doc/go1.11: explain new vet typechecking behaviour in release notes
Since Go1.10, go test runs vet on the tests before executing them.
Moreover, the vet tool typechecks the package under analysis with
go/types before running. In Go1.10, a typechecking failure just caused
a warning to be printed. In Go1.11, a typechecking failure will cause
vet to exit with a fatal error (see Issue #21287).
This means that starting with Go1.11, tests that don't typecheck will
fail immediately. This would not normally be an issue, since a test
that doesn't typecheck shouldn't even compile, and it should already
be broken.
Unfortunately, there's a bug in gc that makes it accept programs with
unused variables inside a closure (Issue #3059). This means that a
test with an unused variable inside a closure, that compiled and
passed in Go1.10, will fail in the typechecking step of vet starting
with Go1.11.
Explain this in the 1.11 release notes.
Fixes #26109
Change-Id: I970c1033ab6bc985d8c64bd24f56e854af155f96
Reviewed-on: https://go-review.googlesource.com/121455 Reviewed-by: Ian Lance Taylor <iant@golang.org>