Fix phpstan/phpstan#6830: Variable inside loop might not be defined.

[phpstan-bot]

Summary

When a variable is conditionally defined before a loop and used inside the loop under the same condition, PHPStan incorrectly reports "Variable $x might not be defined." This happens because the conditional expression tracking ("if $do is true then $x is defined") is lost during loop scope merging when the variable's type changes across iterations.

Changes

• Modified MutatingScope::intersectConditionalExpressions() in src/Analyser/MutatingScope.php to add a fallback merging path when exact key matching fails
• When two conditional expression holders have the same conditions (e.g., $do=true) but different result types (e.g., int(9999) vs int(123)), they are now merged by taking the union of result types and the minimum certainty
• Extended regression test in tests/PHPStan/Rules/Variables/data/bug-6830.php with the original issue's reproduction case (variable defined before loop, used inside loop)

Root cause

intersectConditionalExpressions() uses ConditionalExpressionHolder::getKey() for matching, which includes the result type in the key string (e.g., $do=true => 9999 (Yes)). When a variable's type changes across loop iterations (e.g., $x goes from 9999 to 123), the keys differ and the intersection drops the conditional expression entirely. This means the knowledge that "$do being true implies $x is defined" is lost, causing the false positive.

The fix adds a fallback: when exact key matching fails, it matches holders by their condition expressions only, then merges the result types using TypeCombinator::union() and takes the minimum certainty via TrinaryLogic::maxMin().

Test

Extended the existing tests/PHPStan/Rules/Variables/data/bug-6830.php with two additional test functions:

• test2(): reproduces the original issue — variable $x defined conditionally before a foreach loop, reassigned inside the loop under the same condition
• test3(): simpler variant — variable $x defined conditionally before a foreach loop, read inside the loop under the same condition

Both expect no errors (previously test2 reported "Variable $x might not be defined" on line 28).

Fixes phpstan/phpstan#6830