main ul li { margin: 0.5em 0; }

<h2 id="introduction">DRAFT RELEASE NOTES — Introduction to Go 1.18</h2>

<strong>

Go 1.18 is not yet released. These are work-in-progress

release notes. Go 1.18 is expected to be released in February 2022.

</strong>

<h2 id="language">Changes to the language</h2>

<h3 id="generics">Generics</h3>

Go 1.18 includes an implementation of generic features as described by the

<a href="https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md">Type

Parameters Proposal</a>.

This includes major - but fully backward-compatible - changes to the language.

These new language changes required a large amount of new code that

has not had significant testing in production settings. That will

only happen as more people write and use generic code. We believe

that this feature is well implemented and high quality. However,

unlike most aspects of Go, we can't back up that belief with real

world experience. Therefore, while we encourage the use of generics

where it makes sense, please use appropriate caution when deploying

generic code in production.

The following is a list of the most visible changes. For a more comprehensive overview, see the

<a href="https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md">proposal</a>.

For details see the <a href="/ref/spec">language spec</a>.

<li>

The syntax for

<a href="/ref/spec#Function_declarations">function</a> and

<a href="/ref/spec#Type_declarations">type declarations</a>

now accepts

<a href="/ref/spec#Type_parameters">type parameters</a>.

</li>

<li>

Parameterized functions and types can be instantiated by following them with a list of

type arguments in square brackets.

</li>

<li>

The new token <code>~</code> has been added to the set of

<a href="/ref/spec#Operators_and_punctuation">operators and punctuation</a>.

</li>

<li>

The syntax for

<a href="/ref/spec#Interface_types">Interface types</a>

now permits the embedding of arbitrary types (not just type names of interfaces)

as well as union and <code>~T</code> type elements. Such interfaces may only be used

as type constraints.

An interface now defines a set of types as well as a set of methods.

</li>

<li>

The new

<a href="/ref/spec#Predeclared_identifiers">predeclared identifier</a>

<code>any</code> is an alias for the empty interface. It may be used instead of

<code>interface{}</code>.

</li>

<li>

The new

<a href="/ref/spec#Predeclared_identifiers">predeclared identifier</a>

<code>comparable</code> is an interface the denotes the set of all types which can be

compared using <code>==</code> or <code>!=</code>. It may only be used as (or embedded in)

a type constraint.

</li>

The current generics implementation has the following limitations:

<ul>

<li><!-- https://golang.org/issue/47631 -->

The Go compiler cannot currently handle type declarations inside generic functions

or methods. We hope to provide support for this feature in Go 1.19.

</li>

<li><!-- https://golang.org/issue/49030 -->

Embedding a type parameter, or a pointer to a type parameter, as

an unnamed field in a struct type is not permitted. Similarly

embedding a type parameter in an interface type is not permitted.

Whether these will ever be permitted is unclear at present.

</li>

<li>

A union element with more than one term may not contain an

interface type with a non-empty method set. Whether this will

ever be permitted is unclear at present.

</li>

</ul>

<h3 id="bug_fixes">Bug fixes</h3>

The Go 1.18 compiler now correctly reports <code>declared but not used</code> errors

for variables that are set inside a function literal but are never used. Before Go 1.18,

the compiler did not report an error in such cases. This fixes long-outstanding compiler

issue <a href="https://golang.org/issue/8560">#8560</a>. As a result of this change,

(possibly incorrect) programs may not compile anymore. The necessary fix is

straightforward: fix the program if it was in fact incorrect, or use the offending

variable, for instance by assigning it to the blank identifier <code>_</code>.

Since <code>go vet</code> always pointed out this error, the number of affected

programs is likely very small.

The Go 1.18 compiler now reports an overflow when passing a rune constant expression

such as <code>'1' &lt;&lt; 32</code> as an argument to the predeclared functions

<code>print</code> and <code>println</code>, consistent with the behavior of

user-defined functions. Before Go 1.18, the compiler did not report an error

in such cases but silently accepted such constant arguments if they fit into an

<code>int64</code>. As a result of this change, (possibly incorrect) programs

may not compile anymore. The necessary fix is straightforward: fix the program if it

was in fact incorrect, or explicitly convert the offending argument to the correct type.

Since <code>go vet</code> always pointed out this error, the number of affected

programs is likely very small.

