unordered enum for better interoperability with C (#23585)

ref https://forum.nim-lang.org/t/11564
```nim
block: # unordered enum
  block:
    type
      unordered_enum = enum
        a = 1
        b = 0

    doAssert (ord(a), ord(b)) == (1, 0)

  block:
    type
      unordered_enum = enum
        a = 1
        b = 0
        c

    doAssert (ord(a), ord(b), ord(c)) == (1, 0, 2)

  block:
    type
      unordered_enum = enum
        a = 100
        b
        c = 50
        d

    doAssert (ord(a), ord(b), ord(c), ord(d)) == (100, 101, 50, 51)

  block:
    type
      unordered_enum = enum
        a = 7
        b = 6
        c = 5
        d

    doAssert (ord(a), ord(b), ord(c), ord(d)) == (7, 6, 5, 8)
```

2 files changed, 93 insertions, 0 deletions

diff --git a/tests/enum/tenum.nim b/tests/enum/tenum.nim
index 8046c6589..5348fa528 100644
--- a/tests/enum/tenum.nim
+++ b/tests/enum/tenum.nim
@@ -184,3 +184,86 @@ block: # bug #12589
           A = int64.high()
 
       doAssert ord(A) == int64.high()
+
+import std/enumutils
+from std/sequtils import toSeq
+import std/macros
+
+block: # unordered enum
+  block:
+    type
+      unordered_enum = enum
+        a = 1
+        b = 0
+
+    doAssert (ord(a), ord(b)) == (1, 0)
+    when false: # TODO: fixme pre-existing issues # bug #23586
+      doAssert unordered_enum.toSeq == @[a, b]
+
+  block:
+    type
+      unordered_enum = enum
+        a = 1
+        b = 0
+        c
+
+    doAssert (ord(a), ord(b), ord(c)) == (1, 0, 2)
+
+  block:
+    type
+      unordered_enum = enum
+        a = 100
+        b
+        c = 50
+        d
+
+    doAssert (ord(a), ord(b), ord(c), ord(d)) == (100, 101, 50, 51)
+
+  block:
+    type
+      unordered_enum = enum
+        a = 7
+        b = 6
+        c = 5
+        d
+
+    doAssert (ord(a), ord(b), ord(c), ord(d)) == (7, 6, 5, 8)
+    when false:
+      doAssert unordered_enum.toSeq == @[a, b, c, d]
+
+  block:
+    type
+      unordered_enum = enum
+        a = 100
+        b
+        c = 500
+        d
+        e
+        f = 50
+        g
+        h
+
+    doAssert (ord(a), ord(b), ord(c), ord(d), ord(e), ord(f), ord(g), ord(h)) == 
+             (100, 101, 500, 501, 502, 50, 51, 52)
+
+  block:
+    type
+      unordered_enum = enum
+        A
+        B
+        C = -1
+        D
+        E
+        G = -999
+
+    doAssert (ord(A), ord(B), ord(C), ord(D), ord(E), ord(G)) ==
+             (0, 1, -1, 2, 3, -999)
+
+  block:
+    type
+      SomeEnum = enum
+        seA = 3
+        seB = 2
+        seC = "foo"
+
+    doAssert (ord(seA), ord(seB), ord(seC)) == (3, 2, 4)
diff --git a/tests/enum/tenum_duplicate.nim b/tests/enum/tenum_duplicate.nim
new file mode 100644
index 000000000..4bcad7f6f
--- /dev/null
+++ b/tests/enum/tenum_duplicate.nim
@@ -0,0 +1,10 @@
+discard """
+  errormsg: "duplicate value in enum 'd'"
+"""
+
+type
+  unordered_enum = enum
+    a = 1
+    b = 0
+    c
+    d = 2