CC @rsc, @bcmills, @matloob.

Is it possible to have this fixed in a go1.21 patch release? I see this was moved to go1.22, but this looks like a regression to me.

This issue still needs to be investigated. It it's determined that the current behavior is buggy, the fix will need to be developed for Go 1.22 first before it can be considered for backport to a minor release of Go 1.21.

To me, it seems that at least the go version aspect is working as intended. See the last two paragraphs of https://go.dev/doc/toolchain#select:

Running go version prints the selected Go toolchain’s version (by running the selected toolchain’s implementation of go version).

Running GOTOOLCHAIN=local go version prints the bundled Go toolchain’s version.

Consider a local toolchain version of "go1.21rc4" and a go.mod file whose "go" line is "go1.21.0". Running go version inside such a module correctly prints "go1.21.0", the version of the toolchain that is selected.

Similarly, it probably makes sense that go --help in the same module prints the output of running go1.21.0's go with "--help" flag, rather than that of go1.21rc4.

@thaJeztah, I think it is important that we print an error and return with nonzero exit status in this case, but perhaps the error message we print in case of a failed upgrade could be more helpful?

• Perhaps it could print the file and line number containing the erroneous go directive.
• Perhaps it could suggest setting GOTOOLCHAIN=local?

But it's tricky, because additional information risks adding to the confusion instead of clarifying it.