<h2 id="ports">Ports</h2>

<h3 id="amd64">AMD64</h3>

Go 1.18 introduces the new <code>GOAMD64</code> environment variable, which selects

a mininum target version of the AMD64 architecture. Allowed values are <code>v1</code>,

<code>v2</code>, <code>v3</code>, or <code>v4</code>. Each higher level requires,

and takes advantage of, additional processor features. A detailed

description can be found

<a href="https://golang.org/wiki/MinimumRequirements#amd64">here</a>.

The <code>GOAMD64</code> environment variable defaults to <code>v1</code>.

<h3 id="riscv">RISC-V</h3>

The 64-bit RISC-V architecture on Linux (the <code>linux/riscv64</code> port)

now supports the <code>c-archive</code> and <code>c-shared</code> build modes.

<h3 id="windows">Windows</h3>

The <code>windows/arm</code> and <code>windows/arm64</code> ports now support

non-cooperative preemption, bringing that capability to all four Windows

ports, which should hopefully address subtle bugs encountered when calling

into Win32 functions that block for extended periods of time.

<h3 id="ios">iOS</h3>

On iOS (the <code>ios/arm64</code> port)

and iOS simulator running on AMD64-based macOS (the <code>ios/amd64</code> port),

Go 1.18 now requires iOS 12 or later; support for previous versions has been discontinued.

<h3 id="freebsd">FreeBSD</h3>

Go 1.18 is the last release that is supported on FreeBSD 11.x, which has

already reached end-of-life. Go 1.19 will require FreeBSD 12.2+ or FreeBSD

13.0+.

FreeBSD 13.0+ will require a kernel with the COMPAT_FREEBSD12 option set (this is the default).

<h2 id="tools">Tools</h2>

<h3 id="fuzzing">Fuzzing</h3>

Go 1.18 includes an implementation of fuzzing as described by

<a href="https://golang.org/issue/44551">the fuzzing proposal</a>.

See the <a href="https://go.dev/doc/fuzz">fuzzing landing page</a> to get

started.

Please be aware that fuzzing can consume a lot of memory and may impact your

machine’s performance while it runs. Also be aware that the fuzzing engine

writes values that expand test coverage to a fuzz cache directory within

<code>$GOCACHE/fuzz</code> while it runs. There is currently no limit to the

number of files or total bytes that may be written to the fuzz cache, so it

may occupy a large amount of storage (possibly several GBs).

<h3 id="go-command">Go command</h3>

<code>go</code> <code>get</code> no longer builds or installs packages in

module-aware mode. <code>go</code> <code>get</code> is now dedicated to

adjusting dependencies in <code>go.mod</code>. Effectively, the

<code>-d</code> flag is always enabled. To install the latest version

of an executable outside the context of the current module, use

<a href="https://golang.org/ref/mod#go-install"><code>go</code>

<code>install</code> <code>example.com/cmd@latest</code></a>. Any

<a href="https://golang.org/ref/mod#version-queries">version query</a>

may be used instead of <code>latest</code>. This form of <code>go</code>

<code>install</code> was added in Go 1.16, so projects supporting older

versions may need to provide install instructions for both <code>go</code>

<code>install</code> and <code>go</code> <code>get</code>. <code>go</code>

<code>get</code> now reports an error when used outside a module, since there

is no <code>go.mod</code> file to update. In GOPATH mode (with

<code>GO111MODULE=off</code>), <code>go</code> <code>get</code> still builds

and installs packages, as before.

The <code>go</code> command now embeds version control information in

binaries including the currently checked-out revision, commit time, and a

flag indicating whether edited or untracked files are present. Version

control information is embedded if the <code>go</code> command is invoked in

a directory within a Git, Mercurial, Fossil, or Bazaar repository, and the

<code>main</code> package and its containing main module are in the same

repository. This information may be omitted using the flag

<code>-buildvcs=false</code>.

Additionally, the <code>go</code> command embeds information about the build

including build and tool tags (set with <code>-tags</code>), compiler,

assembler, and linker flags (like <code>-gcflags</code>), whether cgo was

enabled, and if it was, the values of the cgo environment variables

(like <code>CGO_CFLAGS</code>). This information may be omitted using the

flag <code>-buildinfo=false</code>. Both VCS and build information may be

read together with module information using <code>go</code>

<code>version</code> <code>-m</code> <code>file</code> or

<code>runtime/debug.ReadBuildInfo</code> (for the currently running binary)

or the new <a href="#debug/buildinfo"><code>debug/buildinfo</code></a>

package.

