From fa1b7f12bdcf9fef69f16287a39f68261071babd Mon Sep 17 00:00:00 2001
From: mununki <woonki.moon@gmail.com>
Date: Wed, 4 Nov 2026 14:00:54 +0900
Subject: [PATCH] Reject coercions and variant spreads that treat bool and unit
 as variants
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Why:
    `bool` and `unit` are declared as variants, with the constructors
    `false`, `true` and `()` and no `@as`, but their values are `false`,
    `true` and `undefined`. The coercion between a variant and a primitive
    left out `bool` but took `unit` for a variant whose constructor is the
    string "()", so `(() :> string)` compiled to nothing and left
    `undefined` typed as a `string`. The coercion from a closed polymorphic
    variant left out neither type and matched `#"true"` to the constructor
    `true` by name. A variant that spread either type built `true`, `false`
    and `undefined`, but matched and coerced them as strings.

`Variant_coercion.is_bool_or_unit` now keeps both types out of the
coercions between a variant and a primitive, the coercion from a
polymorphic variant (`type_is_variant`, renamed
`can_try_coerce_polyvariant_to_variant`) and variant spreads.
`Ctype.subtype_rec` also rejects a coercion between either type and another
variant or record before the variant checks, whose messages described them
as variants.

Diagnostic added (unit coerced to string):
Example:
    let s: string = (() :> string)
    let n = s->String.length
Before (js): throws a TypeError when the module loads
    let n = undefined.length;

    let s;
After (diagnostic):
    A.res:1:18-29
    Type unit is not a subtype of string

Diagnostic added (polymorphic variant coerced to bool):
Example:
    let toBool = (p: [#"true" | #"false"]): bool => (p :> bool)
    let isFalse = toBool(#"false") == false
Before (js):
    function toBool(p) {
      return p;
    }

    let isFalse = "false" === false;
After (diagnostic):
    A.res:1:50-58
    Type [#"false" | #"true"] is not a subtype of bool

Diagnostic added (bool spread into a variant):
Example:
    type t = | ...bool | Other
    let x: t = true
    let r = switch x { | true => "yes" | false => "no" | Other => "other" }
Before (js): `r` stays undefined
    switch (true) {
      case "false" :
        r = "no";
        break;
      case "true" :
        r = "yes";
        break;
      …
    }
After (diagnostic):
    A.res:1:15-18
    This type is not a valid type to spread. It's only possible to spread other variants.

Diagnostic modified (unit coerced to int):
Example:
    let i: int = (() :> int)
Before (diagnostic):
    Type unit is not a subtype of int

    The constructor () of variant unit has no @as payload, which makes it a string at runtime.
    That runtime representation is not compatible with the expected runtime representation of int.

    Fix this by making sure all constructors in variant unit has a runtime representation of int.
After (diagnostic):
    Type unit is not a subtype of int

Diagnostic modified (coercion suggested for unit):
Example:
    let u = (): unit => ()
    let s: string = u()
Before (diagnostic):
    This has type: unit
    But it's expected to have type: string

    Possible solutions:
    - These types are compatible at runtime. You can use the coercion operator to convert to the expected type: (u() :> string)
After (diagnostic):
    This has type: unit
    But it's expected to have type: string

Review: Does a working program stop compiling? One that coerces `unit` to
    `string` or a polymorphic variant to `bool` or `unit`, which gave a
    value of another type than its own, or that spreads `bool` or `unit`
    into a variant, whose switches never matched `true`, `false` or `()`.
    A variant that only used its other constructors can declare them
    without the spread.

Review: Which coercions still work? `bool` and `unit` coerce to themselves,
    from their aliases and from a private type defined as one, and a
    variant `False | True` still coerces to `string`. `option`, `list` and
    `result` have type parameters, so these coercions never applied to
    them. No existing snapshot or test output changes.

