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phpstan-type-trace

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See the full type-inference chain of any value in PHPStan, not just a single snapshot.

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# phpstan-type-trace

🌐 **[Live examples →](https://kayw-geek.github.io/phpstan-type-trace/)** — read 5 real chains in 10 seconds, no install.

When PHPStan tells you:

Parameter #1 $amount of method format() expects float, float|null given.

You know the final type at the call site. You don't know which assign, which param, or which missing narrow put the `null` there. You scroll up, guess, get it wrong, repeat.

This extension prints the full chain — every event that shaped the variable up to that line:

```
$amount · App\PriceCalculator::format [src/PriceCalculator.php] (up to L25)
  L16  param   float|null
  L20  narrow  Webmozart\Assert\Assert::notNull($amount)  =>  float    via AssertTypeSpecifyingExtension
  L25  read    float
```

One command, zero source edits, third-party extensions attributed.

![Hero](https://raw.githubusercontent.com/kayw-geek/phpstan-type-trace/main/docs/hero.png)

Above: a longer chain from real larastan code — nine events including three `narrow` rows that show *why* the type tightened.

## Install

```bash
composer require --dev kayw-geek/phpstan-type-trace
```

Auto-registered via [phpstan-extension-installer](https://github.com/phpstan/extension-installer). Otherwise add to `phpstan.neon`:

```neon
includes:
    - vendor/kayw-geek/phpstan-type-trace/extension.neon
```

## Usage

### CLI — inspect any line, no source edits

```bash
./vendor/bin/phpstan-trace inspect src/Foo.php:42 myVar
```

Variable name is optional — if only one variable has events at the target line, it's auto-picked. Otherwise the candidates are listed.

Pass `--json` for machine-readable output (handy for IDE plugins, agents, and CI). Schema is versioned and documented in [`docs/json-api.md`](docs/json-api.md); pin with `--api-version=N`.

### `traceType()` — drop in a marker, get the chain on your next phpstan run

No extra command. Just call `traceType($var)` anywhere, then run `vendor/bin/phpstan analyse` like you always do — the chain shows up as a phpstan error at that line.

```php
function compute(?float $discount = null): float
{
    $discount ??= 0.1;
    traceType($discount, 'after ??=');
    return 1 - $discount;
}
```

```
 ------ -----------------------------------------------------------
  Line   PriceCalculator.php
 ------ -----------------------------------------------------------
  5      Type chain for $discount in compute — after ??=
           L3   param      float|null
           L4   assign-op  float
 ------ -----------------------------------------------------------
```

`traceType()` is a runtime no-op (autoloaded from `src/runtime.php`), so leaving a stray call in production code does nothing — it only emits during static analysis.

Signature:

```php
function traceType(mixed $value, ?string $reason = null): void
```

`$value` accepts a variable, property fetch (`$this->x`), or static property (`Foo::$bar`). For arbitrary expressions, only the snapshot type is printed. `$reason` is a string literal shown in the chain header.

## What gets captured

| Source                    | Origin label  | Example                                  | `via` |
| ------------------------- | ------------- | ---------------------------------------- | :---: |
| Function/method params    | `param`       | `function f(int $x)`                     |       |
| Closure / arrow-fn params | `param`       | `fn(int $x) => ...`                      |       |
| Variable assignment       | `assign`      | `$x = 5;`                                | ✓     |
| Compound assignment       | `assign-op`   | `$x += 1; $x ??= 'def';`                 | ✓     |
| Reference assignment      | `assign-ref`  | `$x = &$other;`                          |       |
| Array write               | `array-write` | `$x[] = 'y'; $x['k'] = $v;`              |       |
| Property fetch            | `read`        | `$this->foo`                             | ✓     |
| Static property fetch     | `read`        | `Foo::$bar`                              | ✓     |
| Variable read             | `read`        | bare `$x` usage                          |       |
| If / ternary narrowing    | `narrow`      | `if (is_string($x))`, `$x ?? 'd'`, etc.  | ✓     |

`narrow` events carry a `reason` showing the predicate that justified the narrowing (`is_string($x)`, `$x instanceof Foo`, `$x !== null`, ...), anchored to the branch where the narrow takes effect. Same-line events are ordered by source position, so an inline ternary reads cause → effect: the cond-read first, then the narrow, then the then-branch read.