The underlying data format of the embedded build information can change with

new go releases, so an older version of <code>go</code> may not handle the

build information produced with a newer version of <code>go</code>.

To read the version information from a binary built with <code>go</code> 1.18,

use the <code>go</code> <code>version</code> command and the

<code>debug/buildinfo</code> package from <code>go</code> 1.18+.

If the main module's <code>go.mod</code> file

specifies <a href="/ref/mod#go-mod-file-go"><code>go</code> <code>1.17</code></a>

or higher, <code>go</code> <code>mod</code> <code>download</code> without

arguments now downloads source code for only the modules

explicitly <a href="/ref/mod#go-mod-file-require">required</a> in the main

module's <code>go.mod</code> file. (In a <code>go</code> <code>1.17</code> or

higher module, that set already includes all dependencies needed to build the

packages and tests in the main module.)

To also download source code for transitive dependencies, use

<code>go</code> <code>mod</code> <code>download</code> <code>all</code>.

The <code>go</code> <code>mod</code> <code>vendor</code> subcommand now

supports a <code>-o</code> flag to set the output directory.

(Other <code>go</code> commands still read from the <code>vendor</code>

directory at the module root when loading packages

with <code>-mod=vendor</code>, so the main use for this flag is for

third-party tools that need to collect package source code.)

The <code>go</code> <code>build</code> command and related commands

now support an <code>-asan</code> flag that enables interoperation

with C (or C++) code compiled with the address sanitizer (C compiler

option <code>-fsanitize=address</code>).

The <code>go</code> command now supports a "Workspace" mode. If a

<code>go.work</code> file is found in the working directory or a

parent directory, or one is specified using the <code>-workfile</code>

flag, it will put the <code>go</code> command into workspace mode.

In workspace mode, the <code>go.work</code> file will be used to

determine the set of main modules used as the roots for module

resolution, instead of using the normally-found <code>go.mod</code>

file to specify the single main module.

The <code>go</code> command now supports additional command line

options for the new <a href="#fuzzing">fuzzing support described

above</a>:

<ul>

<li>

<code>go test</code> supports

<code>-fuzz</code>, <code>-fuzztime</code>, and

<code>-fuzzminimizetime</code> options.

For documentation on these see

<a href="/pkg/cmd/go#hdr-Testing_flags"><code>go help testflag</code></a>.

</li>

<li>

<code>go clean</code> supports a <code>-fuzzcache</code>

option.

For documentation see

<a href="/pkg/cmd/go#hdr-Remove_object_files_and_cached_files"><code>go help clean</code></a>.

</li>

</ul>

<h3 id="gofmt"><code>gofmt</code></h3>

<code>gofmt</code> now reads and formats input files concurrently, with a

memory limit proportional to <code>GOMAXPROCS</code>. On a machine with

multiple CPUs, <code>gofmt</code> should now be significantly faster.

<h3 id="vet"><code>vet</code></h3>

<h4 id="vet-generics">Updates for Generics</h4>

The <code>vet</code> tool is updated to support generic code. In most cases,

it reports an error in generic code whenever it would report an error in the

equivalent non-generic code after substituting for type parameters with a

type from their

<a href="https://golang.org/ref/spec#Interface_types">type set</a>.

For example, <code>vet</code> reports a format error in

<pre>func Print[T ~int|~string](t T) {

fmt.Printf("%d", t)

}</pre>

because it would report a format error in the non-generic equivalent of

<code>Print[string]</code>:

<pre>func PrintString(x string) {

fmt.Printf("%d", x)

}</pre>

</p>

<h4 id="vet-precision">Precision improvements for existing checkers</h4>

The <code>cmd/vet</code> checkers <code>copylock</code>, <code>printf</code>,

<code>sortslice</code>, <code>testinggoroutine</code>, and <code>tests</code>

have all had moderate precision improvements to handle additional code patterns.

This may lead to newly reported errors in existing packages. For example, the

<code>printf</code> checker now tracks formatting strings created by

concatenating string constants. So <code>vet</code> will report an error in:

// fmt.Printf formatting directive %d is being passed to Println.

fmt.Println("%d"+` ≡ x (mod 2)`+"\n", x%2)

</p>

<h2 id="runtime">Runtime</h2>

The garbage collector now includes non-heap sources of garbage collector work

(e.g., stack scanning) when determining how frequently to run. As a result,

garbage collector overhead is more predictable when these sources are

significant. For most applications these changes will be negligible; however,

some Go applications may now use less memory and spend more time on garbage

collection, or vice versa, than before. The intended workaround is to tweak

<code>GOGC</code> where necessary.

