bindgen panicked with message "Not an item: ItemId(...)" when parsing recursive union templates (breaks recent LLVM libc++ std::aligned_union) #3397

We noticed that bindgen is panicing on our code base after we rolled our clang toolchain with libc++. We bisect the LLVM and found out the panic starts to appear after llvm/llvm-project#185449 was landed. This PR revised the aligned_union with a recursive union template.

We created a small reproducer:

• Create a header file:

// test_union.h
template <class A0, class... As> union RUnion { A0 arg; RUnion<As...> u; };
template <class A> union RUnion<A> { A arg; };
struct Wrap { RUnion<int, float> u; };

• Run bindgen test_union.h -- -x c++ -std=c++20

Expected Behavior

bindgen should successfully generate FFI bindings (or gracefully treat the recursive template union as opaque/unsupported) without crashing or panicking.

Actual Behavior

bindgen immediately crashes with the following panic:

    panicked at bindgen/ir/context.rs:1495:21:
    Not an item: ItemId(1)

The bindgen we use is 9372688 .

Root Cause Analysis

The panic occurs due to an interaction between self-referential / cyclic type references in the AST and bindgen's temporary item borrowing mechanism (with_loaned_item):

1. Self-Referential Type Reference: When bindgen parses union RUnion with member RUnion<As...> u, field u is represented as an UnresolvedTypeRef whose target type ID resolves back to union RUnion's own ItemId.
2. IR Traversal Loaning (with_loaned_item): During post-parsing IR analysis and modification passes (such as compute_bitfield_units or deanonymize_fields in bindgen/ir/context.rs / bindgen/ir/comp.rs),
   bindgen mutably borrows items from BindgenContext using with_loaned_item:

    fn with_loaned_item<F, T>(&mut self, id: ItemId, f: F) -> T {
        let mut item = self.items[id.0].take().unwrap(); // Temporarily removes Item from self.items
        // Executes closure on &mut item...
    }

3. The Panic: When with_loaned_item runs on union RUnion (ItemId(1)), ItemId(1) is temporarily removed from self.items (self.items[1] = None). As the pass inspects the members of union RUnion, it traverses
   field u and attempts to resolve its target type (union RUnion's ItemId(1)) by calling ctx.resolve_item(ItemId(1)). Because ItemId(1) is currently loaned out, self.items[1] is None, causing resolve_item to panic with Not an item: ItemId(1).