cmd/go: inconsistent reporting of default Go version

[rsc]

In the transcript below, there is no go line in the go.mod.
Builds say I can't use post-Go 1.16 features, but then when I say
"go get go@1.21.0" (using a sufficiently old version so as not to break any users),
it claims I am downgrading from Go 1.24.

% go install
# rsc.io/tmp/jsonfmt
./jsonfmt.go:95:9: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
% cat go.mod
module rsc.io/tmp/jsonfmt
% go get go@go1.21.0
go: downgraded go 1.24 => 1.21.0
go: added toolchain go1.24
% cat go.mod
module rsc.io/tmp/jsonfmt

go 1.21.0

toolchain go1.24
% 

[gabyhelp]

Related Issues and Documentation

• cmd/go: using go get go@{version} in a module without a go directive can print a misleading "downgraded" message #63507
• cmd/go: Go modules initialised by go1.21.1 cannot be built using go1.20.8 #63279 (closed)
• cmd/go: need to backport relaxing of go.mod go verb constraints to 1.11 series #28974 (closed)
• cmd/go: breaking change in 1.23rc2 with version constraints in GOPATH mode #68658 (closed)
• cmd/go: go toolchain version switch doesn't work when only language version is set in go.mod #65580 (closed)
• cmd/go: on malformed "go mod edit -go", suggest current version of go, not hardcoded 1.12 #32286 (closed)
• cmd/go: automatic toolchain upgrades make the combination of `go version` and `go install <package>@<version>` confusing #66518
• cmd/go: `go env GOMOD` tries to download newer toolchain #61455 (closed)
• cmd/fix: fails to run on modules whose go directive value is in "1.n.m" format introduced in Go 1.21.0 #62584 (closed)
• cmd/compile: Go 1.22 changes support for modules that declare `go 1.0` #65528 (closed)

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[cagedmantis]

cc @matloob @samthanawalla

[gopherbot]

Change https://go.dev/cl/624635 mentions this issue: cmd/go: check missing go line in go.mod when 'go get go@1.N.P'