fix fn_sig ice

2 files changed, 13 insertions, 2 deletions

diff --git a/compiler/rustc_const_eval/src/transform/check_consts/check.rs b/compiler/rustc_const_eval/src/transform/check_consts/check.rs
index 38212496ec8..d4c75cd55ce 100644
--- a/compiler/rustc_const_eval/src/transform/check_consts/check.rs
+++ b/compiler/rustc_const_eval/src/transform/check_consts/check.rs
@@ -242,7 +242,7 @@ impl<'mir, 'tcx> Checker<'mir, 'tcx> {
             // impl trait is gone in MIR, so check the return type of a const fn by its signature
             // instead of the type of the return place.
             self.span = body.local_decls[RETURN_PLACE].source_info.span;
-            let return_ty = tcx.fn_sig(def_id).output();
+            let return_ty = self.ccx.fn_sig().output();
             self.check_local_or_return_ty(return_ty.skip_binder(), RETURN_PLACE);
         }
 
diff --git a/compiler/rustc_const_eval/src/transform/check_consts/mod.rs b/compiler/rustc_const_eval/src/transform/check_consts/mod.rs
index 0a90572d39e..b528bc92330 100644
--- a/compiler/rustc_const_eval/src/transform/check_consts/mod.rs
+++ b/compiler/rustc_const_eval/src/transform/check_consts/mod.rs
@@ -8,7 +8,7 @@ use rustc_attr as attr;
 use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_middle::mir;
-use rustc_middle::ty::{self, TyCtxt};
+use rustc_middle::ty::{self, TyCtxt, PolyFnSig};
 use rustc_span::Symbol;
 
 pub use self::qualifs::Qualif;
@@ -64,6 +64,17 @@ impl<'mir, 'tcx> ConstCx<'mir, 'tcx> {
     fn is_async(&self) -> bool {
         self.tcx.asyncness(self.def_id()).is_async()
     }
+
+    pub fn fn_sig(&self) -> PolyFnSig<'tcx> {
+        let did = self.def_id().to_def_id();
+        if self.tcx.is_closure(did) {
+            let ty = self.tcx.type_of(did);
+            let ty::Closure(_, substs) = ty.kind() else { bug!("type_of closure not ty::Closure") };
+            substs.as_closure().sig()
+        } else {
+            self.tcx.fn_sig(did)
+        }
+    }
 }
 
 pub fn rustc_allow_const_fn_unstable(