Description
Copilot doesn't understand how to write fourslash tests. Read a representative sample of tests in tests/cases/fourslash, read the relevant test harness code to understand what various syntax forms are called, and come up with a short summary (two printed pages or less) on how to make an idiomatic fourslash test so that Copilot will be able to quickly write its own tests in the future.
Focus on documenting the simplest form of each syntax and prefer forms that validate over baselines.
Do the same for compiler testscases found in tests/cases/compiler
Put this in .github/copilot-instructions.md along with your summary of the relevant info from CONTRIBUTING.md (how to build, run, run a single testcase, etc - things that will be useful for SWE agent)