Fix phpstan/phpstan#14349: Missing "Fatal error: Cannot re-assign $this"

[phpstan-bot]

Summary

PHP produces a fatal error "Cannot re-assign $this" when $this is used as a foreach value or key variable, but PHPStan did not report this. This PR adds a new rule to detect this error.

Changes

• Added src/Rules/Variables/InvalidForeachVariableRule.php — a new level 0 rule that checks foreach statements for $this as value or key variable
• Added tests/PHPStan/Rules/Variables/InvalidForeachVariableRuleTest.php — test case for the new rule
• Added tests/PHPStan/Rules/Variables/data/bug-14349.php — test data covering $this as value, by-ref value, and key in foreach

Root cause

There was no rule checking foreach variable names for $this. The existing InvalidParameterNameRule covered function/method parameters and InvalidLexicalVariablesInClosureUseRule covered closure use() clauses, but foreach was a gap.

Test

The regression test verifies that foreach ($a as $this), foreach ($a as &$this), and foreach ($a as $this => $v) all produce "Cannot re-assign $this." errors, while foreach ($a as $ok) produces no error.

Fixes phpstan/phpstan#14349

[ondrejmirtes]

Create a rule for VariableAssignNode. Extract the part about assigning $this from InvalidAssignVarRule there. And also make sure the same rule for VariableAssignNode also covers this foreach scenario.

[phpstan-bot]

All done. Here's a summary of the changes:

1. Created src/Rules/Variables/InvalidVariableAssignRule.php - A new level 0 rule that listens on VariableAssignNode and reports "Cannot re-assign $this." when $this is the target of any variable assignment (regular assignments, foreach variables, list destructuring).

2. Modified src/Rules/Operators/InvalidAssignVarRule.php - Removed the $this check from containsNonAssignableExpression() (now returns false for all variables, since $this is handled by the new rule).

3. Deleted src/Rules/Variables/InvalidForeachVariableRule.php - No longer needed since the new VariableAssignNode-based rule covers foreach scenarios.

4. Updated tests - The InvalidVariableAssignRuleTest covers all scenarios: foreach value/key/by-ref, regular assignment, compound assignment, and list destructuring with $this. The InvalidAssignVarRuleTest::testBug3585 now expects no errors since those were all $this assignments.

All 11,634 tests pass and make phpstan reports no errors.

[ondrejmirtes]

Add bug-3585.php to the test of the new rule

[phpstan-bot]

Already handled — the change has been committed and pushed. All tests and static analysis passed.

[ondrejmirtes]

tests/PHPStan/Rules/Variables/data/bug-14349.php should be added to excluded files in make lint in Makefile

[phpstan-bot]

Done. Added tests/PHPStan/Rules/Variables/data/bug-14349.php to the excluded files in make lint in the Makefile, since that test file contains intentionally invalid PHP ($this reassignment) that would fail the linter. The lint target passes cleanly.