Regression: throw expression non-throwing branch returns empty — ?? RHS must yield value (Zend/zend_compile.c)

[PurHur]

Category

language · php-src-strict

Problem

PHP 8.0+ throw expressions work in throwing branches ($x ?? throw ... when $x is null throws), but when the non-throwing branch is taken the expression must return the left-hand value.

Verified locally (2026-06-18):

Call	Zend PHP 8.2	This compiler VM
f(1) for return $x ?? throw new Exception("e");	int(1)	empty string / no value (echo f(1) prints x= only)
f(null)	throws	throws (OK)

Distinct from parse rejection — syntax compiles; runtime lowering of ZEND_AST_THROW in coalesce/ternary contexts loses the RHS value.

php-src reference

• php/php-src Zend/zend_compile.c — throw-expression compile nodes in coalesce
• php/php-src Zend/zend_execute.c — ZEND_COALESCE, ZEND_THROW interaction
• Tests: Zend/tests/throw/throw_as_expression*.phpt

Repro

./script/docker-exec.sh -- bash -lc 'source script/php-env.sh
php bin/vm.php -r "
function f(?int \$x): int {
    return \$x ?? throw new Exception(\"e\");
}
try {
    echo \"null=\", f(null), PHP_EOL;
} catch (Throwable \$e) {
    echo \"caught\n\";
}
echo \"one=\", f(1), PHP_EOL;
"'
# Zend: caught / one=1
# VM today: caught / one=   (empty)

Scope (this repo)

Path	Work
lib/Compiler.php	Throw-expression coalesce/ternary lowering
lib/VM.php	Opcode execution for coalesce + throw expr
lib/JIT/	Match VM when JIT compiles throw expressions

Done when

• Repro prints caught then one=1 on VM (and JIT if applicable)
• Compliance test ported from Zend/tests/throw/ minimal case
• Language: throw as expression (?? / ?: contexts) — parse/compile/runtime parity (Zend) #9209 umbrella can close when all throw-expression contexts green (this is the concrete runtime bug)

Related

• Language: throw as expression (?? / ?: contexts) — parse/compile/runtime parity (Zend) #9209 throw-expression umbrella · Language: throw as expression (?? / ?: contexts) — parse/compile/runtime parity (Zend) #9209 parse contexts already compile

[PurHur]

claim: compile-link-runtime-worker — starting this run

[PurHur]

Merged via #9469.

Fix: isUnreachableAfterThrow() no longer skips Terminal_Return when a Coalesce sits between the RHS throw and the return — the merge block now gets TYPE_RETURN.

Verification (Docker):

null=caught
one=1
php vendor/bin/phpunit test/unit/ThrowCoalesceReturnTest.php  → OK
php vendor/bin/phpunit --filter throw_coalesce_return          → OK (VM+JIT)