I would love this to be true, but it isn't. I've done generating types for the frontend multiple times, sometimes from C# (around 2016, using typelite), Java (openapi template generator) and most recently straight from OpenAPI spec files (.yaml) using Orval.
It always has been a shitshow. It works well for the 90% cases, but in the 10% edge cases, things break. It becomes impossible to fix generation issues, you will often resort in working around issues in your backend/openapi code. Sometimes you report bugs upstream and hope it gets fixed. In the current project we are stuck on a ~2year old Orval version (a typescript generator from openapi) because some features broke or were removed in the latest version, and the entire monorepo (15+ LoB apps) wouldn't compile and would require major changes. This simply because a never version of the generator was broken/removed features previously present.
No, that's not true. If you share code like this then you can do things like put the same validation code in the frontend and the backend: frontend to give a nice user experience, and backend to protect the endpoint.
OpenAPI does support patterns for fields and nullables/non-nullables - that already gets you very far regarding validation. A decently sophisticated generator (which don't exist IMHO) would generate the validation code for your respective language.
Still one lang on both ends is nice: there are some bits of code you want to run on both ends (like templating for SSR/SEO/caching; but also using them in the browser).
