regression test for #103626

Author: kadiwa4

tests/ui/traits/object/issue-103626.rs

@@ -0,0 +1,15 @@
+trait FromResidual<R = <Self as Try>::Residual> {
+    fn from_residual(residual: R) -> Self;
+}
+
+trait Try {
+    type Residual;
+}
+
+fn w<'a, T: 'a, F: Fn(&'a T)>() {
+    let b: &dyn FromResidual = &();
+    //~^ ERROR: the trait `FromResidual` cannot be made into an object
+    //~| ERROR: the trait `FromResidual` cannot be made into an object
+}
+
+fn main() {}

tests/ui/traits/object/issue-103626.stderr

@@ -0,0 +1,33 @@
+error[E0038]: the trait `FromResidual` cannot be made into an object
+  --> $DIR/issue-103626.rs:10:17
+   |
+LL |     let b: &dyn FromResidual = &();
+   |                 ^^^^^^^^^^^^
+   |
+   = note: it cannot use `Self` as a type parameter in a supertrait or `where`-clause
+
+error[E0038]: the trait `FromResidual` cannot be made into an object
+  --> $DIR/issue-103626.rs:10:12
+   |
+LL |     let b: &dyn FromResidual = &();
+   |            ^^^^^^^^^^^^^^^^^ `FromResidual` cannot be made into an object
+   |
+note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
+  --> $DIR/issue-103626.rs:2:8
+   |
+LL | trait FromResidual<R = <Self as Try>::Residual> {
+   |       ------------ this trait cannot be made into an object...
+LL |     fn from_residual(residual: R) -> Self;
+   |        ^^^^^^^^^^^^^ ...because associated function `from_residual` has no `self` parameter
+help: consider turning `from_residual` into a method by giving it a `&self` argument
+   |
+LL |     fn from_residual(&self, residual: R) -> Self;
+   |                      ++++++
+help: alternatively, consider constraining `from_residual` so it does not apply to trait objects
+   |
+LL |     fn from_residual(residual: R) -> Self where Self: Sized;
+   |                                           +++++++++++++++++
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0038`.