The runtime now returns memory to the operating system more efficiently and has

been tuned to work more aggressively as a result.

Go 1.17 generally improved the formatting of arguments in stack traces,

but could print inaccurate values for arguments passed in registers.

This is improved in Go 1.18 by printing a question mark (<code>?</code>)

after each value that may be inaccurate.

<h2 id="compiler">Compiler</h2>

Go 1.17 <a href="go1.17#compiler">implemented</a> a new way of passing

function arguments and results using registers instead of the stack

on 64-bit x86 architecture on selected operating systems.

Go 1.18 expands the supported platforms to include 64-bit ARM (<code>GOARCH=arm64</code>),

big- and little-endian 64-bit PowerPC (<code>GOARCH=ppc64</code>, <code>ppc64le</code>),

as well as 64-bit x86 architecture (<code>GOARCH=amd64</code>)

on all operating systems.

On 64-bit ARM and 64-bit PowerPC systems, benchmarking shows

typical performance improvements of 10% or more.

As <a href="go1.17#compiler">mentioned</a> in the Go 1.17 release notes,

this change does not affect the functionality of any safe Go code and

is designed to have no impact on most assembly code. See the

<a href="go1.17#compiler">Go 1.17 release notes</a> for more details.

The compiler now can inline functions that contain range loops or

labeled for loops.

The new compiler <code>-asan</code> option supports the

new <code>go</code> command <code>-asan</code> option.

<p> <!-- https://github.com/golang/go/issues/49569 -->

Because of changes in the compiler related to supporting generics, the

Go 1.18 compile speed can be roughly 15% slower than the Go 1.17 compile speed.

The execution time of the compiled code is not affected. We

intend to improve the speed of the compiler in Go 1.19.

<h2 id="linker">Linker</h2>

The new linker <code>-asan</code> option supports the

new <code>go</code> command <code>-asan</code> option.

<h2 id="library">Core library</h2>

<h3 id="constraints">New <code>constraints</code> package</h3>

The new <a href="/pkg/constraints/"><code>constraints</code></a> package

defines a set of useful constraints that can be used with type parameters of

generic functions.

<h3 id="debug/buildinfo">New <code>debug/buildinfo</code> package</h3>

The new <a href="/pkg/debug/buildinfo"><code>debug/buildinfo</code></a> package

provides access to module versions, version control information, and build

flags embedded in executable files built by the <code>go</code> command.

The same information is also available via

<a href="/pkg/runtime/debug#ReadBuildInfo"><code>runtime/debug.ReadBuildInfo</code></a>

for the currently running binary and via <code>go</code>

<code>version</code> <code>-m</code> on the command line.

<h3 id="netip">New <code>net/netip</code> package</h3>

The new <a href="/pkg/net/netip/"><code>net/netip</code></a>

package defines a new IP address type, <a href="/pkg/net/netip/#Addr"><code>Addr</code></a>.

Compared to the existing

<a href="/pkg/net/#IP"><code>net.IP</code></a> type, the <code>netip.Addr</code> type takes less

memory, is immutable, and is comparable so it supports <code>==</code>

and can be used as a map key.

In addition to <code>Addr</code>, the package defines

<a href="/pkg/net/netip/#AddrPort"><code>AddrPort</code></a>, representing

an IP and port, and

<a href="/pkg/net/netip/#Prefix"><code>Prefix</code></a>, representing

a network CIDR prefix.

The package also defines several functions to create and examine

these new types:

<a href="/pkg/net/netip#AddrFrom4"><code>AddrFrom4</code></a>,

<a href="/pkg/net/netip#AddrFrom16"><code>AddrFrom16</code></a>,

<a href="/pkg/net/netip#AddrFromSlice"><code>AddrFromSlice</code></a>,

<a href="/pkg/net/netip#AddrPortFrom"><code>AddrPortFrom</code></a>,

<a href="/pkg/net/netip#IPv4Unspecified"><code>IPv4Unspecified</code></a>,

<a href="/pkg/net/netip#IPv6LinkLocalAllNodes"><code>IPv6LinkLocalAllNodes</code></a>,

