mitigates #12815 (#18557)

* mitigates #12815

* Update doc/nimc.rst

Co-authored-by: konsumlamm <44230978+konsumlamm@users.noreply.github.com>

Co-authored-by: konsumlamm <44230978+konsumlamm@users.noreply.github.com>

compiler/lineinfos.nim

@@ -72,6 +72,7 @@ type
     warnFileChanged = "FileChanged",
     warnSuspiciousEnumConv = "EnumConv",
     warnAnyEnumConv = "AnyEnumConv",
+    warnHoleEnumConv = "HoleEnumConv",
     warnCstringConv = "CStringConv",
     warnUser = "User",
     # hints
@@ -158,6 +159,7 @@ const
     warnFileChanged: "file changed: $1",
     warnSuspiciousEnumConv: "$1",
     warnAnyEnumConv: "$1",
+    warnHoleEnumConv: "$1",
     warnCstringConv: "$1",
     warnUser: "$1",
     hintSuccess: "operation successful: $#",

compiler/sempass2.nim

@@ -1173,8 +1173,12 @@ proc track(tracked: PEffects, n: PNode) =
         "implicit conversion to 'cstring' from a non-const location: $1; this will become a compile time error in the future" %
           $n[1])
 
-    if n.typ.skipTypes(abstractInst).kind == tyEnum:
-      message(tracked.config, n.info, warnAnyEnumConv, "enum conversion: $1" % $n[1])
+    let t = n.typ.skipTypes(abstractInst)
+    if t.kind == tyEnum:
+      if tfEnumHasHoles in t.flags:
+        message(tracked.config, n.info, warnHoleEnumConv, "conversion to enum with holes is unsafe: $1" % $n)
+      else:
+        message(tracked.config, n.info, warnAnyEnumConv, "enum conversion: $1" % $n)
 
     if n.len == 2:
       track(tracked, n[1])

doc/nimc.rst

@@ -74,6 +74,8 @@ CStringConv                      Warn about dangerous implicit conversions
                                  to `cstring`.
 EnumConv                         Warn about conversions from enum to enum.
 AnyEnumConv                      Warn about any conversions to an enum type.
+HoleEnumConv                     Warn about conversion to an enum with
+                                 holes. These conversions are unsafe.
 ResultUsed                       Warn about the usage of the
                                  built-in `result` variable.
 User                             Some user-defined warning.

tests/misc/tconv.nim

@@ -95,13 +95,26 @@ reject:
 {.push warning[AnyEnumConv]:on, warningAsError[AnyEnumConv]:on.}
 
 reject:
-  # bug #12815
   type
     Foo = enum
-      one = 1
-      three = 3
+      one
+      three
 
   var va = 2
   var vb = va.Foo
 
 {.pop.}
+
+{.push warningAsError[HoleEnumConv]:on.}
+
+reject:
+  # bug #12815
+  type
+    Hole = enum
+      one = 1
+      three = 3
+
+  var va = 2
+  var vb = va.Hole
+
+{.pop.}