diff --git a/tests/ui/c-variadic/roundtrip.rs b/tests/ui/c-variadic/roundtrip.rs
new file mode 100644
index 000000000000..b6a858c715df
--- /dev/null
+++ b/tests/ui/c-variadic/roundtrip.rs
@@ -0,0 +1,78 @@
+//@ run-pass
+//@ ignore-backends: gcc
+#![feature(c_variadic, const_c_variadic, const_destruct, const_raw_ptr_comparison)]
+
+use std::ffi::*;
+
+// In rustc we implement `va_arg` for the callee reading from a VaList, but still rely on LLVM
+// for exactly how to pass c-variadic arguments and for constructing the VaList. Here we test
+// that the rustc implementation works with what LLVM gives us.
+
+#[allow(improper_ctypes_definitions)]
+const unsafe extern "C" fn variadic<T: VaArgSafe>(mut ap: ...) -> (T, T) {
+    let x = ap.arg::<T>();
+    // Intersperse a small type to test alignment logic. A `u32` (i.e. `c_uint`) is the smallest
+    // type that implements `VaArgSafe`: smaller types would automatically be promoted.
+    assert!(ap.arg::<u32>() == 0xAAAA_AAAA);
+    let y = ap.arg::<T>();
+
+    (x, y)
+}
+
+macro_rules! roundtrip {
+    ($ty:ty, $a:expr, $b:expr) => {
+        const {
+            let a: $ty = $a;
+            let b: $ty = $b;
+            let (x, y) = variadic::<$ty>(a, 0xAAAA_AAAAu32, b);
+            assert!(a == x);
+            assert!(b == y);
+        }
+
+        let a: $ty = $a;
+        let b: $ty = $b;
+        assert_eq!(variadic::<$ty>(a, 0xAAAA_AAAAu32, b), (a, b))
+    };
+}
+
+macro_rules! roundtrip_ptr {
+    ($ty:ty, $a:expr, $b:expr) => {
+        const {
+            let a: $ty = $a;
+            let b: $ty = $b;
+            let (x, y) = variadic::<$ty>(a, 0xAAAA_AAAAu32, b);
+            assert!(a.guaranteed_eq(x).unwrap());
+            assert!(b.guaranteed_eq(y).unwrap());
+        }
+
+        let a: $ty = $a;
+        let b: $ty = $b;
+        assert_eq!(variadic::<$ty>(a, 0xAAAA_AAAAu32, b), (a, b))
+    };
+}
+
+fn main() {
+    unsafe {
+        roundtrip!(i32, -1, -2);
+        roundtrip!(i64, -1, -2);
+        roundtrip!(isize, -1, -2);
+        roundtrip!(c_int, -1, -2);
+        roundtrip!(c_long, -1, -2);
+        roundtrip!(c_longlong, -1, -2);
+
+        roundtrip!(u32, 1, 2);
+        roundtrip!(u64, 1, 2);
+        roundtrip!(usize, 1, 2);
+        roundtrip!(c_uint, 1, 2);
+        roundtrip!(c_ulong, 1, 2);
+        roundtrip!(c_ulonglong, 1, 2);
+
+        roundtrip!(f64, 3.14, 6.28);
+        roundtrip!(c_double, 3.14, 6.28);
+
+        static mut A: u32 = 1u32;
+        static mut B: u32 = 2u32;
+        roundtrip_ptr!(*const u32, &raw const A, &raw const B);
+        roundtrip_ptr!(*mut u32, &raw mut A, &raw mut B);
+    }
+}