Summary

When a cast expression like (string)$x is used in a comparison ((string)$x !== ''), PHPStan correctly narrows the type of the cast result but does not propagate the narrowing back to the inner expression $x. For example, with $x: string|null, (string)$x !== '' should narrow $x to non-empty-string (removing null since (string)null === ''), but previously $x remained string|null.

Changes

• src/Analyser/TypeSpecifier.php:

  • Added Cast\String_ handling in specifyTypesForConstantStringBinaryExpression: when (string)$x is compared to a constant string, propagates narrowing to $x. For empty string comparison, removes null, false, and '' from $x's type. For non-empty string comparisons in true context, removes null and false.
  • Added Cast\Int_ and Cast\Double handling in specifyTypesForConstantBinaryExpression: when (int)$x or (float)$x is compared to a constant, removes null and false from $x's type (only when comparison with zero value in false context, or non-zero value in true context).
  • Extended resolveIdentical normalization to also swap Cast expressions to the left side (matching existing FuncCall normalization), ensuring reversed orderings like '' !== (string)$x work correctly.
  • Added inner expression narrowing in resolveEqual's == '' handler for Cast\String_ expressions, fixing loose comparison narrowing.

• Analogous cases probed:

  • (bool) cast: already works correctly through existing specifyTypesForConstantBinaryExpression → specifyTypesInCondition flow (verified with test)
  • (int) cast: fixed, narrowing now propagates for (int)$x !== 0 and (int)$x === 5 patterns
  • (float) cast: fixed, same pattern as int cast
  • Loose comparisons (!=, ==): fixed for string casts via resolveEqual handler

Root cause

The TypeSpecifier creates type specifications (narrowing) for the expression used in a comparison, but when that expression is a cast like (string)$x, the narrowing was only applied to the cast result — not to the inner expression $x. The fix adds explicit handlers that detect cast expressions in comparisons and create additional type specifications for the inner expression based on the semantics of the cast (e.g., (string)null === '', (int)null === 0).

Test

• tests/PHPStan/Analyser/nsrt/bug-8231.php: regression test for the reported bug with (string) cast comparisons including !==, ===, reversed ordering, and various type combinations (string|null, int|null, string|false, bool|null)
• tests/PHPStan/Analyser/nsrt/bug-8231-analogous.php: tests for analogous cases with (int) cast, (float) cast, (bool) cast (already working), non-zero constant comparisons, reversed orderings, and loose string comparisons

Fixes phpstan/phpstan#8231