Stdlib: Exception::getPrevious() / Throwable chain — missing VM method (zend_exceptions.c)

[PurHur]

Category

stdlib | implementation-ready (partial — see status)

Status (2026-06-06 audit)

Method	VM	Notes
getPrevious()	green	lib/VM/Builtin/ExceptionGetPrevious.php — three-arg ctor chain works
getTrace()	missing	tracked in #7159
getTraceAsString()	missing	tracked in #7159
__toString()	missing	echo $e fatals — #7159

Keep this issue for constructor $previous wiring + regression tests; implement remaining Throwable surface in #7159.

Problem

Exception / Throwable instances need full Zend exception chain semantics. getPrevious() now works, but trace accessors and string casting remain broken, blocking real error handlers and echo $exception formatting.

php-src reference

• php/php-src Zend/zend_exceptions.c — zend_exception_get_previous(), zend_exception_get_trace(), zend_std_cast_object_tostring
• Constructor $previous argument wiring in zend_exception_ctor()

Repro — getPrevious (green)

test/repro-maintainer/exception_get_previous.php:

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php test/repro-maintainer/exception_get_previous.php
php bin/vm.php test/repro-maintainer/exception_get_previous.php'

Check	Zend	VM
getPrevious() instanceof Exception	true	true
getPrevious()->getMessage()	'inner'	'inner'

Repro — remaining gaps (see #7159)

<?php
$e = new Exception('outer', 0, new Exception('inner'));
echo $e, "\n"; // Zend: chained text; VM: fatal

Scope

Path	Work
lib/VM/Builtin/ExceptionGetPrevious.php	done
lib/VM/Builtin/ExceptionConstruct.php	ensure 3rd $previous stored on all Throwable types
lib/Compiler.php	pass previous into throw / new Exception with 3rd arg
#7159	getTrace, getTraceAsString, __toString

Done when

• Three-arg new Exception($msg, $code, $previous) preserves chain
• getPrevious() returns null or previous Throwable
• Stdlib: Throwable getTrace()/getTraceAsString()/__toString() — VM introspection missing (zend_exceptions.c) #7159 closed — trace + __toString parity
• Compliance: test/compliance/cases/stdlib/exception_previous_chain.phpt

Verification

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh && ./vendor/bin/phpunit --filter exception_get_previous'

Links

• Stdlib: Throwable getTrace()/getTraceAsString()/__toString() — VM introspection missing (zend_exceptions.c) #7159 — Throwable introspection (primary worker issue for traces/__toString)
• Stdlib: builtin argument count errors should be ArgumentCountError (not LogicException) #4145 builtin error types · North star Epic: Self-host critical path — compiler compiles itself (M3→M5) #1492

[PurHur]

claim: cursor-agent-lane-b — starting this run (getPrevious regression re-verified on master)

[PurHur]

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

[PurHur]

Merged via #17712

Status: Throwable chain parity was already green on master (#7159 closed getTrace/getTraceAsString/__toString). This PR adds the missing compliance guard from the original done-when checklist.

Verification:
```bash
./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php bin/vm.php test/repro/exception_previous_chain.php
php vendor/bin/phpunit test/compliance/VMTest.php --filter exception_previous_chain'
```