Fix phpstan/phpstan#10786: Invalid return type definition

[phpstan-bot]

Summary

When checking is_null($a->value) && is_null($b->value) and throwing, PHPStan should know that at least one property is non-null afterward. This already worked for plain variables but not for property fetches and other general expressions. The fix extends conditional expression holders to work with property fetches and fixes the isset handling for nullable native typed properties.

Changes

• src/Analyser/TypeSpecifier.php: Replaced instanceof Expr\Variable checks in processBooleanSureConditionalTypes and processBooleanNotSureConditionalTypes with a new canBeConditionalExpressionHolder() helper that also accepts PropertyFetch, StaticPropertyFetch, and ArrayDimFetch. Simplified the self-exclusion loop to a simple unset($conditions[$exprString]).
• src/Analyser/MutatingScope.php: In issetCheck for native typed properties not tracked in expression types, nullable properties now fall through to the type callback instead of returning issetCheckUndefined(). This allows isset() falsey to properly produce specified types that trigger conditional expression holders. Non-nullable properties retain the old behavior (may be uninitialized).

Root cause

Two issues combined:

1. processBooleanSureConditionalTypes and processBooleanNotSureConditionalTypes in TypeSpecifier only created conditional expression holders for Expr\Variable, skipping property fetches entirely.
2. issetCheck in MutatingScope returned null (via issetCheckUndefined) for nullable native typed properties not tracked in the scope, causing filterByFalseyValue(isset($a->value)) to produce empty specified types. Without specified types, the conditional holders could never fire.

Test

Added tests/PHPStan/Analyser/nsrt/bug-10786.php with two test methods verifying that $a->value ?? $b->value resolves to int (not int|null) after checking both properties for null and throwing.

Fixes phpstan/phpstan#10786