flambda-backend: Produce Invalid upon mode mismatch in partial application (#2548)

Author: mshinwell

testsuite/tests/typing-local/partial_app_dead_gadt_case.ml

@@ -0,0 +1,27 @@
+(* TEST
+   {
+     stack-allocation;
+     ocamlopt_flags = "-Oclassic";
+   }{
+     stack-allocation;
+     ocamlopt_flags = "-O3";
+   }
+*)
+
+type (_,_) eq = Refl : ('a, 'a) eq
+
+let apply (type a) (equ : (a, unit -> unit) eq option) (f : a) =
+  match equ with
+  | None -> ()
+  | Some Refl -> f ()
+
+let fn (local_ x) (local_ y) = ()
+
+let bad p = apply p fn
+
+(* More minimal example: *)
+
+let cast (type a b) (Refl : (a, b) eq) (f : a) : b = f
+
+let fn (local_ x) (local_ y) = ()
+let bad2 p = cast p fn ()