From 31975bfa4e628ea1724adb708714d0cddf4593e3 Mon Sep 17 00:00:00 2001
From: Alexey Stukalov <astukalov@gmail.com>
Date: Thu, 4 May 2017 21:19:07 +0200
Subject: [PATCH] ntuple(f, -1) throws ArgumentError

---
 NEWS.md       | 3 +++
 base/tuple.jl | 8 ++++++--
 test/tuple.jl | 2 ++
 3 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/NEWS.md b/NEWS.md
index 7c1418f258cd27..2857ade5692b9f 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -12,6 +12,9 @@ Breaking changes
 
 This section lists changes that do not have deprecation warnings.
 
+  * `ntuple(f, n::Integer)` throws `ArgumentError` if `n` is negative.
+    Previously an empty tuple was returned ([#21697]).
+
 Library improvements
 --------------------
 
diff --git a/base/tuple.jl b/base/tuple.jl
index 4c7687f5a52be7..dc041a47937fb6 100644
--- a/base/tuple.jl
+++ b/base/tuple.jl
@@ -105,7 +105,7 @@ julia> ntuple(i -> 2*i, 4)
 ```
 """
 function ntuple(f::F, n::Integer) where F
-    t = n <= 0  ? () :
+    t = n == 0  ? () :
         n == 1  ? (f(1),) :
         n == 2  ? (f(1), f(2)) :
         n == 3  ? (f(1), f(2), f(3)) :
@@ -120,7 +120,11 @@ function ntuple(f::F, n::Integer) where F
     return t
 end
 
-_ntuple(f, n) = (@_noinline_meta; ([f(i) for i = 1:n]...))
+function _ntuple(f, n)
+    @_noinline_meta
+    (n >= 0) || throw(ArgumentError(string("tuple length should be ≥0, got ", n)))
+    ([f(i) for i = 1:n]...)
+end
 
 # inferrable ntuple
 ntuple(f, ::Type{Val{0}}) = (@_inline_meta; ())
diff --git a/test/tuple.jl b/test/tuple.jl
index d59b0497cdc9f8..2385e52a9ef4fa 100644
--- a/test/tuple.jl
+++ b/test/tuple.jl
@@ -181,6 +181,8 @@ for n = 0:20
         @test t[i] == i
     end
 end
+# issue #21697
+@test_throws ArgumentError ntuple(identity, -1)
 
 # issue #19719
 @test_throws BoundsError (1,2,3)[falses(4)]