cmd/go: assume Go 1.16 instead of Go 1.11 for dependencies that lack explicit 'go' directives

Fixes #45109
Updates #44976
Updates #36876

Change-Id: Icb00f8b6e0d4e076d82da1697e7058b9e7603916
Reviewed-on: https://go-review.googlesource.com/c/go/+/303229
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

doc/go1.17.html

@@ -67,6 +67,18 @@ Do not send CLs removing the interior tags from such phrases.
   Go 1.12</a>.)
 </p>
 
+<p><!-- golang.org/issue/44976 -->
+  If a module dependency lacks an explicit <code>go.mod</code> file, or
+  its <code>go.mod</code> file does not contain
+  a <a href="/doc/modules/gomod-ref#go"><code>go</code> directive</a>,
+  the <code>go</code> command now assumes <code>go 1.16</code> for that
+  dependency instead of the current release. (Dependencies developed in GOPATH
+  mode may lack a <code>go.mod</code> file, and
+  the <code>vendor/modules.txt</code> has to date never recorded
+  the <code>go</code> versions indicated by dependencies' <code>go.mod</code>
+  files.)
+</p>
+
 <h2 id="runtime">Runtime</h2>
 
 <p>

src/cmd/go/internal/modload/init.go

@@ -405,9 +405,38 @@ func LoadModFile(ctx context.Context) {
 		readVendorList()
 		checkVendorConsistency()
 	}
-	if index.goVersionV == "" && cfg.BuildMod == "mod" {
-		addGoStmt()
-		WriteGoMod()
+	if index.goVersionV == "" {
+		// The main module necessarily has a go.mod file, and that file lacks a
+		// 'go' directive. The 'go' command has been adding that directive
+		// automatically since Go 1.12, so this module either dates to Go 1.11 or
+		// has been erroneously hand-edited.
+		//
+		// If we are able to modify the go.mod file, we will add a 'go' directive
+		// to at least make the situation explicit going forward.
+		if cfg.BuildMod == "mod" {
+			// TODO(#44976): If we implicitly upgrade to the latest Go version once
+			// lazy loading is implemented, we could accidentally prune out
+			// dependencies from what was formerly a Go 1.11 module, resulting in
+			// downgrades (if only lower requirements on that module remain) and/or
+			// upgrades (if no requirement remains and we end up re-resolving to
+			// latest).
+			//
+			// We should probably instead load the dependencies using Go 1.11
+			// semantics to ensure that we capture everything that is relevant, or
+			// perhaps error out and let the user tell us which version they intend.
+			//
+			// If we are running 'go mod tidy' in particular, we will have enough
+			// information to upgrade the 'go' version after loading is complete.
+			addGoStmt(latestGoVersion())
+			WriteGoMod()
+		} else {
+			// Reproducibility requires that if we change the semantics of a module,
+			// we write some explicit change to its go.mod file. We cannot write to
+			// the go.mod file (because we are in readonly or vendor mode), so we must
+			// not change its semantics either. The go.mod file looks as if it were
+			// created by Go 1.11, so assume Go 1.11 semantics.
+			rawGoVersion.Store(Target, "1.11")
+		}
 	}
 }
 
@@ -442,7 +471,7 @@ func CreateModFile(ctx context.Context, modPath string) {
 	modFile = new(modfile.File)
 	modFile.AddModuleStmt(modPath)
 	initTarget(modFile.Module.Mod)
-	addGoStmt() // Add the go directive before converted module requirements.
+	addGoStmt(latestGoVersion()) // Add the go directive before converted module requirements.
 
 	convertedFrom, err := convertLegacyConfig(modPath)
 	if convertedFrom != "" {
@@ -680,11 +709,10 @@ func convertLegacyConfig(modPath string) (from string, err error) {
 // addGoStmt adds a go directive to the go.mod file if it does not already
 // include one. The 'go' version added, if any, is the latest version supported
 // by this toolchain.
-func addGoStmt() {
+func addGoStmt(v string) {
 	if modFile.Go != nil && modFile.Go.Version != "" {
 		return
 	}
-	v := latestGoVersion()
 	if err := modFile.AddGoStmt(v); err != nil {
 		base.Fatalf("go: internal error: %v", err)
 	}
@@ -692,7 +720,7 @@ func addGoStmt() {
 }
 
 // latestGoVersion returns the latest version of the Go language supported by
-// this toolchain.
+// this toolchain, like "1.17".
 func latestGoVersion() string {
 	tags := build.Default.ReleaseTags
 	version := tags[len(tags)-1]

src/cmd/go/internal/work/gc.go

@@ -70,7 +70,15 @@ func (gcToolchain) gc(b *Builder, a *Action, archive string, importcfg, embedcfg
 			// as of Go 1.12, so any module that still lacks such a directive must
 			// either have been authored before then, or have a hand-edited go.mod
 			// file that hasn't been updated by cmd/go since that edit.
-			v = "1.11"
+			//
+			// Unfortunately, through at least Go 1.16 we didn't add versions to
+			// vendor/modules.txt. So this could also be a vendored 1.16 dependency.
+			//
+			// Fortunately, there were no breaking changes to the language between Go
+			// 1.11 and 1.16, so if we assume Go 1.16 semantics we will not introduce
+			// any spurious errors — we will only mask errors, and not particularly
+			// important ones at that.
+			v = "1.16"
 		}
 		if allowedVersion(v) {
 			gcargs = append(gcargs, "-lang=go"+v)

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

@@ -0,0 +1,36 @@
+# Regression test for https://golang.org/issue/45109:
+# Dependencies that use post-1.11 Go features should build
+# when compiled as vendored dependencies of Go 1.16 modules.
+
+[short] skip
+
+go mod init example.com/foo
+go mod edit -replace=example.com/use113@v0.1.0=./use113
+
+go mod vendor
+! grep 1.13 vendor/modules.txt  # TODO(#36876): record dependency versions.
+go build .
+
+
+# In Go 1.16 and earlier, 'go mod vendor' did not record dependency versions.
+# That still should not cause a build failure.
+
+go mod edit -go=1.16
+go mod vendor
+! grep 1.13 vendor/modules.txt
+go build .
+
+
+-- foo.go --
+package foo
+
+import _ "example.com/use113"
+
+-- use113/go.mod --
+module example.com/use113
+
+go 1.13
+-- use113/use113.go --
+package use113
+
+const x = 1_000