custom_target: Output must not contain a path segment

[Jason Crain]

I'm trying to generate some data files that will be used by a test program.  I'd like to keep these files organized in directories to group test cases together.  I'm using a custom target to generate these files, but I'm running into a problem where meson complains "Output must not contain a path segment." or "Depfile must be a plain filename without a subdirectory.".  I'm using something like this in a meson.build:

sources = [
  'dir1/file1',
  'dir1/file2',
  'dir2/file3',
  'dir3/dir/file4'
]
foreach s : sources
  custom_target(
    s + '.out',
    input : s + '.in',
    output : s + '.out',
    command : ['touch', '@OUTPUT@', '@DEPFILE@'],
    depfile : s + '.dep',
    build_by_default : true)
endforeach

Though of course it doesn't use 'touch' and it can generate a real depfile.  If I modify meson and remove the following lines, things work as I expected, at least on linux:

diff --git a/mesonbuild/build.py b/mesonbuild/build.py
index bf692e17..b3bc9863 100644
--- a/mesonbuild/build.py
+++ b/mesonbuild/build.py
@@ -1320,8 +1320,6 @@ class CustomTarget(Target):
         for i in self.output:
             if not(isinstance(i, str)):
                 raise InvalidArguments('Output argument not a string.')
-            if '/' in i:
-                raise InvalidArguments('Output must not contain a path segment.')
             if '@INPUT@' in i or '@INPUT0@' in i:
                 m = 'Output cannot contain @INPUT@ or @INPUT0@, did you ' \
                     'mean @PLAINNAME@ or @BASENAME@?'
@@ -1342,8 +1340,6 @@ class CustomTarget(Target):
             depfile = kwargs['depfile']
             if not isinstance(depfile, str):
                 raise InvalidArguments('Depfile must be a string.')
-            if os.path.split(depfile)[1] != depfile:
-                raise InvalidArguments('Depfile must be a plain filename without a subdirectory.')
             self.depfile = depfile
         self.command = self.flatten_command(kwargs['command'])
         if self.capture:

It looks like meson is intentionally preventing a custom target from writing to a subdirectory.  Is this something that can be supported?