Add test for issue #15703, bounds error for NTuple (#20151)

Author: colinfang

test/tuple.jl

@@ -178,3 +178,28 @@ end
 @test_throws BoundsError (1,2,3)[falses(2)]
 @test_throws BoundsError ()[[false]]
 @test_throws BoundsError ()[[true]]
+
+# issue #15703
+let
+    immutable A_15703{N}
+        keys::NTuple{N, Int}
+    end
+
+    immutable B_15703
+        x::A_15703
+    end
+
+    function bug_15703(xs...)
+        [x for x in xs]
+    end
+
+    function test_15703()
+        s = (1,)
+        a = A_15703(s)
+        ss = B_15703(a).x.keys
+        @test ss === s
+        bug_15703(ss...)
+    end
+
+    test_15703()
+end