# Type inference

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## Table of contents

-   [Overview](#overview)
-   [Open questions](#open-questions)
    -   [Inferring a variable type from literals](#inferring-a-variable-type-from-literals)
-   [Alternatives considered](#alternatives-considered)
-   [References](#references)

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## Overview

[Type inference](https://en.wikipedia.org/wiki/Type_inference) occurs in Carbon
when the `auto` keyword is used. This may occur in
[variable declarations](variables.md) or [function declarations](functions.md).

At present, type inference is very simple: given the expression which generates
the value to be used for type inference, the inferred type is the precise type
of that expression. For example, the inferred type for `auto` in
`fn Foo(x: i64) -> auto { return x; }` is `i64`.

Type inference is currently supported for [function return types](functions.md)
and [declared variable types](variables.md).

## Open questions

### Inferring a variable type from literals

Using the type on the right side for `var y: auto = 1` currently results in a
constant `IntLiteral(1)` value, whereas most languages would suggest a variable
integer type, such as `i64`. Carbon might also make it an error. Although type
inference currently only addresses `auto` for variables and function return
types, this is something that will be considered as part of type inference in
general, because it also affects generics, templates, lambdas, and return types.

## Alternatives considered

-   [Use `_` instead of `auto`](/proposals/p0851.md#use-_-instead-of-auto)

## References

-   Proposal
    [#851: auto keyword for vars](https://github.com/carbon-language/carbon-lang/pull/851)