From b71e73bfb58a7c70d6e1561b258b4f648246a374 Mon Sep 17 00:00:00 2001
From: Patrick Walton <pcwalton@fb.com>
Date: Fri, 30 Jul 2021 17:25:08 -0700
Subject: 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.
---
 src/ir/context.rs                                     |  8 ++++++++
 tests/expectations/tests/qualified-dependent-types.rs | 17 +++++++++++++++++
 tests/headers/qualified-dependent-types.hpp           | 16 ++++++++++++++++
 3 files changed, 41 insertions(+)
 create mode 100644 tests/expectations/tests/qualified-dependent-types.rs
 create mode 100644 tests/headers/qualified-dependent-types.hpp

diff --git a/src/ir/context.rs b/src/ir/context.rs
index bd21058c..44df063a 100644
--- a/src/ir/context.rs
+++ b/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))
diff --git a/tests/expectations/tests/qualified-dependent-types.rs b/tests/expectations/tests/qualified-dependent-types.rs
new file mode 100644
index 00000000..f1b2c845
--- /dev/null
+++ b/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,
+}
diff --git a/tests/headers/qualified-dependent-types.hpp b/tests/headers/qualified-dependent-types.hpp
new file mode 100644
index 00000000..fcdfc87c
--- /dev/null
+++ b/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 &) {}
-- 
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