Resolve self parameters in traits against the required class

docs/CHANGELOG.md

@@ -249,6 +249,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 #### Type inference
 
+- **`self` parameters in traits.** A parameter typed `self` inside a trait now sees the class its `@phpstan-require-extends` or `-implements` names, as `$this` already did, so a model trait's `static::saving(static function (self $model) { $model->getTable(); })` no longer reports `getTable()` as unknown.
 - **Constants passed to calls.** Namespaced and imported constants keep their values and types when used as arguments. Contributed by @shuvroroy.
 - **Constants defined from namespaced or imported constants.** A class constant or namespaced constant such as `const MAX = LIMIT;` now holds the value of the constant its own file means, whether imported with `use const`, declared in its namespace, or global, when it is read from another file. A parameter default that names a constant is read the same way when the argument is left out.
 - **Constants holding a class name or an enum case.** A global or namespaced constant such as `const DEFAULT = Widget::class;` or `const SUIT = Suit::Hearts;` now has that type, including when it is passed to a templated function or used as a parameter default.

src/type_engine/variable/forward_walk/param_seeding.rs

@@ -10,7 +10,7 @@ use mago_span::HasSpan;
 use crate::atom::bytes_to_str;
 use crate::parser::extract_hint_type;
 use crate::php_type::PhpType;
-use crate::types::{MethodInfo, ResolvedType};
+use crate::types::{ClassInfo, MethodInfo, ResolvedType};
 
 /// What seeding a function-like body needs to know about its declaration.
 #[derive(Clone, Copy)]
@@ -794,12 +794,33 @@ pub(crate) fn declared_param_type(
         }
     }
 
+    // A `self` parameter in a trait holds a class using the trait, so it gets `$this`'s bounds.
+    if is_the_current_trait(&param_results, ctx.current_class) {
+        crate::type_engine::trait_context::extend_this_with_trait_bounds(
+            &mut param_results,
+            ctx.current_class,
+            ctx.file.all_classes,
+            ctx.file.class_loader,
+            ctx.file.backend,
+        );
+    }
+
     DeclaredParam {
         effective: effective_type,
         results: param_results,
     }
 }
 
+/// Whether a parameter's resolved type is exactly the trait being analysed,
+/// which is what a `self` hint inside a trait resolves to.
+fn is_the_current_trait(param_results: &[ResolvedType], current_class: &ClassInfo) -> bool {
+    current_class.kind == crate::types::ClassLikeKind::Trait
+        && matches!(
+            param_results,
+            [only] if only.class_info.as_ref().is_some_and(|ci| ci.fqn() == current_class.fqn())
+        )
+}
+
 /// Turn a variadic parameter's element types into the array it receives.
 ///
 /// Named arguments land in it under their names, so its keys are

tests/integration/diagnostics_unknown_members.rs

@@ -8313,6 +8313,52 @@ trait MyTrait {
     );
 }
 
+/// A parameter typed `self` inside a trait is an instance of a class using
+/// the trait, so it sees the `@phpstan-require-extends` class's members the
+/// way `$this` does, in a method and in a closure (Laravel model events).
+#[test]
+fn self_typed_parameter_inside_trait_sees_required_class() {
+    let backend = create_test_backend();
+    let uri = "file:///test.php";
+    let text = r#"<?php
+abstract class Model {
+    public function getTable(): string { return ''; }
+    public static function saving(\Closure $callback): void {}
+}
+
+/** @phpstan-require-extends Model */
+trait Stamps {
+    public static function bootStamps(): void {
+        static::saving(static function (self $model): void {
+            $model->getTable();
+            $model->notOnModel();
+        });
+    }
+
+    public static function viaParameter(self $model): string {
+        return $model->getTable();
+    }
+}
+
+class Post extends Model {
+    use Stamps;
+}
+"#;
+    let diags = unknown_member_diagnostics(&backend, uri, text);
+    let messages: Vec<String> = diags
+        .iter()
+        .map(|d| message_text(&d.message).to_string())
+        .collect();
+    assert!(
+        !messages.iter().any(|m| m.contains("getTable")),
+        "getTable() comes from the required class, got: {messages:?}"
+    );
+    assert!(
+        messages.iter().any(|m| m.contains("notOnModel")),
+        "a member neither the trait nor the required class has must still be flagged, got: {messages:?}"
+    );
+}
+
 /// `$this->` and `static::` inside a closure nested within a trait
 /// method should be suppressed just like direct trait method bodies.
 #[test]