Fix phpstan/phpstan#14421: Incorrect type narrowing of superglobal with dependent types

[phpstan-bot]

Summary

When a variable was assigned from a superglobal array offset in one branch and from a function return in another, narrowing that variable (e.g., $b !== null) would incorrectly propagate back to the superglobal, causing PHPStan to think the offset always exists.

Changes

• Modified MutatingScope::createConditionalExpressions() in src/Analyser/MutatingScope.php to skip superglobal-containing expressions from being guarded in conditional expressions
• Added superglobal detection (using the same NodeFinder + isGlobalVariable pattern already used in mergeVariableHolders()) to both loops that create conditional expression holders
• New regression test in tests/PHPStan/Rules/Variables/IssetRuleTest.php and tests/PHPStan/Rules/Variables/data/bug-14421.php

Root cause

createConditionalExpressions() creates type guards that link expressions across merged scopes. When $_SESSION['a'] was checked with isset() in one branch and $b was assigned from it, the method created a conditional expression saying "when $b has type X, then $_SESSION has narrowed type Y". Later, when $b !== null was checked, this conditional triggered and incorrectly narrowed $_SESSION to always have offset 'a' set.

The fix extends the superglobal protection already present in mergeVariableHolders() to also apply in createConditionalExpressions(), preventing superglobal expressions from being targets of conditional type narrowing.

Test

Added a regression test reproducing the exact scenario from the issue: isset($_SESSION['a']) in an if/else with $b assigned from the superglobal in one branch and get_optional_int() in the other, followed by $b !== null check and inner !isset($_SESSION['a']). The test expects no errors (the false positive about offset 'a' always existing is gone).

Fixes phpstan/phpstan#14421