design: add Go 2 transitions background document
Updates golang/go#28221
Change-Id: I16cd63ed1ae8e816a0a991524c6192223506d9b6
Reviewed-on: https://go-review.googlesource.com/c/142417
Reviewed-by: Robert Griesemer <gri@golang.org>
diff --git a/design/28221-go2-transitions.md b/design/28221-go2-transitions.md
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+# Proposal: Go 2 transition
+
+Author: Ian Lance Taylor
+
+Last update: October 15, 2018
+
+## Abstract
+
+A proposal for how to make incompatible changes from Go 1 to Go 2
+while breaking as little as possible.
+
+## Background
+
+Currently the Go language and standard libraries are covered by the
+[Go 1 compatibility guarantee](https://golang.org/doc/go1compat).
+The goal of that document was to promise that new releases of Go would
+not break existing working programs.
+
+Among the goals for the Go 2 process is to consider changes to the
+language and standard libraries that will break the guarantee.
+Since Go is used in a distributed open source environment, we cannot
+rely on a [flag
+day](http://www.catb.org/jargon/html/F/flag-day.html).
+We must permit the interoperation of different packages written using
+different versions of Go.
+
+Every language goes through version transitions.
+As background, here are some notes on what other languages have done.
+Feel free to skip the rest of this section.
+
+### C
+
+C language versions are driven by the ISO standardization process.
+C language development has paid close attention to backward
+compatibility.
+After the first ISO standard, C90, every subsequent standard has
+maintained strict backward compatibility.
+Where new keywords have been introduced, they are introduced in a
+namespace reserved by C90 (an underscore followed by an uppercase
+ASCII letter) and are made more accessible via a `#define` macro in a
+header file that did not previously exist (examples are `_Complex`,
+defined as `complex` in `<complex.h>`, and `_Bool`, defined as `bool`
+in `<stdbool.h>`).
+None of the basic language semantics defined in C90 have changed.
+
+In addition, most C compilers provide options to define precisely
+which version of the C standard the code should be compiled for (for
+example, `-std=c90`).
+Most standard library implementations support feature macros that may
+be #define’d before including the header files to specify exactly
+which version of the library should be provided (for example,
+`_ISOC99_SOURCE`).
+While these features have had bugs, they are fairly reliable and are
+widely used.
+
+A key feature of these options is that code compiled at different
+language/library versions can in general all be linked together and
+work as expected.
+
+The first standard, C90, did introduce breaking changes to the
+previous C language implementations, known informally as K&R C.
+New keywords were introduced, such as `volatile` (actually that might
+have been the only new keyword in C90).
+The precise implementation of integer promotion in integer expressions
+changed from unsigned-preserving to value-preserving.
+Fortunately it was easy to detect code using the new keywords due to
+compilation errors, and easy to adjust that code.
+The change in integer promotion actually made it less surprising to
+naive users, and experienced users mostly used explicit casts to
+ensure portability among systems with different integer sizes, so
+while there was no automatic detection of problems not much code broke
+in practice.
+
+There were also some irritating changes.
+C90 introduced trigraphs, which changed the behavior of some string
+constants.
+Compilers adapted with options like -no-trigraphs and -Wtrigraphs.
+
+More seriously, C90 introduced the notion of undefined behavior, and
+declared that programs that invoked undefined behavior might take
+any action.
+In K&R C, the cases that C90 described as undefined behavior were
+mostly treated as what C90 called implementation-defined behavior: the
+program would take some non-portable but predictable action.
+Compiler writers absorbed the notion of undefined behavior, and
+started writing optimizations that assumed that the behavior would not
+occur.
+This caused effects that surprised people not fluent in the C
+standard.
+I won’t go into the details here, but one example of this (from my
+blog) is [signed overflow](http://www.airs.com/blog/archives/120).
+
+C of course continues to be the preferred language for kernel
+development and the glue language of the computing industry.
+Though it has been partially replaced by newer languages, this is not
+because of any choices made by new versions of C.
+
+The lessons I see here are:
+
+* Backward compatibility matters.
+* Breaking compatibility in small ways is OK, as long as people can
+ spot the breakages through compiler options or compiler errors.
+* Compiler options to select specific language/library versions are
+ useful, provided code compiled using different options can be linked
+ together.
+* Unlimited undefined behavior is confusing for users.
+
+### C++
+
+C++ language versions are also now driven by the ISO standardization process.
