semcheck negative array length (#7518)

compiler/semtypes.nim

@@ -252,6 +252,9 @@ proc semArrayIndex(c: PContext, n: PNode): PType =
     if e.typ.kind == tyFromExpr:
       result = makeRangeWithStaticExpr(c, e.typ.n)
     elif e.kind in {nkIntLit..nkUInt64Lit}:
+      if e.intVal < 0:
+        localError(n[1].info,
+          "Array length can't be negative, but was " & $e.intVal)
       result = makeRangeType(c, 0, e.intVal-1, n.info, e.typ)
     elif e.kind == nkSym and e.typ.kind == tyStatic:
       if e.sym.ast != nil:

tests/array/tarray.nim

@@ -46,3 +46,7 @@ let arr3: array[0, string] = []
 doAssert(arr1.len == 0)
 doAssert(arr2.len == 0)
 doAssert(arr3.len == 0)
+
+# Negative array length is not allowed (#6852)
+doAssert(not compiles(block:
+  var arr: array[-1, int]))