go/types, types2: use max(fileVersion, go1.21) if fileVersion present

Change the rules for how //go:build "file versions" are applied: instead
of considering whether a file version is an upgrade or downgrade from
the -lang version, always use max(fileVersion, go1.21). This prevents
file versions from downgrading the version below go1.21.  Before Go 1.21
the //go:build version did not have the meaning of setting the file's
langage version.

This fixes an issue that was appearing in GOPATH builds: Go 1.23.0
started providing -lang versions to the compiler in GOPATH mode (among
other places) which it wasn't doing before, and it set -lang to the
toolchain version (1.23). Because the -lang version was greater than
go1.21, language version used to compile the file would be set to the
//go:build file version. //go:build file versions below 1.21 could cause
files that could previously build to stop building.

For example, take a Go file with a //go:build line specifying go1.10.
If that file used a 1.18 feature, that use would compile fine with a Go
1.22 toolchain. But it would produce an error when compiling with the
1.23.0 toolchain because it set the language version to 1.10 and
disallowed the 1.18 feature. This breaks backwards compatibility: when
the build tag was added, it did not have the meaning of restricting the
language version.

For #68658

Change-Id: I6cedda81a55bcccffaa3501eef9e2be6541b6ece
Reviewed-on: https://go-review.googlesource.com/c/go/+/607955
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>

0 files changed, 0 insertions, 0 deletions