cmd/go/internal/gover: fix ModIsPrerelease for toolchain versions

We forgot to call the IsPrerelease function on FromToolchain(vers)
rather than on vers itself. IsPrerelase expects a version without the
"go" prefix. See the corresponding code in ModIsValid and ModIsPrefix
that call FromToolchain before passing the versions to IsValid and
IsLang respectively.

Fixes #74786

Change-Id: I3cf055e1348e6a9dc0334e414f06fe85eaf78024
Reviewed-on: https://go-review.googlesource.com/c/go/+/691655
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Michael Matloob <[email protected]>
Reviewed-by: Michael Matloob <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>

Author: matloob

src/cmd/go/internal/gover/mod.go

@@ -109,6 +109,9 @@ func ModIsPrefix(path, vers string) bool {
 // The caller is assumed to have checked that ModIsValid(path, vers) is true.
 func ModIsPrerelease(path, vers string) bool {
 	if IsToolchain(path) {
+		if path == "toolchain" {
+			return IsPrerelease(FromToolchain(vers))
+		}
 		return IsPrerelease(vers)
 	}
 	return semver.Prerelease(vers) != ""

src/cmd/go/testdata/script/mod_get_toolchain.txt

@@ -94,12 +94,14 @@ stderr '^go: added toolchain go1.24rc1$'
 grep 'go 1.22.9' go.mod  # no longer implied
 grep 'toolchain go1.24rc1' go.mod
 
-# go get toolchain@latest finds go1.999testmod.
+# go get toolchain@latest finds go1.23.9.
 cp go.mod.orig go.mod
 go get toolchain@latest
-stderr '^go: added toolchain go1.999testmod$'
+stderr '^go: added toolchain go1.23.9$'
 grep 'go 1.21' go.mod
-grep 'toolchain go1.999testmod' go.mod
+grep 'toolchain go1.23.9' go.mod
+
+
 
 # Bug fixes.
 
@@ -115,7 +117,7 @@ stderr '^go: upgraded go 1.19 => 1.21.0'
 
 # go get [email protected] is OK too.
 go get [email protected]
-stderr '^go: downgraded toolchain go1.999testmod => go1.24rc1$'
+stderr '^go: upgraded toolchain go1.23.9 => go1.24rc1$'
 
 # go get [email protected] should work if we are the Go 1.21 language version,
 # even though there's no toolchain for it.