Compiler-Driven Verification

The execution loop for CDD (philosophy: cdd; Scala toolkit: scala-type-safety). Change the type, follow compiler errors, fix semantically, pin the output, verify:

Step	Command / mechanism
Type check + lint	sbt Test/compile (-Werror, so every warning is fatal)
Exhaustiveness	match over a sealed trait/enum listing every case, enforced by -Ycheck-all-patmat
Generated output	golden expected strings in src/test/scala/scalats/tests/*Test.scala
Generated output type-checks and round-trips	sbt test (needs cd tests && yarn once)
Never silence with	.asInstanceOf, catch-all case _, @nowarn

Workflow

1. Read the current model — TsModel cases, the TsParser.parse branch that produces them, and each TsGenerator renderer (generateCodecType, generateValueType, generateCodecInstance, generateValueInstance) that consumes them.
2. Change the type first. Add or change the TsModel case (or a private ADT in the generator), then sbt Test/compile and follow every error.
3. Write the expected TypeScript first. Add or update the golden string in a CodecTest suite, run sbt 'testOnly scalats.tests.<Suite>', and confirm it fails showing the old output.
4. Implement until the golden test and the ts-node round-trip pass.
5. Full verification per verification-before-completion.

Pitfalls

Pitfall	Correct approach
Golden test passes but the TS doesn't type-check	Only files imported by the suite's decode-encode.ts are type-checked; make the decode-encode type reference the new output
-Werror on unused/value-discard	Remove the binding or consume the value; don't suppress
Changing one renderer only	Codec type, value type and codec instance must agree; satisfies t.Type<X, unknown> in the output catches a mismatch only when ts-node compiles it
Macro change compiles but output is wrong	Macros expand in the test sources; sbt Test/compile recompiles them — inspect the failing golden diff