Fix compiler crash with unboxed types with only 1 constructor. (#6523)

Fixes #6519

Author: cristianoc

CHANGELOG.md

@@ -20,6 +20,7 @@
 - Fix accidental removal of `Belt.Result.Ok` and `Belt.Result.Error` constructors in rc.5 https://github.com/rescript-lang/rescript-compiler/pull/6514
 - Add missing check that the runtime representation of variants matches implementation and interface. https://github.com/rescript-lang/rescript-compiler/pull/6513/files
 - GenType: only export types (not values) from module types. https://github.com/rescript-lang/rescript-compiler/pull/6516
+- Fix compiler crash with unboxed variant definition with only 1 constructor. https://github.com/rescript-lang/rescript-compiler/pull/6523
 
 # 11.0.0-rc.7
 

jscomp/ml/typedecl.ml

@@ -337,6 +337,8 @@ let transl_declaration ~typeRecordAsObject env sdecl id =
   end;
   let unboxed_status =
     match sdecl.ptype_kind with
+    | Ptype_variant [{pcd_args = Pcstr_tuple []; _}] ->
+      unboxed_false_default_false
     | Ptype_variant [{pcd_args = Pcstr_tuple _; _}]
       | Ptype_variant [{pcd_args = Pcstr_record
                           [{pld_mutable = Immutable; _}]; _}]

jscomp/test/UntaggedVariants.js

@@ -427,3 +427,8 @@ module Aliased = {
     }
   }
 }
+
+module OnlyOne = {
+  @unboxed type onlyOne = OnlyOne
+  let onlyOne = OnlyOne
+}