Add a test

tests/testsuite/registry_overlay.rs

@@ -118,3 +118,44 @@ fn multiversion() {
         .overlay_registry(&reg.index_url(), &alt_path)
         .run();
 }
+
+// Test that we can overlay on top of alternate registries, not just crates-io.
+// Since the test framework only supports a single alternate registry, we repurpose
+// the dummy crates-io as the registry to overlay on top.
+#[cargo_test]
+fn alt_registry() {
+    let alt = RegistryBuilder::new().http_index().alternative().build();
+    let crates_io = RegistryBuilder::new().build();
+    let crates_io_path = crates_io
+        .index_url()
+        .to_file_path()
+        .unwrap()
+        .into_os_string()
+        .into_string()
+        .unwrap();
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.0.1"
+                edition = "2015"
+                authors = []
+
+                [dependencies]
+                bar = { version = ">= 0.0.0", registry = "alternative" }
+                baz = { version = ">= 0.1.0", registry = "alternative" }
+            "#,
+        )
+        .file("src/main.rs", "fn main() {}")
+        .build();
+
+    Package::new("bar", "0.0.1").local(true).publish();
+    Package::new("baz", "0.1.1").alternative(true).publish();
+
+    p.cargo("check")
+        .overlay_registry(&alt.index_url(), &crates_io_path)
+        .run();
+}