+Like C, C++ pays close attention to backward compatibility.
+C++ has been historically more free with adding new keywords (there
+are 10 new keywords in C++11).
+This works out OK because the newer keywords tend to be relatively
+long (`constexpr`, `nullptr`, `static_assert`) and compilation errors
+make it easy to find code using the new keywords as identifiers.
+
+C++ uses the same sorts of options for specifying the standard version
+for language and libraries as are found in C.
+It suffers from the same sorts of problems as C with regard to
+undefined behavior.
+
+An example of a breaking change in C++ was the change in the scope of
+a variable declared in the initialization statement of a for loop.
+In the pre-standard versions of C++, the scope of the variable
+extended to the end of the enclosing block, as though it were declared
+immediately before the for loop.
+During the development of the first C++ standard, C++98, this was
+changed so that the scope was only within the for loop itself.
+Compilers adapted by introducing options like `-ffor-scope` so that
+users could control the expected scope of the variable (for a period
+of time, when compiling with neither `-ffor-scope` nor
+`-fno-for-scope`, the GCC compiler used the old scope but warned about
+any code that relied on it).
+
+Despite the relatively strong backward compatibility, code written in
+new versions of C++, like C++11, tends to have a very different feel
+than code written in older versions of C++.
+This is because styles have changed to use new language and library
+features.
+Raw pointers are less commonly used, range loops are used rather than
+standard iterator patterns, new concepts like rvalue references and
+move semantics are used widely, and so forth.
+People familiar with older versions of C++ can struggle to understand
+code written in new versions.
+
+C++ is of course an enormously popular language, and the ongoing
+language revision process has not harmed its popularity.
+
+Besides the lessons from C, I would add:
+
+* A new version may have a very different feel while remaining
+ backward compatible.
+
+### Java
+
+I know less about Java than about the other languages I discuss, so
+there may be more errors here and there are certainly more biases.
+
+Java is largely backward compatible at the byte-code level, meaning
+that Java version N+1 libraries can call code written in, and
+compiled by, Java version N (and N-1, N-2, and so forth).
+Java source code is also mostly backward compatible, although they do
+add new keywords from time to time.
+
+The Java documentation is very detailed about potential compatibility
+issues when moving from one release to another.
+
+The Java standard library is enormous, and new packages are added at
+each new release.
+Packages are also deprecated from time to time.
+Using a deprecated package will cause a warning at compile time (the
+warning may be turned off), and after a few releases the deprecated
+package will be removed (at least in theory).
+
+Java does not seem to have many backward compatibility problems.
+The problems are centered on the JVM: an older JVM generally will not
+run newer releases, so you have to make sure that your JVM is at least
+as new as that required by the newest library you want to use.
+
+Java arguably has something of a forward compatibility problem in
+that JVM bytecodes present a higher level interface than that of a
+CPU, and that makes it harder to introduce new features that cannot
+be directly represented using the existing bytecodes.
+
+This forward compatibility problem is part of the reason that Java
+generics use type erasure.
+Changing the definition of existing bytecodes would have broken
+existing programs that had already been compiled into bytecode.
+Extending bytecodes to support generic types would have required a
+large number of additional bytecodes to be defined.
+
+This forward compatibility problem, to the extent that it is a
+problem, does not exist for Go.
+Since Go compiles to machine code, and implements all required run
+time checks by generating additional machine code, there is no similar
+forward compatibility issue.
+
+But, in general:
+
+* Be aware of how compatibility issues may restrict future changes.
+
+### Python
+
+Python 3.0 (also known as Python 3000) started development in 2006 and
+was initially released in 2008.
+In 2018 the transition is still incomplete.
+Some people continue to use Python 2.7 (released in 2010).
+This is not a path we want to emulate for Go 2.
+
+The main reason for this slow transition appears to be lack of
+backward compatibility.
+Python 3.0 was intentionally incompatible with earlier versions of
+Python.
+Notably, `print` was changed from a statement to a function, and
+strings were changed to use Unicode.
+Python is often used in conjunction with C code, and the latter change
+meant that any code that passed strings from Python to C required
+tweaking the C code.
+
+Because Python is an interpreted language, and because there is no
+backward compatibility, it is impossible to mix Python 2 and Python
+3 code in the same program.
+This means that for a typical program that uses a range of libraries,
+each of those libraries must be converted to Python 3 before the
+program can be converted.
