Expected Behaviour

This should not throw an error:

export function transformer(program: ts.Program) {
  return (context: ts.TransformationContext) => (file: ts.SourceFile) => {
    if (program.getSourceFile(file.fileName).getText() !== file.getText()) {
      throw new Error();
    }
  };
}

Actual Behaviour

The files in the program instance passed in becomes stale when you have a watch running, and program.getSourceFile() will return the file as it looked when the watch started.

I was the one who added the program argument to getCustomTransformers so I'm the one to blame here. 😄 To avoid a breaking change I think we could change it from:

(program: Program) => { before?: TransformerFactory<SourceFile>[]; after?: TransformerFactory<SourceFile>[]; }

to:

(program: Program, getProgram: () => Program) => { before?: TransformerFactory<SourceFile>[]; after?: TransformerFactory<SourceFile>[]; }

And in a new major remove the passed in program and just pass a function that gets the latest program instance.