Signed-off-by: mununki <woonki.moon@gmail.com>
---
 changes/bool-unit-coercion-errors.polish.md       |  1 +
 changes/reject-bool-unit-variant-coercion.fix.md  |  2 ++
 compiler/ml/ctype.ml                              |  8 +++++++-
 compiler/ml/variant_coercion.ml                   | 15 ++++++++++-----
 compiler/ml/variant_type_spread.ml                |  6 +++++-
 .../coercion_hint_unit_to_string.res.expected     | 10 ++++++++++
 .../coercion_polyvariant_to_bool.res.expected     |  9 +++++++++
 .../coercion_polyvariant_to_unit.res.expected     |  9 +++++++++
 .../expected/coercion_string_to_unit.res.expected |  8 ++++++++
 .../expected/coercion_unit_to_int.res.expected    |  8 ++++++++
 .../expected/coercion_unit_to_string.res.expected |  8 ++++++++
 .../coercion_unit_to_unknown.res.expected         |  8 ++++++++
 .../expected/variant_spread_bool.res.expected     | 10 ++++++++++
 .../expected/variant_spread_unit.res.expected     | 10 ++++++++++
 .../fixtures/coercion_hint_unit_to_string.res     |  2 ++
 .../fixtures/coercion_polyvariant_to_bool.res     |  2 ++
 .../fixtures/coercion_polyvariant_to_unit.res     |  2 ++
 .../fixtures/coercion_string_to_unit.res          |  1 +
 .../fixtures/coercion_unit_to_int.res             |  1 +
 .../fixtures/coercion_unit_to_string.res          |  1 +
 .../fixtures/coercion_unit_to_unknown.res         |  1 +
 .../super_errors/fixtures/variant_spread_bool.res |  3 +++
 .../super_errors/fixtures/variant_spread_unit.res |  3 +++
 tests/tests/src/VariantCoercion.mjs               |  7 +++++++
 tests/tests/src/VariantCoercion.res               | 12 ++++++++++++
 25 files changed, 140 insertions(+), 7 deletions(-)
 create mode 100644 changes/bool-unit-coercion-errors.polish.md
 create mode 100644 changes/reject-bool-unit-variant-coercion.fix.md
 create mode 100644 tests/build_tests/super_errors/expected/coercion_hint_unit_to_string.res.expected
 create mode 100644 tests/build_tests/super_errors/expected/coercion_polyvariant_to_bool.res.expected
 create mode 100644 tests/build_tests/super_errors/expected/coercion_polyvariant_to_unit.res.expected
 create mode 100644 tests/build_tests/super_errors/expected/coercion_string_to_unit.res.expected
 create mode 100644 tests/build_tests/super_errors/expected/coercion_unit_to_int.res.expected
 create mode 100644 tests/build_tests/super_errors/expected/coercion_unit_to_string.res.expected
 create mode 100644 tests/build_tests/super_errors/expected/coercion_unit_to_unknown.res.expected
 create mode 100644 tests/build_tests/super_errors/expected/variant_spread_bool.res.expected
 create mode 100644 tests/build_tests/super_errors/expected/variant_spread_unit.res.expected
 create mode 100644 tests/build_tests/super_errors/fixtures/coercion_hint_unit_to_string.res
 create mode 100644 tests/build_tests/super_errors/fixtures/coercion_polyvariant_to_bool.res
 create mode 100644 tests/build_tests/super_errors/fixtures/coercion_polyvariant_to_unit.res
 create mode 100644 tests/build_tests/super_errors/fixtures/coercion_string_to_unit.res
 create mode 100644 tests/build_tests/super_errors/fixtures/coercion_unit_to_int.res
 create mode 100644 tests/build_tests/super_errors/fixtures/coercion_unit_to_string.res
 create mode 100644 tests/build_tests/super_errors/fixtures/coercion_unit_to_unknown.res
 create mode 100644 tests/build_tests/super_errors/fixtures/variant_spread_bool.res
 create mode 100644 tests/build_tests/super_errors/fixtures/variant_spread_unit.res