+Since programs are in various states of conversion, libraries must
+support Python 2 and 3 simultaneously.
+
+Python supports statements of the form `from __future__ import
+FEATURE`.
+A statement like this changes the interpretation of the rest of the
+file in some way.
+For example, `from __future__ import print_function` changes `print`
+from a statement (as in Python 2) to a function (as in Python 3).
+This can be used to take incremental steps toward new language
+versions, and to make it easier to share the same code among different
+language versions.
+
+So, we knew it already, but:
+
+* Backward compatibility is essential.
+* Compatibility of the interface to other languages is important.
+* Upgrading to a new version is limited by the version that your
+ libraries support.
+
+### Perl
+
+The Perl 6 development process began in 2000.
+The first stable version of the Perl 6 spec was announced in 2015.
+This is not a path we want to emulate for Go 2.
+
+There are many reasons for this slow path.
+Perl 6 was intentionally not backward compatible: it was meant to fix
+warts in the language.
+Perl 6 was intended to be represented by a spec rather than, as with
+previous versions of Perl, an implementation.
+Perl 6 started with a set of change proposals, but then continued to
+evolve over time, and then evolve some more.
+
+Perl supports `use feature` which is similar to Python's `from
+__future__ import`.
+It changes the interpretation of the rest of the file to use a
+specified new language feature.
+
+* Don’t be Perl 6.
+* Set and meet deadlines.
+* Don’t change everything at once.
+
+## Proposal
+
+### Language changes
+
+Pedantically speaking, we must have a way to speak about specific
+language versions.
+Each change to the Go language first appears in a Go release.
+We will use Go release numbers to define language versions.
+That is the only reasonable choice, but it can be confusing because
+standard library changes are also associated with Go release numbers.
+When thinking about compatibility, it will be necessary to
+conceptually separate the Go language version from the standard
+library version.
+
+As an example of a specific change, type aliases were first available
+in Go language version 1.9.
+Type aliases were an example of a backward compatible language change.
+All code written in Go language versions 1.0 through 1.8 continued to
+work the same way with Go language 1.9.
+Code using type aliases requires Go language 1.9 or later.
+
+#### Language additions
+
+Type aliases are an example of an addition to the language.
+Code using the type alias syntax `type A = B` did not compile with Go
+versions before 1.9.
+
+Type aliases, and other backward compatible changes since Go 1.0, show
+us that for additions to the language it is not necessary for packages
+to explicitly declare the minimum language version that they require.
+Some packages changed to use type aliases.
+When such a package was compiled with Go 1.8 tools, the package failed
+to compile.
+The package author can simply say: upgrade to Go 1.9, or downgrade to
+an earlier version of the package.
+None of the Go tools need to know about this requirement; it's implied
+by the failure to compile with older versions of the tools.
+
+It's true of course that programmers need to understand language
+additions, but the the tooling does not.
+Neither the Go 1.8 tools nor the Go 1.9 tools need to explicitly know
+that type aliases were added in Go 1.9, other than in the limited
+sense that the Go 1.9 compiler will compile type aliases and the Go
+1.8 compiler will not.
+That said, the possibility of specifying a minimum language version to
+get better error messages for unsupported language features is
+discussed below.
+
+#### Language removals
+
+We must also consider language changes that simply remove features
+from the language.
+For example, [issue 3939](http://golang.org/issue/3939) proposes that
+we remove the conversion `string(i)` for an integer value `i`.
+If we make this change in, say, Go version 1.20, then packages that
+use this syntax will stop compiling in Go 1.20.
+(If you prefer to restrict backward incompatible changes to new major
+versions, then replace 1.20 by 2.0 in this discussion; the problem
+remains the same.)
+
+In this case, packages using the old syntax have no simple recourse.
+While we can provide tooling to convert pre-1.20 code into working
+1.20 code, we can't force package authors to run those tools.
+Some packages may be unmaintained but still useful.
+Some organizations may want to upgrade to 1.20 without having to
+requalify the versions of packages that they rely on.
+Some package authors may want to use 1.20 even though their packages
+now break, but do not have time to modify their package.
+
+These scenarios suggest that we need a mechanism to specify the
+maximum version of the Go language with which a package can be built.
+
+Importantly, specifying the maximum version of the Go language should
+not be taken to imply the maximum version of the Go tools.
+The Go compiler released with Go version 1.20 must be able to build
+packages using Go language 1.19.
