gopls: warn about Go 1.19 and Go 1.20

Update the support table to warn when users install gopls with Go 1.19
or 1.20, or have these older Go versions in their PATH.
Clarify current and future support in the README.

Fixes golang/go#50825
Updates golang/go#65917

Change-Id: I99de1a7717a8cf99cae1a561ced63e9724dfff66
Reviewed-on: https://go-review.googlesource.com/c/tools/+/588763
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

gopls/README.md

@@ -75,32 +75,35 @@ and
 ### Supported Go versions
 
 `gopls` follows the
-[Go Release Policy](https://golang.org/doc/devel/release.html#policy),
-meaning that it officially supports the last 2 major Go releases. Per
-[issue #39146](https://go.dev/issues/39146), we attempt to maintain best-effort
-support for the last 4 major Go releases, but this support extends only to not
-breaking the build and avoiding easily fixable regressions.
-
-In the context of this discussion, gopls "supports" a Go version if it supports
-being built with that Go version as well as integrating with the `go` command
-of that Go version.
-
-The following table shows the final gopls version that supports a given Go
-version. Go releases more recent than any in the table can be used with any
-version of gopls.
+[Go Release Policy](https://golang.org/doc/devel/release.html#policy), meaning
+that it officially supports only the two most recent major Go releases. Until
+August 2024, the Go team will also maintain best-effort support for the last
+4 major Go releases, as described in [issue #39146](https://go.dev/issues/39146).
+
+Starting with the release of Go 1.23.0 and gopls@v0.17.0 in August 2024, the
+gopls build will depend on the latest version of Go. However, due to the
+[forward compatibility](https://go.dev/blog/toolchain) support added to the
+`go` command in Go 1.21, as long as Go 1.21 or later are used to install gopls,
+the toolchain upgrade will be handled automatically, just like any other
+dependency. Gopls will continue to support integrating with the two most recent
+major Go releases of the `go` command, per the Go Release Policy. See
+[issue #65917](https://go.dev/issue/65917) for more details.
+
+Maintaining support for legacy versions of Go caused
+[significant friction](https://go.dev/issue/50825) for gopls maintainers and
+held back other improvements. If you are unable to install a supported version
+of Go on your system, you can still install an older version of gopls. The
+following table shows the final gopls version that supports a given Go version.
+Go releases more recent than those in the table can be used with any version of
+gopls.
 
 | Go Version  | Final gopls version with support (without warnings) |
 | ----------- | --------------------------------------------------- |
 | Go 1.12     | [gopls@v0.7.5](https://github.com/golang/tools/releases/tag/gopls%2Fv0.7.5) |
 | Go 1.15     | [gopls@v0.9.5](https://github.com/golang/tools/releases/tag/gopls%2Fv0.9.5) |
 | Go 1.17     | [gopls@v0.11.0](https://github.com/golang/tools/releases/tag/gopls%2Fv0.11.0) |
 | Go 1.18     | [gopls@v0.14.2](https://github.com/golang/tools/releases/tag/gopls%2Fv0.14.2) |
-
-Our extended support is enforced via [continuous integration with older Go
-versions](doc/contributing.md#ci). This legacy Go CI may not block releases:
-test failures may be skipped rather than fixed. Furthermore, if a regression in
-an older Go version causes irreconcilable CI failures, we may drop support for
-that Go version in CI if it is 3 or 4 Go versions old.
+| Go 1.20     | [gopls@v0.15.3](https://github.com/golang/tools/releases/tag/gopls%2Fv0.15.3) |
 
 ### Supported build systems
 

gopls/internal/util/goversion/goversion.go

@@ -40,7 +40,9 @@ var Supported = []Support{
 	{15, "", "v0.9.5"},
 	{16, "", "v0.11.0"},
 	{17, "", "v0.11.0"},
-	{18, "v0.16.0", "v0.14.2"},
+	{18, "", "v0.14.2"},
+	{19, "v0.17.0", "v0.15.3"},
+	{20, "v0.17.0", "v0.15.3"},
 }
 
 // OldestSupported is the last X in Go 1.X that this version of gopls

gopls/internal/util/goversion/goversion_test.go

@@ -40,20 +40,18 @@ func TestMessage(t *testing.T) {
 		}
 	}
 
-	tests := []struct {
-		goVersion    int
-		fromBuild    bool
-		wantContains []string // string fragments that we expect to see
-		wantIsError  bool     // an error, not a mere warning
-	}{
+	tests := []test{
 		{-1, false, nil, false},
 		deprecated(12, "v0.7.5"),
 		deprecated(13, "v0.9.5"),
 		deprecated(15, "v0.9.5"),
 		deprecated(16, "v0.11.0"),
 		deprecated(17, "v0.11.0"),
-		{18, false, []string{"Found Go version 1.18", "unsupported by gopls v0.16.0", "upgrade to Go 1.19", "install gopls v0.14.2"}, false},
-		{18, true, []string{"Gopls was built with Go version 1.18", "unsupported by gopls v0.16.0", "upgrade to Go 1.19", "install gopls v0.14.2"}, false},
+		deprecated(18, "v0.14.2"),
+		{19, false, []string{"Found Go version 1.19", "unsupported by gopls v0.17.0", "upgrade to Go 1.21", "install gopls v0.15.3"}, false},
+		{19, true, []string{"Gopls was built with Go version 1.19", "unsupported by gopls v0.17.0", "upgrade to Go 1.21", "install gopls v0.15.3"}, false},
+		{20, false, []string{"Found Go version 1.20", "unsupported by gopls v0.17.0", "upgrade to Go 1.21", "install gopls v0.15.3"}, false},
+		{20, true, []string{"Gopls was built with Go version 1.20", "unsupported by gopls v0.17.0", "upgrade to Go 1.21", "install gopls v0.15.3"}, false},
 	}
 
 	for _, test := range tests {