diff --git a/changes/bool-unit-coercion-errors.polish.md b/changes/bool-unit-coercion-errors.polish.md
new file mode 100644
index 00000000000..6611d5d0262
--- /dev/null
+++ b/changes/bool-unit-coercion-errors.polish.md
@@ -0,0 +1 @@
+- Coercion errors no longer describe `bool` and `unit` as variants that could be given `@as` or `@unboxed`.
diff --git a/changes/reject-bool-unit-variant-coercion.fix.md b/changes/reject-bool-unit-variant-coercion.fix.md
new file mode 100644
index 00000000000..368a2b0a50a
--- /dev/null
+++ b/changes/reject-bool-unit-variant-coercion.fix.md
@@ -0,0 +1,2 @@
+- Reject coercing `unit` to `string`, as in `(() :> string)`, and a closed polymorphic variant to `bool` or `unit`, as from `[#"true" | #"false"]` to `bool`. These compiled, but left `undefined` typed as a `string`, or a string typed as a `bool` or `unit`, and type errors suggested them as fixes.
+- Reject spreading `bool` or `unit` into a variant, as in `type t = | ...bool | Other`. The variant held `true`, `false` and `undefined`, but its switches and coercions took them for the strings `"true"`, `"false"` and `"()"`.
diff --git a/compiler/ml/ctype.ml b/compiler/ml/ctype.ml
index 4116d748fe1..ed1f1b21220 100644
--- a/compiler/ml/ctype.ml
+++ b/compiler/ml/ctype.ml
@@ -3698,6 +3698,12 @@ let rec subtype_rec env trace t1 t2 cstrs =
         match
           (extract_concrete_typedecl env t1, extract_concrete_typedecl env t2)
         with
+        | (_, p1, _), (_, p2, _)
+          when Variant_coercion.is_bool_or_unit p1
+               || Variant_coercion.is_bool_or_unit p2 ->
+          (* No other type coerces to or from [bool] or [unit], and the
+             variant errors below would describe them as variants *)
+          (trace, t1, t2, !(current_univar_pairs ()), None) :: cstrs
         | ( ( p1,
               _,
               {
@@ -3918,7 +3924,7 @@ let rec subtype_rec env trace t1 t2 cstrs =
           (trace, t1, t2, !(current_univar_pairs ()), None) :: cstrs)
       | Tvariant {row_closed = true; row_fields}, Tconstr (_, [], _)
         when extract_concrete_typedecl_opt env t2
-             |> Variant_coercion.type_is_variant -> (
+             |> Variant_coercion.can_try_coerce_polyvariant_to_variant -> (
         (* TODO(subtype-errors) Polyvariant to variant *)
         match extract_concrete_typedecl env t2 with
         | ( _,
diff --git a/compiler/ml/variant_coercion.ml b/compiler/ml/variant_coercion.ml
index 11e90a6e71a..314fa791dc6 100644
--- a/compiler/ml/variant_coercion.ml
+++ b/compiler/ml/variant_coercion.ml
@@ -19,6 +19,12 @@ type variant_runtime_representation_issue =
       expected_typename: Path.t;
     }
 
+(* [bool] and [unit] are declared as variants, but their values are [true],
+   [false] and [undefined] at runtime, not what the constructors of a declared
+   variant compile to. No variant coercion or spread applies to them. *)
+let is_bool_or_unit (path : Path.t) =
+  Path.same path Predef.path_bool || Path.same path Predef.path_unit
+
 (* Right now we only allow coercing to primitives string/int/float *)
 let can_coerce_primitive (path : Path.t) =
   Path.same path Predef.path_string
@@ -122,9 +128,7 @@ let can_try_coerce_variant_to_primitive
    type_params = [];
    type_representation;
   }
-    when not (Path.same p Predef.path_bool) ->
-    (* bool is represented as a variant internally, so we need to account for that *)
-    (* TODO(subtype-errors) Report about bool? *)
+    when not (is_bool_or_unit p) ->
     let layout = Variant_runtime.get_layout layout_ref in
     Some
       ( p,
@@ -268,7 +272,8 @@ let can_coerce_polyvariant_to_variant ~row_fields ~variant_constructors ~unboxed
   then Ok ()
   else Error `Unknown
 
