str_decrement_single_a.php

Zend (PHP 8.3+): prints ` and exits 0.

This compiler: Error: … "a" is out of decrement range, exit 1.

php-src reference

• ext/standard/string.c — php_str_decrement() single-character path decrements 'a' to `

Implementation (PHP-in-PHP)

• ext/standard/VmString.php — strDecrement() carry/trim logic mishandles lone 'a' (wraps to 'Z' then rejects). Match php-src per-byte decrement for length-1 strings.
• ext/standard/str_decrement.php + JIT lowering if present.

Done when (php-src-strict)

• php bin/vm.php test/repro/maintainer_gap_str_decrement_single_a.php exits 0.
• Compliance guard for str_decrement('a') === `.

Related: #4218, #9277 (other str_increment/decrement edges).

Refs #1492

[PurHur]

claim: cursor-agent-lane-b — starting this run

[PurHur]

Merged via #10853

Fix: Underflow paths in VmString::strDecrement() and JitStrIncdec now raise ValueError (not Error), matching php-src zend_argument_value_error() for str_decrement('a') / leading '0'.

Note: php-src master rejects single-char 'a' with ValueError — it does not return a backtick. Issue repro updated accordingly.

Verification:

./script/docker-exec.sh -- bash -lc 'php bin/vm.php test/repro/maintainer_gap_str_decrement_single_a.php'
./script/docker-exec.sh -- bash -lc './vendor/bin/phpunit test/compliance/VMTest.php --filter str_decrement'