fixes #12310 [backport] (#12470)

compiler/vmgen.nim

@@ -948,9 +948,13 @@ proc genMagic(c: PCtx; n: PNode; dest: var TDest; m: TMagic) =
     c.genAddSubInt(n, dest, opcAddInt)
   of mInc, mDec:
     unused(c, n, dest)
-    let opc = if m == mInc: opcAddInt else: opcSubInt
+    let isUnsigned = n.sons[1].typ.skipTypes(abstractVarRange).kind in {tyUInt..tyUInt64}
+    let opc = if not isUnsigned:
+                if m == mInc: opcAddInt else: opcSubInt
+              else:
+                if m == mInc: opcAddu else: opcSubu
     let d = c.genx(n.sons[1])
-    if n.sons[2].isInt8Lit:
+    if n.sons[2].isInt8Lit and not isUnsigned:
       c.gABI(n, succ(opc), d, d, n.sons[2].intVal)
     else:
       let tmp = c.genx(n.sons[2])

tests/vm/tmisc_vm.nim

@@ -200,3 +200,17 @@ const
   vmCrash = oh_no()
 
 debugEcho vmCrash
+
+
+# bug #12310
+
+proc someTransform(s: var array[8, uint64]) =
+  var s1 = 5982491417506315008'u64
+  s[1] += s1
+
+static:
+  var state: array[8, uint64]
+  state[1] = 7105036623409894663'u64
+  someTransform(state)
+
+  doAssert state[1] == 13087528040916209671'u64