diff --git a/compiler/rustc_borrowck/src/universal_regions.rs b/compiler/rustc_borrowck/src/universal_regions.rs
index a0d5e1ce780d7..fda4719fc3f5e 100644
--- a/compiler/rustc_borrowck/src/universal_regions.rs
+++ b/compiler/rustc_borrowck/src/universal_regions.rs
@@ -543,38 +543,9 @@ impl<'cx, 'tcx> UniversalRegionsBuilder<'cx, 'tcx> {
             &indices,
         );
 
-        let (unnormalized_output_ty, mut unnormalized_input_tys) =
+        let (unnormalized_output_ty, unnormalized_input_tys) =
             inputs_and_output.split_last().unwrap();
 
-        // C-variadic fns also have a `VaList` input that's not listed in the signature
-        // (as it's created inside the body itself, not passed in from outside).
-        if let DefiningTy::FnDef(def_id, _) = defining_ty {
-            if self.infcx.tcx.fn_sig(def_id).skip_binder().c_variadic() {
-                let va_list_did = self
-                    .infcx
-                    .tcx
-                    .require_lang_item(LangItem::VaList, self.infcx.tcx.def_span(self.mir_def));
-
-                let reg_vid = self
-                    .infcx
-                    .next_nll_region_var(NllRegionVariableOrigin::FreeRegion, || {
-                        RegionCtxt::Free(sym::c_dash_variadic)
-                    })
-                    .as_var();
-
-                let region = ty::Region::new_var(self.infcx.tcx, reg_vid);
-                let va_list_ty = self
-                    .infcx
-                    .tcx
-                    .type_of(va_list_did)
-                    .instantiate(self.infcx.tcx, &[region.into()]);
-
-                unnormalized_input_tys = self.infcx.tcx.mk_type_list_from_iter(
-                    unnormalized_input_tys.iter().copied().chain(iter::once(va_list_ty)),
-                );
-            }
-        }
-
         let fr_fn_body = self
             .infcx
             .next_nll_region_var(NllRegionVariableOrigin::FreeRegion, || {
@@ -816,7 +787,40 @@ impl<'cx, 'tcx> UniversalRegionsBuilder<'cx, 'tcx> {
             DefiningTy::FnDef(def_id, _) => {
                 let sig = tcx.fn_sig(def_id).instantiate_identity();
                 let sig = indices.fold_to_region_vids(tcx, sig);
-                sig.inputs_and_output()
+                let inputs_and_output = sig.inputs_and_output();
+
+                // C-variadic fns also have a `VaList` input that's not listed in the signature
+                // (as it's created inside the body itself, not passed in from outside).
+                if self.infcx.tcx.fn_sig(def_id).skip_binder().c_variadic() {
+                    let va_list_did = self
+                        .infcx
+                        .tcx
+                        .require_lang_item(LangItem::VaList, self.infcx.tcx.def_span(self.mir_def));
+
+                    let reg_vid = self
+                        .infcx
+                        .next_nll_region_var(NllRegionVariableOrigin::FreeRegion, || {
+                            RegionCtxt::Free(sym::c_dash_variadic)
+                        })
+                        .as_var();
+
+                    let region = ty::Region::new_var(self.infcx.tcx, reg_vid);
+                    let va_list_ty = self
+                        .infcx
+                        .tcx
+                        .type_of(va_list_did)
+                        .instantiate(self.infcx.tcx, &[region.into()]);
+
+                    // The signature needs to follow the order [input_tys, va_list_ty, output_ty]
+                    return inputs_and_output.map_bound(|tys| {
+                        let (output_ty, input_tys) = tys.split_last().unwrap();
+                        tcx.mk_type_list_from_iter(
+                            input_tys.iter().copied().chain([va_list_ty, *output_ty]),
+                        )
+                    });
+                }
+
+                inputs_and_output
             }
 
             DefiningTy::Const(def_id, _) => {
diff --git a/tests/ui/c-variadic/variadic-ffi-4.stderr b/tests/ui/c-variadic/variadic-ffi-4.stderr
index a230bb6f5861e..01ace5c796800 100644
--- a/tests/ui/c-variadic/variadic-ffi-4.stderr
+++ b/tests/ui/c-variadic/variadic-ffi-4.stderr
@@ -30,9 +30,9 @@ error: lifetime may not live long enough
   --> $DIR/variadic-ffi-4.rs:22:5
    |
 LL | pub unsafe extern "C" fn no_escape4(_: usize, mut ap0: &mut VaList, mut ap1: ...) {
-   |                                               -------               ------- has type `VaList<'2>`
+   |                                               -------               ------- has type `VaList<'1>`
    |                                               |
-   |                                               has type `&mut VaList<'1>`
+   |                                               has type `&mut VaList<'2>`
 LL |     ap0 = &mut ap1;
    |     ^^^^^^^^^^^^^^ assignment requires that `'1` must outlive `'2`
    |
@@ -44,9 +44,9 @@ error: lifetime may not live long enough
   --> $DIR/variadic-ffi-4.rs:22:5
    |
 LL | pub unsafe extern "C" fn no_escape4(_: usize, mut ap0: &mut VaList, mut ap1: ...) {
-   |                                               -------               ------- has type `VaList<'2>`
+   |                                               -------               ------- has type `VaList<'1>`
    |                                               |
-   |                                               has type `&mut VaList<'1>`
+   |                                               has type `&mut VaList<'2>`
 LL |     ap0 = &mut ap1;
    |     ^^^^^^^^^^^^^^ assignment requires that `'2` must outlive `'1`
    |