<a href="/pkg/net/netip#IPv6Unspecified"><code>IPv6Unspecified</code></a>,

<a href="/pkg/net/netip#MustParseAddr"><code>MustParseAddr</code></a>,

<a href="/pkg/net/netip#MustParseAddrPort"><code>MustParseAddrPort</code></a>,

<a href="/pkg/net/netip#MustParsePrefix"><code>MustParsePrefix</code></a>,

<a href="/pkg/net/netip#ParseAddr"><code>ParseAddr</code></a>,

<a href="/pkg/net/netip#ParseAddrPort"><code>ParseAddrPort</code></a>,

<a href="/pkg/net/netip#ParsePrefix"><code>ParsePrefix</code></a>,

<a href="/pkg/net/netip#PrefixFrom"><code>PrefixFrom</code></a>.

The <a href="/pkg/net/"><code>net</code></a> package includes new

methods that parallel existing methods, but

return <code>netip.AddrPort</code> instead of the

heavier-weight <a href="/pkg/net/#IP"><code>net.IP</code></a> or

<a href="/pkg/net/#UDPAddr"><code>*net.UDPAddr</code></a> types:

<a href="/pkg/net/#Resolver.LookupNetIP"><code>Resolver.LookupNetIP</code></a>,

<a href="/pkg/net/#UDPConn.ReadFromUDPAddrPort"><code>UDPConn.ReadFromUDPAddrPort</code></a>,

<a href="/pkg/net/#UDPConn.ReadMsgUDPAddrPort"><code>UDPConn.ReadMsgUDPAddrPort</code></a>,

<a href="/pkg/net/#UDPConn.WriteToUDPAddrPort"><code>UDPConn.WriteToUDPAddrPort</code></a>,

<a href="/pkg/net/#UDPConn.WriteMsgUDPAddrPort"><code>UDPConn.WriteMsgUDPAddrPort</code></a>.

The new <code>UDPConn</code> methods support allocation-free I/O.

The <code>net</code> package also now includes functions and methods

to convert between the existing

<a href="/pkg/net/#TCPAddr"><code>TCPAddr</code></a>/<a href="/pkg/net/#UDPAddr"><code>UDPAddr</code></a>

types and <code>netip.AddrPort</code>:

<a href="/pkg/net/#TCPAddrFromAddrPort"><code>TCPAddrFromAddrPort</code></a>,

<a href="/pkg/net/#UDPAddrFromAddrPort"><code>UDPAddrFromAddrPort</code></a>,

<a href="/pkg/net/#TCPAddr.AddrPort"><code>TCPAddr.AddrPort</code></a>,

<a href="/pkg/net/#UDPAddr.AddrPort"><code>UDPAddr.AddrPort</code></a>.

<h3 id="minor_library_changes">Minor changes to the library</h3>

As always, there are various minor changes and updates to the library,

made with the Go 1 <a href="/doc/go1compat">promise of compatibility</a>

in mind.

<dl id="bufio"><dt><a href="/pkg/bufio/">bufio</a></dt>

<dd>

<p><!-- CL 345569 -->

The new <a href="/pkg/bufio#Writer.AvailableBuffer"><code>Writer.AvailableBuffer</code></a>

method returns an empty buffer with a possibly non-empty capacity for use

with append-like APIs. After appending, the buffer can be provided to a

succeeding <code>Write</code> call and possibly avoid any copying.

</p>

<p><!-- CL 345570 -->

The methods <a href="/pkg/bufio#Reader.Reset"><code>Reader.Reset</code></a> and

<a href="/pkg/bufio#Writer.Reset"><code>Writer.Reset</code></a>

now use the default buffer size when called on objects with a

<code>nil</code> buffer.

</p>

</dd>

<dl id="bytes"><dt><a href="/pkg/bytes/">bytes</a></dt>

<dd>

<p><!-- CL 351710 -->

The new <a href="/pkg/bytes/#Cut"><code>Cut</code></a> function

slices a <code>[]byte</code> around a separator. It can replace

and simplify many common uses of

<a href="/pkg/bytes/#Index"><code>Index</code></a>,

<a href="/pkg/bytes/#IndexByte"><code>IndexByte</code></a>,

<a href="/pkg/bytes/#IndexRune"><code>IndexRune</code></a>,

and <a href="/pkg/bytes/#SplitN"><code>SplitN</code></a>.

