cmd/go: suppress errors for 'go get' of module paths that are also constrained-out packages

Fixes #33526

Change-Id: Iedd2d6dbe440499bf074ac632513319a22f2d648
Reviewed-on: https://go-review.googlesource.com/c/go/+/297009
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>

src/cmd/go/internal/modget/get.go

@@ -369,7 +369,23 @@ func runGet(ctx context.Context, cmd *base.Command, args []string) {
 	// directory.
 	if !*getD && len(pkgPatterns) > 0 {
 		work.BuildInit()
-		pkgs := load.PackagesAndErrors(ctx, pkgPatterns)
+
+		var pkgs []*load.Package
+		for _, pkg := range load.PackagesAndErrors(ctx, pkgPatterns) {
+			if pkg.Error != nil {
+				var noGo *load.NoGoError
+				if errors.As(pkg.Error.Err, &noGo) {
+					if m := modload.PackageModule(pkg.ImportPath); m.Path == pkg.ImportPath {
+						// pkg is at the root of a module, and doesn't exist with the current
+						// build tags. Probably the user just wanted to change the version of
+						// that module — not also build the package — so suppress the error.
+						// (See https://golang.org/issue/33526.)
+						continue
+					}
+				}
+			}
+			pkgs = append(pkgs, pkg)
+		}
 		load.CheckPackageErrors(pkgs)
 		work.InstallPackages(ctx, pkgPatterns, pkgs)
 		// TODO(#40276): After Go 1.16, print a deprecation notice when building and
@@ -1453,6 +1469,7 @@ func (r *resolver) checkPackagesAndRetractions(ctx context.Context, pkgPatterns
 			LoadTests:                *getT,
 			ResolveMissingImports:    false,
 			AllowErrors:              true,
+			SilenceNoGoErrors:        true,
 		}
 		matches, pkgs := modload.LoadPackages(ctx, pkgOpts, pkgPatterns...)
 		for _, m := range matches {
@@ -1468,9 +1485,9 @@ func (r *resolver) checkPackagesAndRetractions(ctx context.Context, pkgPatterns
 					// associated with either the package or its test — ErrNoGo must
 					// indicate that none of those source files happen to apply in this
 					// configuration. If we are actually building the package (no -d
-					// flag), the compiler will report the problem; otherwise, assume that
-					// the user is going to build or test it in some other configuration
-					// and suppress the error.
+					// flag), we will report the problem then; otherwise, assume that the
+					// user is going to build or test this package in some other
+					// configuration and suppress the error.
 					continue
 				}
 

src/cmd/go/internal/modload/load.go

@@ -181,6 +181,15 @@ type PackageOpts struct {
 	// future version).
 	SilenceMissingStdImports bool
 
+	// SilenceNoGoErrors indicates that LoadPackages should not print
+	// imports.ErrNoGo errors.
+	// This allows the caller to invoke LoadPackages (and report other errors)
+	// without knowing whether the requested packages exist for the given tags.
+	//
+	// Note that if a requested package does not exist *at all*, it will fail
+	// during module resolution and the error will not be suppressed.
+	SilenceNoGoErrors bool
+
 	// SilenceUnmatchedWarnings suppresses the warnings normally emitted for
 	// patterns that did not match any packages.
 	SilenceUnmatchedWarnings bool
@@ -290,6 +299,10 @@ func LoadPackages(ctx context.Context, opts PackageOpts, patterns ...string) (ma
 	// Report errors, if any.
 	checkMultiplePaths()
 	for _, pkg := range loaded.pkgs {
+		if !pkg.isTest() {
+			loadedPackages = append(loadedPackages, pkg.path)
+		}
+
 		if pkg.err != nil {
 			if sumErr := (*ImportMissingSumError)(nil); errors.As(pkg.err, &sumErr) {
 				if importer := pkg.stack; importer != nil {
@@ -298,23 +311,24 @@ func LoadPackages(ctx context.Context, opts PackageOpts, patterns ...string) (ma
 					sumErr.importerIsTest = importer.testOf != nil
 				}
 			}
-			silence := opts.SilenceErrors
+
+			if opts.SilenceErrors {
+				continue
+			}
 			if stdErr := (*ImportMissingError)(nil); errors.As(pkg.err, &stdErr) &&
 				stdErr.isStd && opts.SilenceMissingStdImports {
-				silence = true
+				continue
+			}
+			if opts.SilenceNoGoErrors && errors.Is(pkg.err, imports.ErrNoGo) {
+				continue
 			}
 
-			if !silence {
-				if opts.AllowErrors {
-					fmt.Fprintf(os.Stderr, "%s: %v\n", pkg.stackText(), pkg.err)
-				} else {
-					base.Errorf("%s: %v", pkg.stackText(), pkg.err)
-				}
+			if opts.AllowErrors {
+				fmt.Fprintf(os.Stderr, "%s: %v\n", pkg.stackText(), pkg.err)
+			} else {
+				base.Errorf("%s: %v", pkg.stackText(), pkg.err)
 			}
 		}
-		if !pkg.isTest() {
-			loadedPackages = append(loadedPackages, pkg.path)
-		}
 	}
 	if !opts.SilenceErrors {
 		// Also list errors in matching patterns (such as directory permission

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

@@ -16,6 +16,7 @@ go mod edit -droprequire example.net/tools
 # error out if dependencies of tag-guarded files are missing.
 
 go get -d example.net/tools@v0.1.0
+! stderr 'no Go source files'
 
 ! go list example.net/tools
 stderr '^package example.net/tools: build constraints exclude all Go files in .*[/\\]tools$'
@@ -30,6 +31,19 @@ go list -deps example.net/cmd/tool
 stderr '^no required module provides package example.net/missing; to add it:\n\tgo get example.net/missing$'
 
 
+# https://golang.org/issue/33526: 'go get' without '-d' should succeed
+# for a module whose root is a constrained-out package.
+#
+# Ideally it should silently succeed, but today it logs the "no Go source files"
+# error and succeeds anyway.
+
+go get example.net/tools@v0.1.0
+! stderr .
+
+! go build example.net/tools
+stderr '^package example.net/tools: build constraints exclude all Go files in .*[/\\]tools$'
+
+
 # https://golang.org/issue/29268
 # 'go get' should fetch modules whose roots contain test-only packages, but
 # without the -t flag shouldn't error out if the test has missing dependencies.