Add regression test for #152415

tests/ui/traits/next-solver/unconstrained-const-param-supertrait-projection-152415.rs

@@ -0,0 +1,22 @@
+//! Reporting the unconstrained const param error used to hit
+//! `assertion failed: !origin.span.is_dummy()` with the new solver.
+//!
+//! Regression test for <https://github.com/rust-lang/rust/issues/152415>.
+//@ compile-flags: -Znext-solver
+
+trait Super<T> {
+    type Assoc;
+}
+
+trait Mirror {
+    type Assoc;
+}
+
+impl<T, const N: usize> Mirror for T {
+    //~^ ERROR the const parameter `N` is not constrained by the impl trait, self type, or predicates
+    type Assoc = T;
+}
+
+trait Foo<A>: Super<<A as Mirror>::Assoc, Assoc = A> {}
+
+fn main() {}

tests/ui/traits/next-solver/unconstrained-const-param-supertrait-projection-152415.stderr

@@ -0,0 +1,12 @@
+error[E0207]: the const parameter `N` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/unconstrained-const-param-supertrait-projection-152415.rs:15:9
+   |
+LL | impl<T, const N: usize> Mirror for T {
+   |         ^^^^^^^^^^^^^^ unconstrained const parameter
+   |
+   = note: expressions using a const parameter must map each value to a distinct output value
+   = note: proving the result of expressions other than the parameter are unique is not supported
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0207`.