fix deepcopy on 0-field mutable structs (#31825)

Author: JeffBezanson

base/deepcopy.jl

@@ -56,16 +56,15 @@ 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
     y = ccall(:jl_new_struct_uninit, Any, (Any,), T)
     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))

test/copy.jl

@@ -186,3 +186,10 @@ end
         end
     end
 end
+
+# issue #17149
+mutable struct Bar17149
+end
+let x = Bar17149()
+    @test deepcopy(x) !== x
+end