+This can be done by adding an option to cmd/compile (and, if
+necessary, cmd/asm and cmd/link) along the lines of the `-std` option
+supported by C compilers.
+When cmd/compile sees the option, perhaps `-lang=go1.19`, it will
+compile the code using the Go 1.19 syntax.
+
+This requires cmd/compile to support all previous versions, one way or
+another.
+If supporting old syntaxes proves to be troublesome, the `-lang`
+option could perhaps be implemented by passing the code through a
+convertor from the old version to the current.
+That would keep support of old versions out of cmd/compile proper, and
+the convertor could be useful for people who want to update their
+code.
+But it is unlikely that supporting old language versions will be a
+significant problem.
+
+Naturally, even though the package is built with the language version
+1.19 syntax, it must in other respects be a 1.20 package: it must link
+with 1.20 code, be able to call and be called by 1.20 code, and so
+forth.
+
+The go tool will need to know the maximum language version so that it
+knows how to invoke cmd/compile.
+Assuming we continue with the modules experiment, the logical place
+for this information is the go.mod file.
+The go.mod file for a module M can specify the maximum language
+version for the packages that it defines.
+This would be honored when M is downloaded as a dependency by some
+other module.
+
+The maximum language version is not a minimum language version.
+If a module require features in language 1.19, but can be built with
+1.20, we can say that the maximum language version is 1.20.
+If we build with Go release 1.19, we will see that we are at less than
+the maximum, and simply build with language version 1.19.
+Maximum language versions greater than that supported by the current
+tools can simply be ignored.
+If we later build with Go release 1.21, we will build the module with
+`-lang=go1.20`.
+
+This means that the tools can set the maximum language version
+automatically.
+When we use Go release 1.30 to release a module, we can mark the
+module as having maximum language version 1.30.
+All users of the module will see this maximum version and do the right
+thing.
+
+This implies that we will have to support old versions of the language
+indefinitely.
+If we remove a language feature after version 1.25, version 1.26 and
+all later versions will still have to support that feature if invoked
+with the `-lang=go1.25` option (or `-lang=go1.24` or any other earlier
+version in which the feature is supported).
+Of course, if no `-lang` option is used, or if the option is
+`-lang=go1.26` or later, the feature will not be available.
+Since we do not expect wholesale removals of existing language
+features, this should be a manageable burden.
+
+I believe that this approach suffices for language removals.
+
+#### Minimum language version
+
+For better error messages it may be useful to permit the module file
+to specify a minimum language version.
+This is not required: if a module uses features introduced in
+language version 1.N, then building it with 1.N-1 will fail at compile
+time.
+This may be confusing, but in practice it will likely be obvious what
+the problem is.
+
+That said, if modules can specify a minimum language version, the go
+tool could produce an immediate, clear error message when building
+with 1.N-1.
+
+The minimum language version could potentially be set by the compiler
+or some other tool.
+When compiling each file, see which features it uses, and use that to
+determine the minimum version.
+It need not be precisely accurate.
+
+This is just a suggestion, not a requirement.
+It would likely provide a better user experience as the language
+changes.
+
+#### Language redefinitions
+
+The Go language can also change in ways that are not additions or
+removals, but are instead changes to the way a specific language
+construct works.
+For example, in Go 1.1 the size of the type `int` on 64-bit hosts
+changed from 32 bits to 64 bits.
+This change was relatively harmless, as the language does not specify
+the exact size of `int`.
+Potentially, though, some Go 1.0 programs continued to compile with Go
+1.1 but stopped working.
+
+A redefinition is a case where we have code that compiles successfully
+with both versions 1.N and version 1.M, where M > N, and where the
+meaning of the code is different in the two versions.
+For example, [issue 20733](https://golang.org/issue/20733) proposes
+that variables in a range loop should be redefined in each iteration.
+Though in practice this change seems more likely to fix programs than
+to break them, in principle this change might break working programs.
+
+Note that a new keyword normally cannot cause a redefinition, though
+we must be careful to ensure that that is true before introducing
+one.
+For example, if we introduce the keyword `check` as suggested in [the
+error handling draft
+design](https://go.googlesource.com/proposal/+/master/design/go2draft-error-handling.md),
+and we permit code like `check(f())`, that might seem to be a
+redefinition if `check` is defined as a function in the same package.
+But after the keyword is introduced, any attempt to define such a
+function will fail.