</p>

<p><!-- CL 323318, CL 332771 -->

<a href="/pkg/bytes/#Trim"><code>Trim</code></a>, <a href="/pkg/bytes/#TrimLeft"><code>TrimLeft</code></a>,

and <a href="/pkg/bytes/#TrimRight"><code>TrimRight</code></a> are now allocation free and, especially for

small ASCII cutsets, up to 10 times faster.

</p>

<p><!-- CL 359485 -->

The <a href="/pkg/bytes/#Title"><code>Title</code></a> function is now deprecated. It doesn't

handle Unicode punctuation and language-specific capitalization rules, and is superseded by the

<a href="https://golang.org/x/text/cases">golang.org/x/text/cases</a> package.

</p>

</dd>

<dl id="crypto/tls"><dt><a href="/pkg/crypto/tls/">crypto/tls</a></dt>

<dd>

<p><!-- CL 325250 -->

The new <a href="/pkg/crypto/tls/#Conn.NetConn"><code>Conn.NetConn</code></a>

method allows access to the underlying

<a href="/pkg/net#Conn"><code>net.Conn</code></a>.

</p>

</dd>

<dl id="debug/elf"><dt><a href="/pkg/debug/elf/">debug/elf</a></dt>

<dd>

<p><!-- CL 352829 -->

The <a href="/pkg/debug/elf/#R_PPC64_RELATIVE"><code>R_PPC64_RELATIVE</code></a>

constant has been added.

</p>

</dd>

<dl id="debug/plan9obj"><dt><a href="/pkg/debug/plan9obj/">debug/plan9obj</a></dt>

<dd>

<p><!-- CL 350229 -->

The <a href="/pkg/debug/plan9obj#File.Symbols">File.Symbols</a>

method now returns the new exported error

value <a href="/pkg/debug/plan9obj#ErrNoSymbols">ErrNoSymbols</a>

if the file has no symbol section.

</p>

</dd>

<dl id="go/ast"><dt><a href="/pkg/go/ast/">go/ast</a></dt>

<dd>

<p><!-- https://golang.org/issue/47781, CL 325689, CL 327149, CL 348375, CL 348609 -->

Per the proposal

<a href="https://go.googlesource.com/proposal/+/master/design/47781-parameterized-go-ast.md">

Additions to go/ast and go/token to support parameterized functions and types

</a>

the following additions are made to the <a href="/pkg/go/ast"><code>go/ast</code></a> package:

<ul>

<li>

the <a href="/pkg/go/ast/#FuncType"><code>FuncType</code></a>

and <a href="/pkg/go/ast/#TypeSpec"><code>TypeSpec</code></a>

nodes have a new field <code>TypeParams</code> to hold type parameters, if any.

</li>

<li>

The new expression node <a href="/pkg/go/ast/#IndexListExpr"><code>IndexListExpr</code></a>

represents index expressions with multiple indices, used for function and type instantiations

with more than one explicit type argument.

</li>

</ul>

</p>

</dd>

<dl id="go/constant"><dt><a href="/pkg/go/constant/">go/constant</a></dt>

<dd>

<p><!-- https://golang.org/issue/46211, CL 320491 -->

The new <a href="/pkg/go/constant/#Kind.String"><code>Kind.String</code></a>

method returns a human-readable name for the receiver kind.

</p>

</dd>

<dl id="go/token"><dt><a href="/pkg/go/token/">go/token</a></dt>

<dd>

<p><!-- https://golang.org/issue/47781, CL 324992 -->

The new constant <a href="/pkg/go/token/#TILDE"><code>TILDE</code></a>

represents the <code>~</code> token per the proposal

<a href="https://go.googlesource.com/proposal/+/master/design/47781-parameterized-go-ast.md">

Additions to go/ast and go/token to support parameterized functions and types

</a>.

</p>

</dd>

<dl id="go/types"><dt><a href="/pkg/go/types/">go/types</a></dt>

<dd>

<p><!-- https://golang.org/issue/46648 -->

The new <a href="/pkg/go/types/#Config.GoVersion"><code>Config.GoVersion</code></a>

field sets the accepted Go language version.

</p>

<p><!-- https://golang.org/issue/47916 -->

Per the proposal

<a href="https://go.googlesource.com/proposal/+/master/design/47916-parameterized-go-types.md">

Additions to go/types to support type parameters

</a>

the following additions are made to the <a href="/pkg/go/types"><code>go/types</code></a> package:

</p>

<ul>

<li>

The new type

<a href="/pkg/go/types/#TypeParam"><code>TypeParam</code></a>, factory function

<a href="/pkg/go/types/#NewTypeParam"><code>NewTypeParam</code></a>,

and associated methods are added to represent a type parameter.

