Use equality when relating formal and expected type in arg checking

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -291,21 +291,20 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
             let coerce_error =
                 self.coerce(provided_arg, checked_ty, coerced_ty, AllowTwoPhase::Yes, None).err();
-
             if coerce_error.is_some() {
                 return Compatibility::Incompatible(coerce_error);
             }
 
-            // 3. Check if the formal type is a supertype of the checked one
-            //    and register any such obligations for future type checks
-            let supertype_error = self.at(&self.misc(provided_arg.span), self.param_env).sup(
+            // 3. Check if the formal type is actually equal to the checked one
+            //    and register any such obligations for future type checks.
+            let formal_ty_error = self.at(&self.misc(provided_arg.span), self.param_env).eq(
                 DefineOpaqueTypes::Yes,
                 formal_input_ty,
                 coerced_ty,
             );
 
             // If neither check failed, the types are compatible
-            match supertype_error {
+            match formal_ty_error {
                 Ok(InferOk { obligations, value: () }) => {
                     self.register_predicates(obligations);
                     Compatibility::Compatible

tests/ui/coercion/constrain-expectation-in-arg.rs

@@ -0,0 +1,15 @@
+trait Trait {
+    type Item;
+}
+
+struct Struct<A: Trait<Item = B>, B> {
+    pub field: A,
+}
+
+fn identity<T>(x: T) -> T {
+    x
+}
+
+fn test<A: Trait<Item = B>, B>(x: &Struct<A, B>) {
+    let x: &Struct<_, _> = identity(x);
+}