Detect and avoid cycles when resolving items.

These can happen in certain cases involving incomplete qualified dependent
types. To avoid looping forever, we need to check for them.

Closes #2085.

Author: pcwalton

src/ir/context.rs

@@ -2723,8 +2723,16 @@ impl ItemResolver {
         assert!(ctx.collected_typerefs());
 
         let mut id = self.id;
+        let mut seen_ids = HashSet::default();
         loop {
             let item = ctx.resolve_item(id);
+
+            // Detect cycles and bail out. These can happen in certain cases
+            // involving incomplete qualified dependent types (#2085).
+            if !seen_ids.insert(id) {
+                return item;
+            }
+
             let ty_kind = item.as_type().map(|t| t.kind());
             match ty_kind {
                 Some(&TypeKind::ResolvedTypeRef(next_id))

tests/expectations/tests/qualified-dependent-types.rs

@@ -0,0 +1,17 @@
+#![allow(
+    dead_code,
+    non_snake_case,
+    non_camel_case_types,
+    non_upper_case_globals
+)]
+
+#[repr(C)]
+#[derive(Debug, Default, Copy, Clone)]
+pub struct Foo {
+    pub _address: u8,
+}
+#[repr(C)]
+#[derive(Debug, Default, Copy, Clone)]
+pub struct Bar {
+    pub _address: u8,
+}

tests/headers/qualified-dependent-types.hpp

@@ -0,0 +1,16 @@
+// Issue #2085.
+
+template<typename T>
+struct Foo;
+
+template<typename T, typename U>
+struct Bar {};
+
+template<typename T>
+struct Bar<T, void> {
+    using BarDependent = typename Foo<T>::Dependent;
+    void method(const BarDependent &);
+};
+
+template<typename T>
+void Bar<T, void>::method(const BarDependent &) {}