From 45a1e9b989263161861b3fa6888731610210b8b6 Mon Sep 17 00:00:00 2001
From: Jeff Bezanson <jeff.bezanson@gmail.com>
Date: Wed, 24 Apr 2019 17:30:10 -0400
Subject: [PATCH] fix `deepcopy` on 0-field mutable structs

---
 base/deepcopy.jl | 5 ++---
 test/copy.jl     | 7 +++++++
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/base/deepcopy.jl b/base/deepcopy.jl
index bc9802f0b36ce..9ec829e41558c 100644
--- a/base/deepcopy.jl
+++ b/base/deepcopy.jl
@@ -53,8 +53,7 @@ end
 
 function deepcopy_internal(@nospecialize(x), stackdict::IdDict)
     T = typeof(x)::DataType
-    nf = nfields(x)
-    (isbitstype(T) || nf == 0) && return x
+    isbitstype(T) && return x
     if haskey(stackdict, x)
         return stackdict[x]
     end
@@ -62,7 +61,7 @@ function deepcopy_internal(@nospecialize(x), stackdict::IdDict)
     if T.mutable
         stackdict[x] = y
     end
-    for i in 1:nf
+    for i in 1:nfields(x)
         if isdefined(x,i)
             ccall(:jl_set_nth_field, Cvoid, (Any, Csize_t, Any), y, i-1,
                   deepcopy_internal(getfield(x,i), stackdict))
diff --git a/test/copy.jl b/test/copy.jl
index 10bdca12522df..44504709db0a9 100644
--- a/test/copy.jl
+++ b/test/copy.jl
@@ -176,3 +176,10 @@ end
         end
     end
 end
+
+# issue #17149
+mutable struct Bar17149
+end
+let x = Bar17149()
+    @test deepcopy(x) !== x
+end