[api-extractor] lodash.merge() counterintuitively applies `$extends` for `reportVariants` setting

[jason-ha]

Summary

Using a base configuration with long reportVariants such as

	"apiReport": {
		"reportVariants": ["public", "beta"]
	},

cannot be shortened in a derived config.

If the derived specifies:

	"apiReport": {
		"reportVariants": ["alpha"]
	},

The result using loadash.merge is

	"apiReport": {
		"reportVariants": ["alpha", "beta"]
	},

Workaround

Use array with duplicate entries to mask base config. "reportVariants": ["alpha", "alpha"]

will produce internal "reportConfigs":

  "reportConfigs": [
    {
      "fileName": "fluid-framework.alpha.api.md",
      "variant": "alpha"
    },
    {
      "fileName": "fluid-framework.alpha.api.md",
      "variant": "alpha"
    }
  ],

but won't fail.

Details

Problem occurs in ExtractorConfig.ts line 614:

        // Merge extractorConfig into baseConfig, mutating baseConfig
        lodash.merge(baseConfig, configObject);
        configObject = baseConfig;

Standard questions

Please answer these questions to help us investigate your issue more quickly:

Question	Answer
@microsoft/api-extractor version?	7.45.1
Operating system?	Windows
API Extractor scenario?	reporting (.api.md)
Would you consider contributing a PR?	No
TypeScript compiler version?	5.4.2
Node.js version (node -v)?	18.20.2