Crash when using custom extensions outside of Babel pipeline with enhancement compilation disabled

[novemberborn]

From #1822 (comment):

  "ava": {
    "compileEnhancements": false,
    "extensions": [
      "ts"
    ],
    "require": [
      "ts-node/register",
      "tsconfig-paths/register"
    ]
  },

Because of how ts is specified, AVA won't apply its Babel pipeline to it. However, note that the babel option is not disabled.

The logic in these lines is broken:

const {projectDir, babelConfig} = this.options;
const compileEnhancements = this.options.compileEnhancements !== false;
const precompileFull = babelConfig ?
  babelPipeline.build(projectDir, cacheDir, babelConfig, compileEnhancements) :
  filename => {
    throw new Error(`Cannot apply full precompilation, possible bad usage: ${filename}`);
  };
const precompileEnhancementsOnly = compileEnhancements && this.options.extensions.enhancementsOnly.length > 0 ?
  babelPipeline.build(projectDir, cacheDir, null, compileEnhancements) :
  filename => {
    throw new Error(`Cannot apply enhancement-only precompilation, possible bad usage: ${filename}`);
  };


this._precompiler = {
  cacheDir,
  enabled: babelConfig || compileEnhancements,
  precompileEnhancementsOnly,
  precompileFull
};

Note how this._precompiler.enabled ends up being true. However precompileEnhancementsOnly is set to a function that always throws. Consequently, when compiling TypeScript files, precompileEnhancementsOnly is called and AVA crashes.

I think the logic should be such that a () => null function is when compileEnhancements is false, and the error-throwing function is used when it's not and this.options.extensions.enhancementsOnly.length is 0.