-let type_is_variant (typ : (Path.t * Path.t * Types.type_declaration) option) =
+let can_try_coerce_polyvariant_to_variant
+    (typ : (Path.t * Path.t * Types.type_declaration) option) =
   match typ with
-  | Some (_, _, {type_kind = Type_variant _; _}) -> true
+  | Some (_, p, {type_kind = Type_variant _}) -> not (is_bool_or_unit p)
   | _ -> false
diff --git a/compiler/ml/variant_type_spread.ml b/compiler/ml/variant_type_spread.ml
index a1524ac7958..446f5c4f47f 100644
--- a/compiler/ml/variant_type_spread.ml
+++ b/compiler/ml/variant_type_spread.ml
@@ -36,7 +36,7 @@ let map_constructors ~(sdecl : Parsetree.type_declaration) ~all_constructors env
    pcd_args = Pcstr_tuple [{ptyp_loc; ptyp_desc = Ptyp_constr (loc, _)}];
   } -> (
     (* This is a variant type spread constructor. Look up its type *)
-    let _, type_decl =
+    let path, type_decl =
       try Typetexp.find_type env ptyp_loc loc.txt
       with _ -> raise (VariantTypeSpreadError (loc.loc, CouldNotFindType))
     in
@@ -44,6 +44,10 @@ let map_constructors ~(sdecl : Parsetree.type_declaration) ~all_constructors env
     match type_decl with
     | {type_kind = Type_variant ([], _)} ->
       raise (VariantTypeSpreadError (loc.loc, InvalidType))
+    | {type_kind = Type_variant _} when Variant_coercion.is_bool_or_unit path ->
+      (* Their constructors would build [true], [false] and [undefined], but
+         would be matched and coerced as the strings of their names *)
+      raise (VariantTypeSpreadError (loc.loc, InvalidType))
     | {type_kind = Type_variant (cstrs, _); type_attributes; type_params} ->
       if List.length type_params > 0 then
         raise (VariantTypeSpreadError (loc.loc, HasTypeParams));
diff --git a/tests/build_tests/super_errors/expected/coercion_hint_unit_to_string.res.expected b/tests/build_tests/super_errors/expected/coercion_hint_unit_to_string.res.expected
new file mode 100644
index 00000000000..6532e1e9c28
--- /dev/null
+++ b/tests/build_tests/super_errors/expected/coercion_hint_unit_to_string.res.expected
@@ -0,0 +1,10 @@
+
+  We've found a bug for you!
+  /.../fixtures/coercion_hint_unit_to_string.res:2:17-19
+
+  1 │ let u = (): unit => ()
+  2 │ let s: string = u()
+  3 │ 
+
+  This has type: unit
+  But it's expected to have type: string
\ No newline at end of file
diff --git a/tests/build_tests/super_errors/expected/coercion_polyvariant_to_bool.res.expected b/tests/build_tests/super_errors/expected/coercion_polyvariant_to_bool.res.expected
new file mode 100644
index 00000000000..1be2d51ecdb
--- /dev/null
+++ b/tests/build_tests/super_errors/expected/coercion_polyvariant_to_bool.res.expected
@@ -0,0 +1,9 @@
+
+  We've found a bug for you!
+  /.../fixtures/coercion_polyvariant_to_bool.res:2:16-24
+
+  1 │ let p: [#"true"] = #"true"
+  2 │ let b: bool = (p :> bool)
+  3 │ 
+
+  Type [#"true"] is not a subtype of bool
\ No newline at end of file
diff --git a/tests/build_tests/super_errors/expected/coercion_polyvariant_to_unit.res.expected b/tests/build_tests/super_errors/expected/coercion_polyvariant_to_unit.res.expected
new file mode 100644
index 00000000000..a36cc8b60aa
--- /dev/null
+++ b/tests/build_tests/super_errors/expected/coercion_polyvariant_to_unit.res.expected
@@ -0,0 +1,9 @@
+
+  We've found a bug for you!
+  /.../fixtures/coercion_polyvariant_to_unit.res:2:16-24
+
+  1 │ let p: [#"()"] = #"()"
+  2 │ let u: unit = (p :> unit)
+  3 │ 
+
+  Type [#"()"] is not a subtype of unit
\ No newline at end of file
diff --git a/tests/build_tests/super_errors/expected/coercion_string_to_unit.res.expected b/tests/build_tests/super_errors/expected/coercion_string_to_unit.res.expected
new file mode 100644
index 00000000000..e83f5c56242
--- /dev/null
+++ b/tests/build_tests/super_errors/expected/coercion_string_to_unit.res.expected
@@ -0,0 +1,8 @@
+
+  We've found a bug for you!
+  /.../fixtures/coercion_string_to_unit.res:1:16-26
+
+  1 │ let u: unit = ("a" :> unit)
+  2 │ 
+
+  Type string is not a subtype of unit
\ No newline at end of file
diff --git a/tests/build_tests/super_errors/expected/coercion_unit_to_int.res.expected b/tests/build_tests/super_errors/expected/coercion_unit_to_int.res.expected
new file mode 100644
index 00000000000..9be7b11fe04
--- /dev/null
+++ b/tests/build_tests/super_errors/expected/coercion_unit_to_int.res.expected
@@ -0,0 +1,8 @@
+
+  We've found a bug for you!
+  /.../fixtures/coercion_unit_to_int.res:1:15-23
+
+  1 │ let i: int = (() :> int)
+  2 │ 
+
+  Type unit is not a subtype of int
\ No newline at end of file
diff --git a/tests/build_tests/super_errors/expected/coercion_unit_to_string.res.expected b/tests/build_tests/super_errors/expected/coercion_unit_to_string.res.expected
new file mode 100644
index 00000000000..f7ba693108b
--- /dev/null
+++ b/tests/build_tests/super_errors/expected/coercion_unit_to_string.res.expected
@@ -0,0 +1,8 @@
+
+  We've found a bug for you!
+  /.../fixtures/coercion_unit_to_string.res:1:18-29
+
+  1 │ let s: string = (() :> string)
+  2 │ 
+
+  Type unit is not a subtype of string
\ No newline at end of file