+So it is not possible for code using `check`, under whichever meaning,
+to compile with both version 1.N and 1.M.
+The new keyword can be handled as a removal (of the non-keyword use of
+`check`) and an addition (of the keyword `check`).
+
+In order for the Go ecosystem to survive a transition to Go 2, we must
+minimize these sorts of redefinitions.
+As discussed earlier, successful languages have generally had
+essentially no redefinitions beyond a certain point.
+
+The complexity of a redefinition is, of course, that we can no longer
+rely on the compiler to detect the problem.
+When looking at a redefined language construct, the compiler cannot
+know which meaning is meant.
+In the presence of redefined language constructs, we cannot determine
+the maximum language version.
+We don't know if the construct is intended to be compiled with the old
+meaning or the new.
+
+The only possibility would be to let programmers set the language
+version.
+In this case it would be either a minimum or maximum language
+version, as appropriate.
+It would have to be set in such a way that it would not be
+automatically updated by any tools.
+Of course, setting such a version would be error prone.
+Over time, a maximum language version would lead to surprising
+results, as people tried to use new language features, and failed.
+
+I think the only feasible safe approach is to not permit language
+redefinitions.
+
+We are stuck with our current semantics.
+This doesn't mean we can't improve them.
+For example, for [issue 20733](https://golang.org/issue/20733), the
+range issue, we could change range loops so that taking the address of
+a range parameter, or referring to it from a function literal, is
+forbidden.
+This would not be a redefinition; it would be a removal.
+That approach might eliminate the bugs without the potential of
+breaking code unexpectedly.
+
+#### Build tags
+
+Build tags are an existing mechanism that can be used by programs to
+choose which files to compile based on the release.
+
+Build tags name release versions, which look just like language
+versions, but, speaking pedantically, are different.
+In the discussion above we've talked about using Go release 1.N to
+compile code with language version 1.N-1.
+That is not possible using build tags.
+
+Build tags can be used to set the maximum or a minimum release, or
+both, that will be used to compile a specific file.
+They can be a convenient way to take advantage of language changes
+that are only available after a certain version; that is, they can be
+used to set a minimum language version when compiling a file.
+
+As discussed above, though, what is most useful for language changes
+is the ability to set a maximum language version.
+Build tags don't provide that in a useful way.
+If you use a build tag to set your current release version as your
+maximum version, your package will not build with later releases.
+Setting a maximum language version is only possible when it is set to
+a version before the current release, and is coupled with an alternate
+implementation that is used for the later versions.
+That is, if you are building with 1.N, it's not helpful to use a build
+tag of `!1.N+1`.
+You could use a build tag of `!1.M` where `M < N`, but in almost all
+cases you will then need a separate file with a build tag of `1.M+1`.
+
+Build tags can be used to handle language redefinitions: if there is a
+language redefinition at language version `1.N`, programmers can write
+one file with a build tag of `!1.N` using the old semantics and a
+different file with a build tag of `1.N` using the new semantics.
+However, these duplicate implementations are a lot of work, it's hard
+to know in general when it is required, and it would be easy to make a
+mistake.
+The availability of build tags is not enough to overcome the earlier
+comments about not permitting any language redefinitions.
+
+#### import "go2"
+
+It would be possible to add a mechanism to Go similar to Python's
+`from __future__ import` and Perl's `use feature`.
+For example, we could use a special import path, such as `import
+"go2/type-aliases"`.
+This would put the required language features in the file that uses
+them, rather than hidden away in the go.mod file.
+
+This would provide a way to describe the set of language additions
+required by the file.
+It's more complicated, because instead of relying on a language
+version, the language is broken up into separate features.
+There is no obvious way to ever remove any of these special imports,
+so they will tend to accumulate over time.
+Python and Perl avoid the accumulation problem by intentionally making
+a backward incompatible change.
+After moving to Python 3 or Perl 6, the accumulated feature requests
+can be discarded.
+Since Go is trying to avoid a large backward incompatible change,
+there would be no clear way to ever remove these imports.
+
+This mechanism does not address language removals.
+We could introduce a removal import, such as `import
+"go2/no-int-to-string"`, but it's not obvious why anyone would ever
+use it.
+In practice, there would be no way to ever remove language features,
+even ones that are confusing and error-prone.
+
+This kind of approach doesn't seem suitable for Go.