</li>

<li>

The new type

<a href="/pkg/go/types/#TypeParamList"><code>TypeParamList</code></a> holds a list of

type parameters.

</li>

<li>

The new type

<a href="/pkg/go/types/#TypeList"><code>TypeList</code></a> holds a list of types.

</li>

<li>

The new factory function

<a href="/pkg/go/types/#NewSignatureType"><code>NewSignatureType</code></a> allocates a

<a href="/pkg/go/types/#Signature"><code>Signature</code></a> with

(receiver or function) type parameters.

To access those type parameters, the <code>Signature</code> type has two new methods

<a href="/pkg/go/types/#Signature.RecvTypeParams"><code>Signature.RecvTypeParams</code></a> and

<a href="/pkg/go/types/#Signature.TypeParams"><code>Signature.TypeParams</code></a>.

</li>

<li>

<a href="/pkg/go/types/#Named"><code>Named</code></a> types have four new methods:

<a href="/pkg/go/types/#Named.Origin"><code>Named.Origin</code></a> to get the original

parameterized types of instantiated types,

<a href="/pkg/go/types/#Named.TypeArgs"><code>Named.TypeArgs</code></a> and

<a href="/pkg/go/types/#Named.TypeParams"><code>Named.TypeParams</code></a> to get the

type arguments or type parameters of an instantiated or parameterized type, and

<a href="/pkg/go/types/#Named.TypeParams"><code>Named.SetTypeParams</code></a> to set the

type parameters (for instance, when importing a named type where allocation of the named

type and setting of type parameters cannot be done simultaneously due to possible cycles).

</li>

<li>

The <a href="/pkg/go/types/#Interface"><code>Interface</code></a> type has four new methods:

<a href="/pkg/go/types/#Interface.IsComparable"><code>Interface.IsComparable</code></a> and

<a href="/pkg/go/types/#Interface.IsMethodSet"><code>Interface.IsMethodSet</code></a> to

query properties of the type set defined by the interface, and

<a href="/pkg/go/types/#Interface.MarkImplicit"><code>Interface.MarkImplicit</code></a> and

<a href="/pkg/go/types/#Interface.IsImplicit"><code>Interface.IsImplicit</code></a> to set

and test whether the interface is an implicit interface around a type constraint literal.

</li>

<li>

The new types

<a href="/pkg/go/types/#Union"><code>Union</code></a> and

<a href="/pkg/go/types/#Term"><code>Term</code></a>, factory functions

<a href="/pkg/go/types/#NewUnion"><code>NewUnion</code></a> and

<a href="/pkg/go/types/#NewTerm"><code>NewTerm</code></a>, and associated

methods are added to represent type sets in interfaces.

</li>

<li>

The new function

<a href="/pkg/go/types/#Instantiate"><code>Instantiate</code></a>

instantiates a parameterized type.

</li>

<li>

The new <a href="/pkg/go/types/#Info.Instances"><code>Info.Instances</code></a>

map records function and type instantiations through the new

<a href="/pkg/go/types/#Instance"><code>Instance</code></a> type.

</li>

<li><!-- CL 342671 -->

The new type <a href="/pkg/go/types/#ArgumentError"><code>ArgumentError</code></a>

and associated methods are added to represent an error related to a type argument.

</li>

<li><!-- CL 353089 -->

The new type <a href="/pkg/go/types/#Context"><code>Context</code></a> and factory function

<a href="/pkg/go/types/#NewContext"><code>NewContext</code></a>

are added to facilitate sharing of identical type instances

across type-checked packages, via the new

<a href="/pkg/go/types/#Config.Context"><code>Config.Context</code></a>

field.

</li>

</ul>

</dd>

<dl id="html/template"><dt><a href="/pkg/html/template/">html/template</a></dt>

<dd>

<p><!-- CL 321491 -->

Within a <code>range</code> pipeline the new

<code>{{break}}</code> command will end the loop early and the

new <code>{{continue}}</code> command will immediately start the

next loop iteration.

