check for -G prefix, error out when -G is used together with --use-ninja

bin/catkin_make

@@ -133,11 +133,13 @@ def main():
             print(fmt('@{yf}Packages @{boldon}"%s"@{boldoff} not found in the workspace - ignoring them' % ', '.join(sorted(unknown_packages))), file=sys.stderr)
             args.pkg = [name for name in args.pkg if name in packages_by_name]
 
-    if '-G' not in cmake_args:
+    if not [arg for arg in cmake_args if arg.startswith('-G')]:
         if not args.use_ninja:
             cmake_args += ['-G', 'Unix Makefiles']
         else:
             cmake_args += ['-G', 'Ninja']
+    elif args.use_ninja:
+        return fmt("@{rf}Error: either specify a generator using '-G...' or '--use-ninja' but not both")
 
     # check if cmake must be run (either for a changed list of package paths or changed cmake arguments)
     force_cmake = cmake_input_changed(packages, build_path, cmake_args=cmake_args)

python/catkin/builder.py

@@ -803,11 +803,15 @@ def build_workspace_isolated(
     else:
         cmake_args = []
 
-    if '-G' not in cmake_args:
+    if not [arg for arg in cmake_args if arg.startswith('-G')]:
         if not use_ninja:
             cmake_args += ['-G', 'Unix Makefiles']
         else:
             cmake_args += ['-G', 'Ninja']
+    elif use_ninja:
+        print(colorize_line("Error: either specify a generator using '-G...' or '--use-ninja' but not both"))
+        sys.exit(1)
+
 
     if make_args:
         print("Additional make Arguments: " + " ".join(make_args))