type_alias_enum_variants: add regression test for #61801.

Author: Centril

src/test/ui/type-alias-enum-variants/issue-61801-path-pattern-can-infer.rs

@@ -0,0 +1,30 @@
+// In this regression test we check that a path pattern referring to a unit variant
+// through a type alias is successful in inferring the generic argument.
+
+// compile-pass
+
+#![feature(type_alias_enum_variants)]
+
+enum Opt<T> {
+    N,
+    S(T),
+}
+
+type OptAlias<T> = Opt<T>;
+
+fn f1(x: OptAlias<u8>) {
+    match x {
+        OptAlias::N // We previously failed to infer `T` to `u8`.
+            => (),
+        _ => (),
+    }
+
+    match x {
+        <
+            OptAlias<_> // And we failed to infer this type also.
+        >::N => (),
+        _ => (),
+    }
+}
+
+fn main() {}