From 8a751e64a3bfab5ae4b309d2ed8e2f75e96fee59 Mon Sep 17 00:00:00 2001
From: Shuhei Kadowaki <40514306+aviatesk@users.noreply.github.com>
Date: Sun, 6 Jun 2021 16:33:47 +0900
Subject: [PATCH] fix #40814, improve type stability of `_totuple` (#41074)

Replaces #40987, as suggested by #40814.
---
 base/tuple.jl | 6 +++++-
 test/tuple.jl | 3 +++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/base/tuple.jl b/base/tuple.jl
index 5ebf36b750cf01..dc777e7497598f 100644
--- a/base/tuple.jl
+++ b/base/tuple.jl
@@ -327,7 +327,11 @@ function _totuple(T, itr, s...)
     @_inline_meta
     y = iterate(itr, s...)
     y === nothing && _totuple_err(T)
-    return (convert(fieldtype(T, 1), y[1]), _totuple(tuple_type_tail(T), itr, y[2])...)
+    t1 = convert(fieldtype(T, 1), y[1])
+    # inference may give up in recursive calls, so annotate here to force accurate return type to be propagated
+    rT = tuple_type_tail(T)
+    ts = _totuple(rT, itr, y[2])::rT
+    return (t1, ts...)
 end
 
 # use iterative algorithm for long tuples
diff --git a/test/tuple.jl b/test/tuple.jl
index e8751e73b7ad45..672a7c6c350ea5 100644
--- a/test/tuple.jl
+++ b/test/tuple.jl
@@ -631,3 +631,6 @@ f38837(xs) = map((F,x)->F(x), (Float32, Float64), xs)
     @test_throws BoundsError (1, 2)[0:2]
     @test_throws ArgumentError (1, 2)[OffsetArrays.IdOffsetRange(1:2, -1)]
 end
+
+# https://github.com/JuliaLang/julia/issues/40814
+@test Base.return_types(NTuple{3,Int}, (Vector{Int},)) == Any[NTuple{3,Int}]