Add test

testsuite/tests/tool-toplevel/pr10712.compilers.reference

@@ -0,0 +1,5 @@
+module A :
+  sig type ('foo, 'bar) t val get_foo : ('foo, 'a) t -> 'foo option end
+- : ('foo, 'a) A.t -> 'foo option = <fun>
+val _bar : ('a, 'b) A.t -> 'a option = <fun>
+

testsuite/tests/tool-toplevel/pr10712.ml

@@ -0,0 +1,27 @@
+(* TEST
+   * toplevel
+   * toplevel.opt
+*)
+
+module A : sig
+  type ('foo, 'bar) t
+
+  val get_foo : ('foo, _) t -> 'foo option
+end = struct
+  type ('foo, 'bar) t =
+    | Foo of 'foo
+    | Bar of 'bar
+
+  let get_foo = function
+    | Foo foo -> Some foo
+    | Bar _ -> None
+end
+;;
+
+(* Type variables should be 'foo and 'a (name persists) *)
+A.get_foo
+;;
+
+(* Type variables be 'a and 'b (original names lost in let-binding) *)
+let _bar = A.get_foo
+;;