diff --git a/tests/build_tests/super_errors/expected/coercion_unit_to_unknown.res.expected b/tests/build_tests/super_errors/expected/coercion_unit_to_unknown.res.expected
new file mode 100644
index 00000000000..f735acd0fd9
--- /dev/null
+++ b/tests/build_tests/super_errors/expected/coercion_unit_to_unknown.res.expected
@@ -0,0 +1,8 @@
+
+  We've found a bug for you!
+  /.../fixtures/coercion_unit_to_unknown.res:1:19-31
+
+  1 │ let u: unknown = (() :> unknown)
+  2 │ 
+
+  Type unit is not a subtype of unknown
\ No newline at end of file
diff --git a/tests/build_tests/super_errors/expected/variant_spread_bool.res.expected b/tests/build_tests/super_errors/expected/variant_spread_bool.res.expected
new file mode 100644
index 00000000000..4b9273360a8
--- /dev/null
+++ b/tests/build_tests/super_errors/expected/variant_spread_bool.res.expected
@@ -0,0 +1,10 @@
+
+  We've found a bug for you!
+  /.../fixtures/variant_spread_bool.res:2:8-11
+
+  1 │ type t =
+  2 │   | ...bool
+  3 │   | Other
+  4 │ 
+
+  This type is not a valid type to spread. It's only possible to spread other variants.
\ No newline at end of file
diff --git a/tests/build_tests/super_errors/expected/variant_spread_unit.res.expected b/tests/build_tests/super_errors/expected/variant_spread_unit.res.expected
new file mode 100644
index 00000000000..f6b1fa51230
--- /dev/null
+++ b/tests/build_tests/super_errors/expected/variant_spread_unit.res.expected
@@ -0,0 +1,10 @@
+
+  We've found a bug for you!
+  /.../fixtures/variant_spread_unit.res:2:8-11
+
+  1 │ type t =
+  2 │   | ...unit
+  3 │   | Other
+  4 │ 
+
+  This type is not a valid type to spread. It's only possible to spread other variants.
\ No newline at end of file
diff --git a/tests/build_tests/super_errors/fixtures/coercion_hint_unit_to_string.res b/tests/build_tests/super_errors/fixtures/coercion_hint_unit_to_string.res
new file mode 100644
index 00000000000..a94231fa8d4
--- /dev/null
+++ b/tests/build_tests/super_errors/fixtures/coercion_hint_unit_to_string.res
@@ -0,0 +1,2 @@
+let u = (): unit => ()
+let s: string = u()
diff --git a/tests/build_tests/super_errors/fixtures/coercion_polyvariant_to_bool.res b/tests/build_tests/super_errors/fixtures/coercion_polyvariant_to_bool.res
new file mode 100644
index 00000000000..a5ac8661da5
--- /dev/null
+++ b/tests/build_tests/super_errors/fixtures/coercion_polyvariant_to_bool.res
@@ -0,0 +1,2 @@
+let p: [#"true"] = #"true"
+let b: bool = (p :> bool)
diff --git a/tests/build_tests/super_errors/fixtures/coercion_polyvariant_to_unit.res b/tests/build_tests/super_errors/fixtures/coercion_polyvariant_to_unit.res
new file mode 100644
index 00000000000..40d0be715c0
--- /dev/null
+++ b/tests/build_tests/super_errors/fixtures/coercion_polyvariant_to_unit.res
@@ -0,0 +1,2 @@
+let p: [#"()"] = #"()"
+let u: unit = (p :> unit)
diff --git a/tests/build_tests/super_errors/fixtures/coercion_string_to_unit.res b/tests/build_tests/super_errors/fixtures/coercion_string_to_unit.res
new file mode 100644
index 00000000000..590a4d3730b
--- /dev/null
+++ b/tests/build_tests/super_errors/fixtures/coercion_string_to_unit.res
@@ -0,0 +1 @@
+let u: unit = ("a" :> unit)
diff --git a/tests/build_tests/super_errors/fixtures/coercion_unit_to_int.res b/tests/build_tests/super_errors/fixtures/coercion_unit_to_int.res
new file mode 100644
index 00000000000..c863192dfd9
--- /dev/null
+++ b/tests/build_tests/super_errors/fixtures/coercion_unit_to_int.res
@@ -0,0 +1 @@
+let i: int = (() :> int)
diff --git a/tests/build_tests/super_errors/fixtures/coercion_unit_to_string.res b/tests/build_tests/super_errors/fixtures/coercion_unit_to_string.res
new file mode 100644
index 00000000000..6d59bcdb66b
--- /dev/null
+++ b/tests/build_tests/super_errors/fixtures/coercion_unit_to_string.res
@@ -0,0 +1 @@
+let s: string = (() :> string)
diff --git a/tests/build_tests/super_errors/fixtures/coercion_unit_to_unknown.res b/tests/build_tests/super_errors/fixtures/coercion_unit_to_unknown.res
new file mode 100644
index 00000000000..c1238810c79
--- /dev/null
+++ b/tests/build_tests/super_errors/fixtures/coercion_unit_to_unknown.res
@@ -0,0 +1 @@
+let u: unknown = (() :> unknown)
diff --git a/tests/build_tests/super_errors/fixtures/variant_spread_bool.res b/tests/build_tests/super_errors/fixtures/variant_spread_bool.res
new file mode 100644
index 00000000000..6ffd2bb8ab9
--- /dev/null
+++ b/tests/build_tests/super_errors/fixtures/variant_spread_bool.res
@@ -0,0 +1,3 @@
+type t =
+  | ...bool
+  | Other
diff --git a/tests/build_tests/super_errors/fixtures/variant_spread_unit.res b/tests/build_tests/super_errors/fixtures/variant_spread_unit.res
new file mode 100644
index 00000000000..3e1dfc5480e
--- /dev/null
+++ b/tests/build_tests/super_errors/fixtures/variant_spread_unit.res
@@ -0,0 +1,3 @@
+type t =
+  | ...unit
+  | Other
diff --git a/tests/tests/src/VariantCoercion.mjs b/tests/tests/src/VariantCoercion.mjs
index 0459f6047e4..5d298c9451c 100644
--- a/tests/tests/src/VariantCoercion.mjs
+++ b/tests/tests/src/VariantCoercion.mjs
@@ -104,6 +104,12 @@ let CoerceVariantBinaryOp = {
   z: z
 };
 
