Unroll foreach over union of constant arrays in tryProcessUnrolledConstantArrayForeach

[phpstan-bot]

Summary

When building an array inside a foreach loop that iterates over a union of constant arrays (e.g. list{'username', 'password'}|list{'app_id', 'app_key'}), PHPStan previously merged all possible keys from all union members into a single array with optional keys, rather than preserving per-array shapes as a union type.

This caused false positives when the built array was passed to a function expecting a union of shaped arrays.

Changes

• Modified tryProcessUnrolledConstantArrayForeach() in src/Analyser/NodeScopeResolver.php to handle unions of multiple ConstantArrayType values
• Removed the count($constantArrays) !== 1 guard that previously rejected unions
• Added a total key count check across all arrays against FOREACH_UNROLL_LIMIT
• Each constant array in the union is now unrolled independently with its own chain scope starting from $originalScope
• Per-array body scopes and chain scopes are computed separately, then merged across arrays for the final result
• Native type matching updated to pair constant arrays by index when counts match

Root cause

The tryProcessUnrolledConstantArrayForeach() method only supported a single constant array (count($constantArrays) !== 1 early return). When the iteratee was a union of constant arrays — which commonly occurs when the outer foreach over a constant array assigns different array values to the loop variable — the method bailed out and the standard iterative foreach processing ran instead. The iterative processing sees the union's combined iterable value type (e.g. 'username'|'password'|'app_id'|'app_key') and produces a single array with all keys as optional, losing the per-array shape information.

The fix processes each constant array in the union independently through the existing unrolling logic, preserving per-array type precision, then merges the results.

Analogous cases probed:

• Direct parameter union of constant arrays (@param list{'a','b'}|list{'x','y'}) — also fixed by this change
• Different-length arrays in the union — also fixed
• Single constant array (original behavior) — unchanged, verified by existing tests

Test

Added tests/PHPStan/Analyser/nsrt/bug-7978.php with three test cases:

1. The original reported pattern: nested foreach over class constant with shaped array values
2. Direct union parameter: @param list{'username','password'}|list{'app_id','app_key'}
3. Different-length arrays: @param list{'a','b','c'}|list{'x'}

All verify the inferred type is a union of the correct shaped arrays rather than a single array with all optional keys.

Fixes phpstan/phpstan#7978