fix: resolve conditional return string branch for interpolated strings

The AST-based conditional return resolver only matched
Expression::Literal(Literal::String) when checking `$key is string`,
so interpolated strings like `"{$prefix}.host"` (parsed as
Expression::CompositeString) fell through to the else branch and
resolved as null. Add CompositeString to the string match so the
correct branch is taken.

Closes #269

Author: calebdw

docs/CHANGELOG.md

@@ -45,6 +45,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 
 - **Completion works after closing a multi-line closure argument on the same line as the next chain operator.** Typing `->` immediately after a call like `->map(function (...) { ... })->` now resolves the full receiver instead of returning no member suggestions until the operator is moved to a new line. Contributed by @calebdw.
+- **Conditional return types recognize interpolated strings as strings.** A function with a conditional return type like `($key is string ? mixed : null)` now correctly resolves the `string` branch when called with an interpolated string argument (e.g. `config("{$prefix}.host")`). Previously the interpolated string was not recognized as a string literal, causing the return type to fall through to the else branch and resolve as `null`. Contributed by @calebdw.
 - **Deprecated enum cases are recognized.** Enum cases annotated with `@deprecated` or `#[Deprecated]` now carry deprecation metadata, so usages like `self::Low` can be highlighted as deprecated in contextual semantic-token mode. Contributed by @calebdw.
 - **Class constant accesses use constant semantic highlighting.** `self::CONSTANT`, `static::CONSTANT`, `parent::CONSTANT`, and `ClassName::CONSTANT` now emit the `enumMember` semantic token instead of being colored as properties, while `ClassName::$property` still emits `property`. Contributed by @calebdw.
 - **PHP attributes use decorator semantic highlighting.** Attribute class names in `#[...]` now emit the `decorator` semantic token instead of `class`, so editor themes can color attributes differently from normal class references. Contributed by @calebdw.

src/type_engine/types/conditional.rs

@@ -1435,7 +1435,13 @@ pub fn resolve_conditional_with_args_and_defaults<'b>(
             } else {
                 // IsType equivalent: check scalar types from AST literals.
                 if condition_is_scalar_type(condition.as_ref(), "string")
-                    && matches!(arg_expr, Some(Expression::Literal(Literal::String(_))))
+                    && matches!(
+                        arg_expr,
+                        Some(
+                            Expression::Literal(Literal::String(_))
+                                | Expression::CompositeString(_)
+                        )
+                    )
                 {
                     let take_then = !*negated;
                     return resolve_conditional_with_args_and_defaults(

tests/integration/hover.rs

@@ -154,6 +154,37 @@ class Service {
     assert!(text.contains("Order"), "should resolve to Order: {}", text);
 }
 
+#[test]
+fn hover_conditional_return_with_interpolated_string_resolves_string_branch() {
+    let backend = create_test_backend();
+    let uri = "file:///test.php";
+    let content = r#"<?php
+/**
+ * @param string|null $key
+ * @return ($key is null ? \stdClass : ($key is string ? mixed : null))
+ */
+function config($key = null, $default = null) {}
+
+function test(string $cache): void {
+    $redisHost = config("{$cache}.host", 'localhost');
+
+    if ($redisHost === null) {
+        return;
+    }
+
+    $redisHost;
+}
+"#;
+
+    let hover = hover_at(&backend, uri, content, 14, 6).expect("expected hover on $redisHost");
+    let text = hover_text(&hover);
+    assert!(
+        text.contains("mixed"),
+        "interpolated string arg should resolve `$key is string` to the `mixed` branch, \
+         not fall through to `null`: {text}"
+    );
+}
+
 #[test]
 fn hover_variable_without_type() {
     let backend = create_test_backend();