+let CoerceBoolLikeVariant = {
+  t: "True",
+  s: "True",
+  fromPoly: "True"
+};
+
 let PrivateColor = {
   red: "Red"
 };
@@ -145,6 +151,7 @@ export {
   CoerceFromBigintToVariant,
   CoerceFromPolyvariantToVariant,
   CoerceVariantBinaryOp,
+  CoerceBoolLikeVariant,
   PrivateColor,
   colorToPlain,
   colorToString,
diff --git a/tests/tests/src/VariantCoercion.res b/tests/tests/src/VariantCoercion.res
index 95467ed5e61..c00ffad29a7 100644
--- a/tests/tests/src/VariantCoercion.res
+++ b/tests/tests/src/VariantCoercion.res
@@ -153,6 +153,18 @@ module CoerceVariantBinaryOp = {
   let z = (X :> float) +. (Y :> float) +. 1.5
 }
 
+// [bool] and [unit] coerce to no primitive, since they are not variants at
+// runtime, but a declared variant whose constructors are named like them does
+module CoerceBoolLikeVariant = {
+  type t = False | True
+
+  let t: t = True
+  let s = (t :> string)
+
+  type polyTrue = [#True]
+  let fromPoly = ((#True: polyTrue) :> t)
+}
+
 // A private variant coerces out of itself, but nothing coerces into it from
 // another type
 module PrivateColor: {