feat(eslint-plugin): Add unified-signature rule (#178)

packages/eslint-plugin/README.md

@@ -152,5 +152,6 @@ Then you should add `airbnb` (or `airbnb-base`) to your `extends` section of `.e
 | [`@typescript-eslint/promise-function-async`](./docs/rules/promise-function-async.md)                     | Requires any function or method that returns a Promise to be marked async. (`promise-function-async` from TSLint)                                   | :heavy_check_mark: |          |
 | [`@typescript-eslint/restrict-plus-operands`](./docs/rules/restrict-plus-operands.md)                     | When adding two variables, operands must both be of type number or of type string. (`restrict-plus-operands` from TSLint)                           |                    |          |
 | [`@typescript-eslint/type-annotation-spacing`](./docs/rules/type-annotation-spacing.md)                   | Require consistent spacing around type annotations (`typedef-whitespace` from TSLint)                                                               | :heavy_check_mark: | :wrench: |
+| [`@typescript-eslint/unified-signatures`](./docs/rules/unified-signatures.md)                             | Warns for any two overloads that could be unified into one. (`unified-signatures` from TSLint)     |    |   |
 
 <!-- end rule list -->

packages/eslint-plugin/ROADMAP.md

@@ -34,7 +34,7 @@
 | [`promise-function-async`]        | ✅  | [`@typescript-eslint/promise-function-async`]        |
 | [`typedef`]                       | 🛑  | N/A                                                  |
 | [`typedef-whitespace`]            | ✅  | [`@typescript-eslint/type-annotation-spacing`]       |
-| [`unified-signatures`]            | 🛑  | N/A                                                  |
+| [`unified-signatures`]            | ✅  | [`@typescript-eslint/unified-signatures`]            |
 
 ### Functionality
 
@@ -589,6 +589,7 @@ Relevant plugins: [`chai-expect-keywords`](https://github.com/gavinaiken/eslint-
 [`@typescript-eslint/no-unnecessary-type-assertion`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/no-unnecessary-type-assertion.md
 [`@typescript-eslint/no-var-requires`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/no-var-requires.md
 [`@typescript-eslint/type-annotation-spacing`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/type-annotation-spacing.md
+[`@typescript-eslint/unified-signatures`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/unified-signatures.md
 [`@typescript-eslint/no-misused-new`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/no-misused-new.md
 [`@typescript-eslint/no-object-literal-type-assertion`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/no-object-literal-type-assertion.md
 [`@typescript-eslint/no-this-alias`]: https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/no-this-alias.md

packages/eslint-plugin/docs/rules/unified-signatures.md

@@ -0,0 +1,33 @@
+# Warns for any two overloads that could be unified into one by using a union or an optional/rest parameter. (unified-signatures)
+
+Warns for any two overloads that could be unified into one by using a union or an optional/rest parameter.
+
+## Rule Details
+
+This rule aims to keep the source code as maintanable as posible by reducing the amount of overloads.
+
+Examples of **incorrect** code for this rule:
+
+```ts
+function f(x: number): void;
+function f(x: string): void;
+```
+
+```ts
+f(): void;
+f(...x: number[]): void;
+```
+
+Examples of **correct** code for this rule:
+
+```ts
+function f(x: number | string): void;
+```
+
+```ts
+function f(x?: ...number[]): void;
+```
+
+## Related to
+
+- TSLint: ['unified-signatures`](https://palantir.github.io/tslint/rules/unified-signatures/)