From d6e6a5dc0c91a95bf46afe4adfa6f59f3b43ed3d Mon Sep 17 00:00:00 2001
From: Joseph Ferano <joseph@ferano.io>
Date: Fri, 25 Sep 2026 17:12:05 +0700
Subject: [PATCH] A program's global named as a prelude function or global
 shadows it for the file, with the shadowing warning a defn gets

---
 lib/check.ml                             | 23 +++++++++++++++--------
 test/programs/shadow-prelude-global.flan | 14 ++++++++++++++
 test/test_acceptance.ml                  |  5 +++++
 test/test_flan.ml                        | 14 ++++++++++++++
 4 files changed, 48 insertions(+), 8 deletions(-)
 create mode 100644 test/programs/shadow-prelude-global.flan

diff --git a/lib/check.ml b/lib/check.ml
index 616992ec..d2193bb4 100644
--- a/lib/check.ml
+++ b/lib/check.ml
@@ -14543,30 +14543,37 @@ let shown_name n =
   else n
 
 let shadow_prelude (prelude : Ast.decl list) (decls : Ast.decl list) =
-  let fn_name (d : Ast.decl) =
+  (* A value name, and whether it is a function's. A program's global takes
+     a prelude function's name over as a program's function does: both are
+     names a call or a read reaches, and the prelude's own uses keep the
+     prelude's. *)
+  let value_name (d : Ast.decl) =
     match d.Ast.d with
-    | Ast.Defn fn | Ast.Declare (fn, _) | Ast.DeclareC (fn, _) -> Some fn.Ast.name
+    | Ast.Defn fn | Ast.Declare (fn, _) | Ast.DeclareC (fn, _) ->
+      Some (fn.Ast.name, true)
+    | Ast.Defvar (n, _, _, _) | Ast.Defconst (n, _, _) -> Some (n, false)
     | _ -> None
   in
-  let theirs = List.filter_map fn_name prelude in
+  let theirs = List.map fst (List.filter_map value_name prelude) in
   let taken =
     List.filter_map
       (fun (d : Ast.decl) ->
-         match fn_name d with
-         | Some n when List.mem n theirs -> Some (n, d.Ast.dloc)
+         match value_name d with
+         | Some (n, f) when List.mem n theirs -> Some (n, d.Ast.dloc, f)
          | _ -> None)
       decls
   in
   let warnings =
     List.map
-      (fun (n, at) ->
+      (fun (n, at, f) ->
          Loc.diag ~kind:"check/shadows-prelude" at
            (Printf.sprintf
-              "%s shadows the prelude's %s — every call in this file now \
+              "%s shadows the prelude's %s — every %s in this file now \
                reaches your definition"
-              n n))
+              n n (if f then "call" else "use")))
       taken
   in
+  let taken = List.map (fun (n, at, _) -> (n, at)) taken in
   let prelude, decls =
     List.fold_left
       (fun (prelude, decls) (n, (at : Loc.t)) ->
diff --git a/test/programs/shadow-prelude-global.flan b/test/programs/shadow-prelude-global.flan
new file mode 100644
index 00000000..9cd578b7
--- /dev/null
+++ b/test/programs/shadow-prelude-global.flan
@@ -0,0 +1,14 @@
+;;;; A program's global named as a prelude function takes the name over for
+;;;; its own file, as a program's function does, and the prelude's own calls
+;;;; keep the prelude's: sort still swaps with the prelude's swap.
+(defonce swap i32 3)
+(defonce clamp i32 4)
+(defconst reverse i32 5)
+
+(defn main [] i32
+  (println (+ swap clamp reverse))               ; 12
+  (let [xs [3 1 2]]
+    (sort (slice xs))
+    (println (at xs 0))                          ; 1
+    (println (at xs 2)))                         ; 3
+  0)
diff --git a/test/test_acceptance.ml b/test/test_acceptance.ml
index 214f522e..4080d400 100644
--- a/test/test_acceptance.ml
+++ b/test/test_acceptance.ml
@@ -592,6 +592,11 @@ let () =
     outputs "a prelude function shadowed" "programs/shadow-prelude.flan" sp_out;
     outputs ~x86:true "a prelude function shadowed, x86"
       "programs/shadow-prelude.flan" sp_out;
+    (* And by a program's global, the same way. *)
+    outputs "a prelude function shadowed by a global"
+      "programs/shadow-prelude-global.flan" "12\n1\n3\n";
+    outputs ~x86:true "a prelude function shadowed by a global, x86"
+      "programs/shadow-prelude-global.flan" "12\n1\n3\n";
     (* (max-value T) and (min-value T), concrete and inside a generic. *)
     let maxof_out =
       "255\n0\n127\n-128\n2147483647\n-9223372036854775808\n\
diff --git a/test/test_flan.ml b/test/test_flan.ml
index fa7c57ab..836e7166 100644
--- a/test/test_flan.ml
+++ b/test/test_flan.ml
@@ -5953,6 +5953,20 @@ let () =
    | _ -> check "a defn of a prelude function's name warns exactly once" false);
   accepts "a defn of a prelude function's name is not defined twice"
     prelude_src;
+  (* A global takes the name over the same way, with the same warning. *)
+  (match
+     snd (Check.shadow_prelude (Parse.program (Prelude.forms ()))
+            (program "(defonce swap i32 3)"))
+   with
+   | [ d ] ->
+     check "a global of a prelude function's name warns once"
+       (d.Loc.kind = "check/shadows-prelude"
+        && d.Loc.dmsg
+           = "swap shadows the prelude's swap — every use in this file now \
+              reaches your definition")
+   | _ -> check "a global of a prelude function's name warns exactly once" false);
+  accepts "a global of a prelude function's name is not defined twice"
+    "(defonce swap i32 3)\n(defn f [] i32 swap)";
   rejects_check "a struct of a prelude type's name is still defined twice"
     "(defstruct Form [x i32])" ~needle:"Form is defined twice";
   (* An operator is a builtin like any other and shadows like any other.