fixes #23925; VM generates wrong cast for negative enum values (#23951)

Follow up of #23927 which solves the build error.

This is still only a partial fix as it doesn't take into account
unordered enums. I'll make a separate issue for those.

---------

Co-authored-by: ringabout <43030857+ringabout@users.noreply.github.com>

compiler/vmgen.nim

@@ -900,9 +900,15 @@ proc genCard(c: PCtx; n: PNode; dest: var TDest) =
   c.freeTemp(tmp)
 
 proc genCastIntFloat(c: PCtx; n: PNode; dest: var TDest) =
+  template isSigned(typ: PType): bool {.dirty.} =
+    typ.kind == tyEnum and firstOrd(c.config, typ) < 0 or
+    typ.kind in {tyInt..tyInt64}
+  template isUnsigned(typ: PType): bool {.dirty.} =
+    typ.kind == tyEnum and firstOrd(c.config, typ) >= 0 or
+    typ.kind in {tyUInt..tyUInt64, tyChar, tyBool}
+
   const allowedIntegers = {tyInt..tyInt64, tyUInt..tyUInt64, tyChar, tyEnum, tyBool}
-  var signedIntegers = {tyInt..tyInt64}
-  var unsignedIntegers = {tyUInt..tyUInt64, tyChar, tyEnum, tyBool}
+
   let src = n[1].typ.skipTypes(abstractRange)#.kind
   let dst = n[0].typ.skipTypes(abstractRange)#.kind
   let srcSize = getSize(c.config, src)
@@ -914,12 +920,12 @@ proc genCastIntFloat(c: PCtx; n: PNode; dest: var TDest) =
     if dest < 0: dest = c.getTemp(n[0].typ)
     c.gABC(n, opcAsgnInt, dest, tmp)
     if dstSize != sizeof(BiggestInt): # don't do anything on biggest int types
-      if dst.kind in signedIntegers: # we need to do sign extensions
+      if isSigned(dst): # we need to do sign extensions
         if dstSize <= srcSize:
           # Sign extension can be omitted when the size increases.
           c.gABC(n, opcSignExtend, dest, TRegister(dstSize*8))
-      elif dst.kind in unsignedIntegers:
-        if src.kind in signedIntegers or dstSize < srcSize:
+      elif isUnsigned(dst):
+        if isSigned(src) or dstSize < srcSize:
           # Cast from signed to unsigned always needs narrowing. Cast
           # from unsigned to unsigned only needs narrowing when target
           # is smaller than source.
@@ -947,7 +953,7 @@ proc genCastIntFloat(c: PCtx; n: PNode; dest: var TDest) =
     if dest < 0: dest = c.getTemp(n[0].typ)
     if src.kind == tyFloat32:
       c.gABC(n, opcCastFloatToInt32, dest, tmp)
-      if dst.kind in unsignedIntegers:
+      if isUnsigned(dst):
         # integers are sign extended by default.
         # since there is no opcCastFloatToUInt32, narrowing should do the trick.
         c.gABC(n, opcNarrowU, dest, TRegister(32))

tests/vm/tvmmisc.nim

@@ -766,3 +766,27 @@ block: # issue #15730
 static: # more nil cstring issues
   let x = cstring(nil)
   doAssert x.len == 0
+
+block: # bug #23925
+  type Foo = enum A = -1
+  proc foo =
+    doAssert cast[Foo](-1) == A
+    doAssert ord(A) == -1
+
+  static: foo()
+  foo()
+
+  type E = enum
+    e0 e1 e2 e3 e4 e5 e6 e7 e8 e9 e10 e11 e12 e13 e14 e15 e16 e17 e18 e19 e20
+    e21 e22 e23 e24 e25 e26 e27 e28 e29 e30 e31 e32 e33 e34 e35 e36 e37 e38
+    e39 e40 e41 e42 e43 e44 e45 e46 e47 e48 e49 e50 e51 e52 e53 e54 e55 e56
+    e57 e58 e59 e60 e61 e62 e63 e64 e65 e66 e67 e68 e69 e70 e71 e72 e73 e74
+    e75 e76 e77 e78 e79 e80 e81 e82 e83 e84 e85 e86 e87 e88 e89 e90 e91 e92
+    e93 e94 e95 e96 e97 e98 e99 e100 e101 e102 e103 e104 e105 e106 e107 e108
+    e109 e110 e111 e112 e113 e114 e115 e116 e117 e118 e119 e120 e121 e122
+    e123 e124 e125 e126 e127 e128
+  proc bar =
+    doAssert cast[E](int(e128)) == e128
+
+  static: bar()
+  bar()