x/tools/go/analysis/passes/stdversion: language vs toolchain version

[dominikh]

In this example

-- go.mod --
module example.com

go 1.23
-- foo.go --
//go:build go1.21

package pkg

import "reflect"

func main() {
	_ = reflect.TypeFor[int]
}

stdversion reports ./foo.go:8:14: reflect.TypeFor requires go1.22 or later (file is go1.21)

I believe that this might be overly strict. Even though the //go:build directive downgrades the language version available in the file, it doesn't change the fact that this file will only be built by a toolchain that is at least verison 1.23, because the module as a whole will only be built by a toolchain that satisfies the go dependency in go.mod.

I thus believe that the standard library available to a file with a build tag is max(file's version, module's version), and not min.

/cc @adonovan

[gabyhelp]

Related Issues

• go/ast, go/build/constraint: support for per-file Go versions using //go:build lines #59033 (closed)
• x/tools/go/analysis/passes/stdversion: spurious reports in files tagged //go:build 1.<old> #67123 (closed)
• doc: mention language downgrading via `//go:build` #61894

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