fixes #6279

tests/ccgbugs/t6279.nim

@@ -0,0 +1,9 @@
+discard """
+cmd: "nim c -r -d:fulldebug -d:smokeCycles --gc:refc $file"
+output: '''@[a]'''
+"""
+
+# bug #6279
+var foo = newSeq[tuple[a: seq[string], b: seq[string]]]()
+foo.add((@["a"], @["b"]))
+echo foo[0].a # Crashes on this line

tests/ccgbugs/taddhigh.nim

@@ -1,5 +1,5 @@
 discard """
-  output: '''@[5, 5, 5]'''
+  output: '''@[5, 5, 5, 5, 5]'''
 """
 
 # bug #1832
@@ -13,4 +13,7 @@ s.add x
 # Causes the 0 to appear:
 s.add s[s.high]
 
+s.add s[s.len-1]
+s.add s[s.xlen-1]
+
 echo s # @[5, 5, 0]