fix nim-lang/RFCs#294 ; disallow enum <=> enum conversion

changelog.md

@@ -66,6 +66,13 @@
 
 - nil dereference is not allowed at compile time. `cast[ptr int](nil)[]` is rejected at compile time.
 
+- An enum now can't be converted to another enum directly, you must use `ord`:
+  ```
+  type A = enum a1, a2
+  type B = enum b1, b2
+  doAssert not compiles(a1.B)
+  doAssert compiles(a1.ord.B)
+  ```
 
 ## Compiler changes
 

compiler/semexprs.nim

@@ -153,7 +153,9 @@ proc checkConvertible(c: PContext, targetTyp: PType, src: PNode): TConvStatus =
     result = checkConversionBetweenObjects(d.skipTypes(abstractInst), s.skipTypes(abstractInst), pointers)
   elif (targetBaseTyp.kind in IntegralTypes) and
       (srcBaseTyp.kind in IntegralTypes):
-    if targetTyp.kind == tyBool:
+    if targetTyp.kind == tyEnum and srcBaseTyp.kind == tyEnum:
+      result = convNotLegal
+    elif targetTyp.kind == tyBool:
       discard "convOk"
     elif targetTyp.isOrdinalType:
       if src.kind in nkCharLit..nkUInt64Lit and

tests/misc/tconv.nim

@@ -1,5 +1,7 @@
 template reject(x) =
-    static: assert(not compiles(x))
+  static: doAssert(not compiles(x))
+template accept(x) =
+  static: doAssert(compiles(x))
 
 reject:
     const x = int8(300)
@@ -55,3 +57,16 @@ block: # issue 3766
     proc r(x: static[R]) =
       echo x
     r 3.R
+
+
+block: # https://github.com/nim-lang/RFCs/issues/294
+  type Koo = enum k1, k2
+  type Goo = enum g1, g2
+
+  reject: Goo(k2)
+  reject: k2.Goo
+  reject: k2.string
+
+  accept: Koo(k2)
+  accept: k2.Koo
+  accept: k2.int.Goo