Fix phpstan/phpstan#10055: Conditional argument type not properly narrowing type (for constant string unions?)

[phpstan-bot]

Summary

Conditional parameter types (@param ($param1 is 'value3' ? bool : int) $param2) fail to narrow correctly when the parameter union type has more than 2 members. The "false" branch condition becomes a UnionType that never matches individual constant types during the equals() check.

Changes

• Modified src/Analyser/MutatingScope.php to split union condition types into individual ConditionalExpressionHolder instances when creating holders for @param conditional types
• Added regression test tests/PHPStan/Analyser/nsrt/bug-10055.php

Root cause

When processing @param ($param1 is 'value3' ? bool : int) $param2 with $param1 typed as 'value1'|'value2'|'value3':

• The "true" branch condition is TypeCombinator::intersect('value1'|'value2'|'value3', 'value3') = 'value3' (works fine)
• The "false" branch condition is TypeCombinator::remove('value1'|'value2'|'value3', 'value3') = 'value1'|'value2' (a UnionType)

Later, when matching conditions in a match arm where $param1 is narrowed to 'value1', the equals() check compares ConstantStringType('value1') against UnionType('value1'|'value2'), which always fails.

The fix splits union condition types into individual holders: instead of one holder with condition 'value1'|'value2', two holders are created — one for 'value1' and one for 'value2' — both mapping to the same result type (int). This allows the equals() check to match correctly.

Test

Added tests/PHPStan/Analyser/nsrt/bug-10055.php which verifies that in a match expression, conditional parameter types correctly narrow $param2 to int for 'value1' and 'value2' arms, and to bool for the 'value3' arm.

Fixes phpstan/phpstan#10055