--- /dev/null
+pub trait SomeTrait {}
+
+impl SomeTrait for () {}
+
+// Adding this `impl` would cause errors in this crate's dependent,
+// so it would be a breaking change. We explicitly don't add this impl,
+// as the dependent crate already assumes this impl exists and thus already
+// does not compile.
+//impl SomeTrait for i32 {}
--- /dev/null
+// aux-build: coherence_cross_crate_trait_decl.rs
+// This test ensures that adding an `impl SomeTrait for i32` within
+// `coherence_cross_crate_trait_decl` is not a breaking change, by
+// making sure that even without such an impl this test fails to compile.
+
+#![feature(type_alias_impl_trait)]
+
+extern crate coherence_cross_crate_trait_decl;
+
+use coherence_cross_crate_trait_decl::SomeTrait;
+
+trait OtherTrait {}
+
+type Alias = impl SomeTrait;
+
+fn constrain() -> Alias {
+ ()
+}
+
+impl OtherTrait for Alias {}
+impl OtherTrait for i32 {}
+//~^ ERROR: conflicting implementations of trait `OtherTrait` for type `Alias`
+
+fn main() {}
--- /dev/null
+error[E0119]: conflicting implementations of trait `OtherTrait` for type `Alias`
+ --> $DIR/coherence_cross_crate.rs:21:1
+ |
+LL | impl OtherTrait for Alias {}
+ | ------------------------- first implementation here
+LL | impl OtherTrait for i32 {}
+ | ^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `Alias`
+ |
+ = note: upstream crates may add a new impl of trait `coherence_cross_crate_trait_decl::SomeTrait` for type `i32` in future versions
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0119`.