Allow custom_target do depend on indexed output of custom_target

Fixes: #3494

mesonbuild/backend/backends.py

@@ -764,7 +764,7 @@ class Backend:
                 fname = [os.path.join(self.build_to_src, target.subdir, i)]
             elif isinstance(i, build.BuildTarget):
                 fname = [self.get_target_filename(i)]
-            elif isinstance(i, build.CustomTarget):
+            elif isinstance(i, (build.CustomTarget, build.CustomTargetIndex)):
                 fname = [os.path.join(self.get_target_dir(i), p) for p in i.get_outputs()]
             elif isinstance(i, build.GeneratedList):
                 fname = [os.path.join(self.get_target_private_dir(target), p) for p in i.get_outputs()]

mesonbuild/build.py

@@ -1974,7 +1974,7 @@ def get_sources_string_names(sources):
             s = s.held_object
         if isinstance(s, str):
             names.append(s)
-        elif isinstance(s, (BuildTarget, CustomTarget, GeneratedList)):
+        elif isinstance(s, (BuildTarget, CustomTarget, CustomTargetIndex, GeneratedList)):
             names += s.get_outputs()
         elif isinstance(s, File):
             names.append(s.fname)

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')