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Turn common bugs into individual sections #294

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Turn common bugs into individual sections #294

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55 changes: 37 additions & 18 deletions FAQ.md
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<!-- START doctoc generated TOC please keep comment here to allow auto update -->
<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->


- [Common "Bugs" That Aren't Bugs](#common-bugs-that-arent-bugs)
- [These two empty classes can be used in place of each other.](#these-two-empty-classes-can-be-used-in-place-of-each-other)
- [I can use a non-`void`-returning function where one returning `void` is expected.](#i-can-use-a-non-void-returning-function-where-one-returning-void-is-expected)
- [I'm allowed to use a shorter parameter list where a longer one is expected.](#im-allowed-to-use-a-shorter-parameter-list-where-a-longer-one-is-expected)
- [`private` class members are actually visible at runtime.](#private-class-members-are-actually-visible-at-runtime)
- [This conditional type returns `never` when it should return the true branch.](#this-conditional-type-returns-never-when-it-should-return-the-true-branch)
- [This mapped type returns a primitive type, not an object type.](#this-mapped-type-returns-a-primitive-type-not-an-object-type)
- [A method and a function property of the same type behave differently.](#a-method-and-a-function-property-of-the-same-type-behave-differently)
- [Common Feature Requests](#common-feature-requests)
- [Type System Behavior](#type-system-behavior)
- [What is structural typing?](#what-is-structural-typing)
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Here are some behaviors that may look like bugs, but aren't.

* These two empty classes can be used in place of each other
* See the [FAQ Entry on this page](#why-do-these-empty-classes-behave-strangely)
* I can use a non-`void`-returning function where one returning `void` is expected
* See the [FAQ Entry on this page](#why-are-functions-returning-non-void-assignable-to-function-returning-void)
* Prior discussion at #4544
* I'm allowed to use a shorter parameter list where a longer one is expected
* See the [FAQ Entry on this page](#why-are-functions-with-fewer-parameters-assignable-to-functions-that-take-more-parameters)
* Prior discussion at #370, #9300, #9765, #9825, #13043, #16871, #13529, #13977, #17868, #20274, #20541, #21868, #26324, #30876
* `private` class members are actually visible at runtime
* See the [FAQ Entry on this page](#you-should-emit-classes-like-this-so-they-have-real-private-members) for a commonly suggested "fix"
* Prior discussion at #564, #1537, #2967, #3151, #6748, #8847, #9733, #11033
* This conditional type returns `never` when it should return the true branch.
* See this [issue](https://github.com/microsoft/TypeScript/issues/31751) for discussion about _distributive conditional types_.
* This mapped type returns a primitive type, not an object type.
* Mapped types declared as `{ [ K in keyof T ]: U }` where T is a type parameter are known as _homomorphic mapped types_, which means that the mapped type is a structure preserving function of `T`. When type parameter `T` is instantiated with a primitive type the mapped type evaluates to the same primitive.
* A method and a function property of the same type behave differently.
* Methods are always bivariant in their argument, while function properties are contravariant in their argument under `strictFunctionTypes`. More discussion [here](https://github.com/microsoft/TypeScript/pull/18654).
### These two empty classes can be used in place of each other.

See the [FAQ Entry on this page](#why-do-these-empty-classes-behave-strangely).

### I can use a non-`void`-returning function where one returning `void` is expected.

* See the [FAQ Entry on this page](#why-are-functions-returning-non-void-assignable-to-function-returning-void)
* Prior discussion at #4544

### I'm allowed to use a shorter parameter list where a longer one is expected.

* See the [FAQ Entry on this page](#why-are-functions-with-fewer-parameters-assignable-to-functions-that-take-more-parameters)
* Prior discussion at #370, #9300, #9765, #9825, #13043, #16871, #13529, #13977, #17868, #20274, #20541, #21868, #26324, #30876

### `private` class members are actually visible at runtime.

* See the [FAQ Entry on this page](#you-should-emit-classes-like-this-so-they-have-real-private-members) for a commonly suggested "fix"
* Prior discussion at #564, #1537, #2967, #3151, #6748, #8847, #9733, #11033

### This conditional type returns `never` when it should return the true branch.

See this [issue](https://github.com/microsoft/TypeScript/issues/31751) for discussion about _distributive conditional types_.

### This mapped type returns a primitive type, not an object type.

Mapped types declared as `{ [ K in keyof T ]: U }` where T is a type parameter are known as _homomorphic mapped types_, which means that the mapped type is a structure preserving function of `T`. When type parameter `T` is instantiated with a primitive type the mapped type evaluates to the same primitive.

### A method and a function property of the same type behave differently.

Methods are always bivariant in their argument, while function properties are contravariant in their argument under `strictFunctionTypes`. More discussion [here](https://github.com/microsoft/TypeScript/pull/18654).

## Common Feature Requests
> I want to request one of the following features...
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