3 files changed, 44 insertions, 3 deletions
diff --git a/compiler/rustc_trait_selection/src/traits/project.rs b/compiler/rustc_trait_selection/src/traits/project.rs
index c8276854016..ad216e5502d 100644
--- a/compiler/rustc_trait_selection/src/traits/project.rs
+++ b/compiler/rustc_trait_selection/src/traits/project.rs
@@ -2192,7 +2192,7 @@ fn confirm_impl_candidate<'cx, 'tcx>(
 // Verify that the trait item and its implementation have compatible substs lists
 fn check_substs_compatible<'tcx>(
     tcx: TyCtxt<'tcx>,
-    assoc_ty: &ty::AssocItem,
+    assoc_item: &ty::AssocItem,
     substs: ty::SubstsRef<'tcx>,
 ) -> bool {
     fn check_substs_compatible_inner<'tcx>(
@@ -2224,7 +2224,10 @@ fn check_substs_compatible<'tcx>(
         true
     }
 
-    check_substs_compatible_inner(tcx, tcx.generics_of(assoc_ty.def_id), substs.as_slice())
+    let generics = tcx.generics_of(assoc_item.def_id);
+    // Chop off any additional substs (RPITIT) substs
+    let substs = &substs[0..generics.count().min(substs.len())];
+    check_substs_compatible_inner(tcx, generics, substs)
 }
 
 fn confirm_impl_trait_in_trait_candidate<'tcx>(
@@ -2253,12 +2256,21 @@ fn confirm_impl_trait_in_trait_candidate<'tcx>(
         };
     }
 
-    let impl_fn_def_id = leaf_def.item.def_id;
     // Rebase from {trait}::{fn}::{opaque} to {impl}::{fn}::{opaque},
     // since `data.substs` are the impl substs.
     let impl_fn_substs =
         obligation.predicate.substs.rebase_onto(tcx, tcx.parent(trait_fn_def_id), data.substs);
 
+    if !check_substs_compatible(tcx, &leaf_def.item, impl_fn_substs) {
+        let err = tcx.ty_error_with_message(
+            obligation.cause.span,
+            "impl method and trait method have different parameters",
+        );
+        return Progress { term: err.into(), obligations };
+    }
+
+    let impl_fn_def_id = leaf_def.item.def_id;
+
     let cause = ObligationCause::new(
         obligation.cause.span,
         obligation.cause.body_id,
diff --git a/src/test/ui/impl-trait/in-trait/generics-mismatch.rs b/src/test/ui/impl-trait/in-trait/generics-mismatch.rs
new file mode 100644
index 00000000000..cc0fc720ebb
--- /dev/null
+++ b/src/test/ui/impl-trait/in-trait/generics-mismatch.rs
@@ -0,0 +1,17 @@
+#![feature(return_position_impl_trait_in_trait)]
+#![allow(incomplete_features)]
+
+struct U;
+
+trait Foo {
+    fn bar(&self) -> impl Sized;
+}
+
+impl Foo for U {
+    fn bar<T>(&self) {}
+    //~^ ERROR method `bar` has 1 type parameter but its trait declaration has 0 type parameters
+}
+
+fn main() {
+    U.bar();
+}
diff --git a/src/test/ui/impl-trait/in-trait/generics-mismatch.stderr b/src/test/ui/impl-trait/in-trait/generics-mismatch.stderr
new file mode 100644
index 00000000000..cd42683e022
--- /dev/null
+++ b/src/test/ui/impl-trait/in-trait/generics-mismatch.stderr
@@ -0,0 +1,12 @@
+error[E0049]: method `bar` has 1 type parameter but its trait declaration has 0 type parameters
+  --> $DIR/generics-mismatch.rs:11:12
+   |
+LL |     fn bar(&self) -> impl Sized;
+   |           - expected 0 type parameters
+...
+LL |     fn bar<T>(&self) {}
+   |            ^ found 1 type parameter
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0049`.