Fix phpstan/phpstan#8231: Missing type narrowing after (string)$value

[phpstan-bot]

Summary

When comparing a string cast expression with an empty string (e.g., (string)$x !== ''), PHPStan was not propagating the type narrowing back to the inner expression. This meant that $x of type string|null was not narrowed to string (or non-empty-string) inside the condition branch, causing false positive errors.

Changes

• Added a new block in src/Analyser/TypeSpecifier.php in resolveNormalizedIdentical() to handle Cast\String_ expressions
• When (string)$expr === '' is encountered, the narrowing is propagated to $expr using the union type null|false|'' (the types that produce '' when cast to string)
• This works in both contexts: === '' narrows to those types, !== '' removes them

Root cause

TypeSpecifier::resolveNormalizedIdentical() already handled unwrapping AlwaysRememberedExpr to propagate type narrowing to inner expressions, but did not handle Expr\Cast\String_. When (string)$x !== '' was analysed, the cast expression (string)$x was correctly narrowed to non-empty-string, but the underlying variable $x retained its original string|null type because the narrowing was never propagated through the cast.

Test

Added tests/PHPStan/Analyser/nsrt/bug-8231.php with assertions covering:

• string|null narrowed to non-empty-string after (string)$x !== ''
• string|null narrowed to ''|null after (string)$x === ''
• int|null narrowed to int (removing null)
• int|string|null narrowed to int|non-empty-string
• bool|string|null narrowed to non-empty-string|true

Fixes phpstan/phpstan#8231