fixes #18643 [backport:1.0] (#18678)

(cherry picked from commit 018465a234)

compiler/ccgexprs.nim

@@ -916,7 +916,7 @@ proc genArrayElem(p: BProc, n, x, y: PNode, d: var TLoc) =
   if optBoundsCheck in p.options and ty.kind != tyUncheckedArray:
     if not isConstExpr(y):
       # semantic pass has already checked for const index expressions
-      if firstOrd(p.config, ty) == 0:
+      if firstOrd(p.config, ty) == 0 and lastOrd(p.config, ty) >= 0:
         if (firstOrd(p.config, b.t) < firstOrd(p.config, ty)) or (lastOrd(p.config, b.t) > lastOrd(p.config, ty)):
           linefmt(p, cpsStmts, "if ((NU)($1) > (NU)($2)){ #raiseIndexError2($1, $2); $3}$n",
                   [rdCharLoc(b), intLiteral(lastOrd(p.config, ty)), raiseInstr(p)])

tests/array/tarray.nim

@@ -590,3 +590,14 @@ block t12466:
     a[0'u16 + i] = i
   for i in 0'u16 ..< 8'u16:
     a[0'u16 + i] = i
+
+block t18643:
+  # https://github.com/nim-lang/Nim/issues/18643
+  let a: array[0, int] = []
+  var caught = false
+  let b = 9999999
+  try:
+    echo a[b]
+  except IndexDefect:
+    caught = true
+  doAssert caught, "IndexDefect not caught!"