Test that we don't add a new kind of breaking change with TAITs

src/test/ui/type-alias-impl-trait/auxiliary/coherence_cross_crate_trait_decl.rs

@@ -0,0 +1,9 @@
+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 {}

src/test/ui/type-alias-impl-trait/coherence_cross_crate.rs

@@ -0,0 +1,24 @@
+// 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() {}

src/test/ui/type-alias-impl-trait/coherence_cross_crate.stderr

@@ -0,0 +1,13 @@
+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`.