+
+### Standard library changes
+
+One of the benefits of a Go 2 transition is the chance to release some
+of the standard library packages from the Go 1 compatibility
+guarantee.
+Another benefit is the chance to move many, perhaps most, of the
+packages out of the six month release cycle.
+If the modules experiment works out it may even be possible to start
+doing this sooner rather than later, with some packages on a faster
+cycle.
+
+I propose that the six month release cycle continue, but that it be
+treated as a compiler/runtime release cycle.
+We want Go releases to be useful out of the box, so releases will
+continue to include the current versions of roughly the same set of
+packages that they contain today.
+However, many of those packages will actually be run on their own
+release cycles.
+People using a given Go release will be able to explicitly choose to
+use newer versions of the standard library packages.
+In fact, in some cases they may be able to use older versions of the
+standard library packages where that seems useful.
+
+Different release cycles would require more resources on the part of
+the package maintainers.
+We can only do this if we have enough people to manage it and enough
+testing resources to test it.
+
+We could also continue using the six month release cycle for
+everything, but make the separable packages available separately for
+use with different, compatible, releases.
+
+#### Core standard library
+
+Still, some parts of the standard library must be treated as core
+libraries.
+These libraries are closely tied to the compiler and other tools, and
+must strictly follow the release cycle.
+Neither older nor newer versions of these libraries may be used.
+
+Ideally, these libraries will remain on the current version 1.
+If it seems necessary to change any of them to version 2, that will
+have to be discussed on a case by case basis.
+At this time I see no reason for it.
+
+The tentative list of core libraries is:
+
+* os/signal
+* plugin
+* reflect
+* runtime
+* runtime/cgo
+* runtime/debug
+* runtime/msan
+* runtime/pprof
+* runtime/race
+* runtime/tsan
+* sync
+* sync/atomic
+* testing
+* time
+* unsafe
+
+I am, perhaps optimistically, omitting the net, os, and syscall
+packages from this list.
+We'll see what we can manage.
+
+#### Penumbra standard library
+
+The penumbra standard library consists of those packages that are
+included with a release but are maintained independently.
+This will be most of the current standard library.
+These packages will follow the same discipline as today, with the
+option to move to a v2 where appropriate.
+It will be possible to use `go get` to upgrade or, possibly, downgrade
+these standard library packages.
+In particular, fixes can be made as minor releases separately from the
+six month core library release cycle.
+
+The go tool will have to be able to distinguish between the core
+library and the penumbra library.
+I don't know precisely how this will work, but it seems feasible.
+
+When moving a standard library package to v2, it will be essential to
+plan for programs that use both v1 and v2 of the package.
+Those programs will have to work as expected, or if that is impossible
+will have to fail cleanly and quickly.
+In some cases this will involve modifying the v1 version to use an
+internal package that is also shared by the v2 package.
+
+Standard library packages will have to compile with older versions of
+the language, at least the two previous release cycles that we
+currently support.
+
+#### Removing packages from the standard library
+
+The ability to support `go get` of standard library packages will
+permit us to remove packages from the releases.
+Those packages will continue to exist and be maintained, and people
+will be able to retrieve them if they need them.
+However, they will not be shipped by default with a Go release.
+
+This will include packages like
+
+* index/suffixarray
+* log/syslog
+* net/http/cgi
+* net/http/fcgi
+
+and perhaps other packages that do not seem to be widely useful.
+
+We should in due course plan a deprecation policy for old packages, to
+move these packages to a point where they are no longer maintained.
+The deprecation policy will also apply to the v1 versions of packages
+that move to v2.
+
+Or this may prove to be too problematic, and we should never deprecate
+any existing package, and never remove them from the standard
+releases.
+
+## Go 2
+
+If the above process works as planned, then in an important sense
+there never will be a Go 2.
+Or, to put it a different way, we will slowly transition to new
+language and library features.
+We could at any point during the transition decide that now we are
+Go 2, which might be good marketing.
+Or we could just skip it (there has never been a C 2.0, why have a Go
+2.0?).
+
+Popular languages like C, C++, and Java never have a version 2.
+In effect, they are always at version 1.N, although they use different
+names for that state.
+I believe that we should emulate them.
+In truth, a Go 2 in the full sense of the word, in the sense of an
+incompatible new version of the language or core libraries, would not
+be a good option for our users.
+A real Go 2 would, perhaps unsurprisingly, be harmful.