</p>

</dd>

<dl id="image/draw"><dt><a href="/pkg/image/draw/">image/draw</a></dt>

<dd>

<p><!-- CL 340049 -->

The <code>Draw</code> and <code>DrawMask</code> fallback implementations

(used when the arguments are not the most common image types) are now

faster when those arguments implement the optional

<a href="/pkg/image/draw/#RGBA64Image"><code>draw.RGBA64Image</code></a>

and <a href="/pkg/image/#RGBA64Image"><code>image.RGBA64Image</code></a>

interfaces that were added in Go 1.17.

</p>

</dd>

<dl id="net"><dt><a href="/pkg/net/">net</a></dt>

<dd>

<p><!-- CL 340261 -->

<a href="/pkg/net#Error"><code>net.Error.Temporary</code></a> has been deprecated.

</p>

</dd>

<dl id="net/http"><dt><a href="/pkg/net/http/">net/http</a></dt>

<dd>

<p><!-- CL 330852 -->

On WebAssembly targets, the <code>Dial</code>, <code>DialContext</code>,

<code>DialTLS</code> and <code>DialTLSContext</code> method fields in

<a href="/pkg/net/http/#Transport"><code>Transport</code></a>

will now be correctly used, if specified, for making HTTP requests.

</p>

<p><!-- CL 338590 -->

The new

<a href="/pkg/net/http#Cookie.Valid"><code>Cookie.Valid</code></a>

method reports whether the cookie is valid.

</p>

<p><!-- CL 346569 -->

The new

<a href="/pkg/net/http#MaxBytesHandler"><code>MaxBytesHandler</code></a>

function creates a <code>Handler</code> that wraps its

<code>ResponseWriter</code> and <code>Request.Body</code> with a

<a href="/pkg/net/http#MaxBytesReader"><code>MaxBytesReader</code></a>.

</p>

</dd>

<dl id="os/user"><dt><a href="/pkg/os/user/">os/user</a></dt>

<dd>

<p><!-- CL 330753 -->

<a href="/pkg/os/user#User.GroupIds"><code>User.GroupIds</code></a>.

now uses a Go native implementation when cgo is not available.

</p>

</dd>

<dl id="reflect"><dt><a href="/pkg/reflect/">reflect</a></dt>

<dd>

<p><!-- CL 356049, CL 320929 -->

The new

<a href="/pkg/reflect/#Value.SetIterKey"><code>Value.SetIterKey</code></a>

and <a href="/pkg/reflect/#Value.SetIterValue"><code>Value.SetIterValue</code></a>

methods set a Value using a map iterator as the source. They are equivalent to

<code>Value.Set(iter.Key())</code> and <code>Value.Set(iter.Value())</code> but

do fewer allocations.

</p>

<p><!-- CL 350691 -->

The new

<a href="/pkg/reflect/#Value.UnsafePointer"><code>Value.UnsafePointer</code></a>

method returns the Value's value as an <a href="/pkg/unsafe/#Pointer"><code>unsafe.Pointer</code></a>.

This allows callers to migrate from <a href="/pkg/reflect/#Value.UnsafeAddr"><code>Value.UnsafeAddr</code></a>

and <a href="/pkg/reflect/#Value.Pointer"><code>Value.Pointer</code></a>

to eliminate the need to perform uintptr to unsafe.Pointer conversions at the callsite (as unsafe.Pointer rules require).

</p>

<p><!-- CL 321891 -->

The new

<a href="/pkg/reflect/#MapIter.Reset"><code>MapIter.Reset</code></a>

method changes its receiver to iterate over a

different map. The use of

<a href="/pkg/reflect/#MapIter.Reset"><code>MapIter.Reset</code></a>

allows allocation-free iteration

over many maps.

</p>

<p><!-- CL 352131 -->

A number of methods (

<a href="/pkg/reflect#Value.CanInt"><code>Value.CanInt</code></a>,

<a href="/pkg/reflect#Value.CanUint"><code>Value.CanUint</code></a>,

<a href="/pkg/reflect#Value.CanFloat"><code>Value.CanFloat</code></a>,

<a href="/pkg/reflect#Value.CanComplex"><code>Value.CanComplex</code></a>

)

have been added to

<a href="/pkg/reflect#Value"><code>Value</code></a>

to test if a conversion is safe.

</p>

<p><!-- CL 357962 -->

<a href="/pkg/reflect#Value.FieldByIndexErr"><code>Value.FieldByIndexErr</code></a>

has been added to avoid the panic that occurs in

<a href="/pkg/reflect#Value.FieldByIndex"><code>Value.FieldByIndex</code></a>