Package overrides (#27)

* Add a package overrides feature.

* Formatting

* Update 0000-package-overrides.md

* Adding an additional question to resolve

* Switching 'override' for 'replace'

* Update 0000-package-overrides.md

* Rename 0000-package-overrides.md to 0009-package-overrides.md

Author: aeschright

accepted/0009-package-overrides.md

@@ -0,0 +1,42 @@
+# Implement a package override option
+
+## Summary
+
+Add a section to package.json that allows the user to designate packages as an 'override' or replacement at a particular version.
+
+## Motivation
+
+In some situations, allowing more than one version of a package to be installed at the same time can cause unwanted behaviors. This will allow users to fix that by explicitly setting a single version to use.
+
+## Detailed Explanation
+
+Users will be able to add an additional dependency section called 'replace' where they give a list of one or more packages to be installed at a specific version. Having this section will ensure that the only version of that package that is eventually installed to the project is the one set as an replacement. This will not affect the name, source, path, or other aspects of the package's installation.
+
+## Rationale and Alternatives
+
+Users don't frequently need to do this sort of override/replacement because npm's nested tree structure allows for a collection of dependencies to require incompatible semvers for the same package. In many cases, the default behavior is acceptable and nothing further is needed. In other, more problematic situations, the user may encounter conflicting binaries or other components that cannot be used simultaneously. The only current options are to attempt to get the maintainers of the dependencies to make changes that will reduce these conflicts, or to use a tool like `https://www.npmjs.com/package/replace-deep-dep` to repair the requirements.
+
+## Implementation
+
+In order to do this, the npm installer will need to read in an replacement section from `package.json`. This will have a simple format such as `replace: { 'foo': '1.2.3' }`. These values will need to be used at the end of the tree-building process to select the final version. It will be applied in a 'flat' way -- the same version will be selected for all sub-dependencies that require that package.
+
+Users will not be allowed to `npm publish` a package with this kind of dependency (we may choose to revise this behavior later). If someone installs a package that does have a 'replace' section, we'll warn them that this may have unexpected results.
+
+## Prior Art
+
+  - [Dart](https://www.dartlang.org/tools/pub/dependencies#dependency-overrides) uses a dependency override option that allows defining a specific source or version.
+  - [Bower](https://github.com/bower/spec/blob/master/json.md#resolutions) has a setting for 'resolutions', which are package versions to fall back to if there's a conflict.
+  - [Yarn](https://yarnpkg.com/en/docs/package-json#toc-resolutions) has a similar resolutions setting, which allows choosing a version or a file source. It allows a more complex system of selecting which packages the override will be applied to.
+
+Our goal in this RFC is to provide the simplest possible solution to a less common but impactful problem. The most important part of this feature is to define a safe fallback for package versions that conflict.
+
+## Questions and Bikeshedding
+
+- Will a warning be sufficient if someone installs a package that contains an override in its own package.json?
+  - Yes, that's fine. Or no warning is fine (just a verbose log or somesuch)
+- Do we want a `--force` option to allow publishing a package with this in place?
+  - Yes.
+- Are there other workarounds users are likely to try, or edge cases to consider?
+  - Not aside from replace-deep-dep. It's fairly tricky to do without it.
+- Should this be applied to devDependencies, optionally or by default?
+  - Yes. By default.