Add test to show issue

test cases/common/161 index customtarget/check_args.py

@@ -0,0 +1,18 @@
+#!python3
+
+import sys
+from pathlib import Path
+
+def main():
+    if len(sys.argv) != 2:
+        print(sys.argv)
+        return 1
+    if sys.argv[1] != 'gen.c':
+        print(sys.argv)
+        return 2
+    Path('foo').touch()
+
+    return 0
+
+if __name__ == '__main__':
+    sys.exit(main())

test cases/common/161 index customtarget/meson.build

@@ -29,4 +29,11 @@ lib = static_library(
   ['lib.c', gen[1]],
 )
 
+custom_target(
+  'foo',
+  input: gen[0],
+  output: 'foo',
+  command: [find_program('check_args.py'), '@INPUT@'],
+)
+
 subdir('subdir')