**`via` — third-party extension attribution.** When the inferred type was shaped by a third-party PHPStan extension, the extension's short class name is appended (`via NewModelQueryDynamicMethodReturnTypeExtension`, `via AssertTypeSpecifyingExtension`, ...). Three categories are attributed today: dynamic return type (assign / assign-op), type-specifying (narrow), and properties class reflection (read). Detection is by source-file location — official add-ons like `phpstan/phpstan-webmozart-assert` that ship under the `PHPStan\` namespace are still attributed; only classes shipped by `phpstan/phpstan` core are filtered out.

When the inferred type surprises you, `via` tells you which extension to blame (or thank) without grepping the vendor tree.

**Not attributed yet:** type-specifying calls used as bare statements (`Assert::notNull($x);` outside any `if` / ternary). PHPStan still narrows the scope but no `narrow` event is emitted. Wrap the call in an `if` if you need the attribution.

## Limitations

- Loops report the post-fixpoint type, not per-iteration deltas.
- Multiple closures inside the same enclosing function share one bucket. Same-named vars across sibling closures may collide.
- Cannot follow values across function boundaries.
- Ref-aliases (`$alias = &$x; $alias[] = 'y';`) show only the snapshot at the call.

## Use it in PhpStorm

[![JetBrains Plugin](https://img.shields.io/jetbrains/plugin/v/31962-phpstan-type-trace?label=PhpStorm%20plugin)](https://plugins.jetbrains.com/plugin/31962-phpstan-type-trace)

Install the companion plugin to read chains inside the editor — caret on a variable, run **Trace Type at Caret**, and the chain renders in a dedicated tool window with clickable line numbers, copyable types, and `via <Extension>` attribution pills.

The plugin shells out to this CLI, so the package above must be installed in your project's `vendor/`. Marketplace page: <https://plugins.jetbrains.com/plugin/31962-phpstan-type-trace>.

## Use it with Claude Code

When Claude Code (or any LLM agent) is chasing PHPStan errors, it usually guesses at types. With this extension installed as a [Claude Code plugin](https://docs.claude.com/claude-code), Claude invokes the trace automatically — fixes are grounded in real upstream type evidence, not pattern-matching.

```
/plugin marketplace add kayw-geek/phpstan-type-trace
/plugin install phpstan-type-trace@kayw-geek
```

Installed into `~/.claude/plugins/cache/`, auto-discovered across every project. Updates: `/plugin marketplace update kayw-geek` then reinstall.

<details>
<summary><strong>How it works</strong></summary>

Two-phase PHPStan pipeline:

1. **Collectors** (one per event kind) record every relevant AST event with `(file, functionKey, path, line, pos, type, origin)`:
   - Param entry: `ParamInFunctionCollector`, `ParamInMethodCollector`, `ParamInClosureCollector`, `ParamInArrowFunctionCollector` — hooked on PHPStan's `In*Node` virtual nodes so scope is already inside the function when params are read.
   - Reads: `VarReadCollector`, `PropertyFetchCollector`, `StaticPropertyFetchCollector`.
   - Writes: `AssignCollector`, `AssignOpCollector` (covers all 13 compound-op subclasses), `AssignRefCollector`, `ArrayWriteCollector`.
   - Narrowing: `NarrowingCollector` (if-statements), `TernaryNarrowingCollector` (ternaries) — anchored to the branch where the narrow holds, with a reason predicate extracted from the guard.
   - Call sites: `TraceCallCollector`.
2. **`TraceReportRule`** runs once at the end on the virtual `CollectedDataNode`. For each `traceType()` call it joins the recorded events on `(functionKey, path)` filtered to lines `<=` the call line, sorts by `line → source position → rank` (so inline ternaries read cond-read → narrow → then-read, not narrow first), collapses repeated reads of the same type *except* right after a narrow (since the narrow is evidence and the read is the usage — they convey different things), and emits the delta chain as a PHPStan error.

The CLI runs the same pipeline with a dump env var set, captures every chain as a JSON sentinel error, then filters to the `(file, line, variable)` you asked about.

</details>

## License

MIT

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