Adding the following

+++ b/dom/webidl/CanvasRenderingContext2D.webidl
@@ -322,6 +322,9 @@ interface mixin CanvasPathMethods {
   undefined rect(double x, double y, double w, double h);
 
   [Throws, LenientFloat]
+  undefined roundRect(double x, double y, double w, double h, optional (double or DOMPointInit or sequence<(double or DOMPointInit)>) radii = 0);
+
+  [Throws, LenientFloat]
   undefined arc(double x, double y, double radius, double startAngle, double endAngle, optional boolean anticlockwise = false);
 
   [Throws, LenientFloat]

will give this Codegen.py AssertionError:

 0:07.10 Traceback (most recent call last):
 0:07.10   File "/usr/lib/python3.10/runpy.py", line 196, in _run_module_as_main
 0:07.10     return _run_code(code, main_globals, None,
 0:07.10   File "/usr/lib/python3.10/runpy.py", line 86, in _run_code
 0:07.10     exec(code, run_globals)
 0:07.10   File "/home/tom/projects/mozilla/python/mozbuild/mozbuild/action/webidl.py", line 19, in <module>
 0:07.10     sys.exit(log_build_task(main, sys.argv[1:]))
 0:07.10   File "/home/tom/projects/mozilla/python/mozbuild/mozbuild/action/util.py", line 16, in log_build_task
 0:07.10     return f(*args, **kwargs)
 0:07.10   File "/home/tom/projects/mozilla/python/mozbuild/mozbuild/action/webidl.py", line 15, in main
 0:07.10     manager.generate_build_files()
 0:07.10   File "/home/tom/projects/mozilla/dom/bindings/mozwebidlcodegen/__init__.py", line 323, in generate_build_files
 0:07.10     written, deps = self._generate_build_files_for_webidl(filename)
 0:07.10   File "/home/tom/projects/mozilla/dom/bindings/mozwebidlcodegen/__init__.py", line 579, in _generate_build_files_for_webidl
 0:07.10     root = CGBindingRoot(self._config, binding_stem, filename)
 0:07.10   File "/home/tom/projects/mozilla/dom/bindings/Codegen.py", line 18729, in __init__
 0:07.10     curr = CGHeaders(
 0:07.10   File "/home/tom/projects/mozilla/dom/bindings/Codegen.py", line 1460, in __init__
 0:07.10     addHeadersForType(t)
 0:07.10   File "/home/tom/projects/mozilla/dom/bindings/Codegen.py", line 1400, in addHeadersForType
 0:07.10     addHeadersForType((t, None))
 0:07.10   File "/home/tom/projects/mozilla/dom/bindings/Codegen.py", line 1398, in addHeadersForType
 0:07.10     headerSet.add(self.getUnionDeclarationFilename(config, unrolled))
 0:07.10   File "/home/tom/projects/mozilla/dom/bindings/Codegen.py", line 1595, in getUnionDeclarationFilename
 0:07.10     assert len(config.filenamesPerUnion[unionType.name]) == 1
 0:07.10 AssertionError
 0:07.52 gmake[4]: *** [Makefile:54: webidl.stub] Error 1

peterv: Could you take a look?

Hmm, this is a problem related to sequences containing unions.

https://hg.mozilla.org/mozilla-central/rev/3a372161bed7