Patch to make go build more consistent, when the -o flag is specified and is a directory

build.diff

diff --git a/src/cmd/go/internal/work/build.go b/src/cmd/go/internal/work/build.go
index a75ace7d4e..c6108a2d0f 100644
--- a/src/cmd/go/internal/work/build.go
+++ b/src/cmd/go/internal/work/build.go
@@ -369,13 +369,6 @@ func runBuild(ctx context.Context, cmd *base.Command, args []string) {
 	pkgs := load.PackagesAndErrors(ctx, load.PackageOpts{}, args)
 	load.CheckPackageErrors(pkgs)
 
-	explicitO := len(cfg.BuildO) > 0
-
-	if len(pkgs) == 1 && pkgs[0].Name == "main" && cfg.BuildO == "" {
-		cfg.BuildO = pkgs[0].DefaultExecName()
-		cfg.BuildO += cfg.ExeSuffix
-	}
-
 	// sanity check some often mis-used options
 	switch cfg.BuildContext.Compiler {
 	case "gccgo":
@@ -399,61 +392,82 @@ func runBuild(ctx context.Context, cmd *base.Command, args []string) {
 
 	pkgs = omitTestOnly(pkgsFilter(pkgs))
 
-	// Special case -o /dev/null by not writing at all.
-	if cfg.BuildO == os.DevNull {
-		cfg.BuildO = ""
-	}
-
-	if cfg.BuildO != "" {
-		// If the -o name exists and is a directory or
-		// ends with a slash or backslash, then
-		// write all main packages to that directory.
-		// Otherwise require only a single package be built.
-		if fi, err := os.Stat(cfg.BuildO); (err == nil && fi.IsDir()) ||
-			strings.HasSuffix(cfg.BuildO, "/") ||
-			strings.HasSuffix(cfg.BuildO, string(os.PathSeparator)) {
-			if !explicitO {
-				base.Fatalf("go build: build output %q already exists and is a directory", cfg.BuildO)
-			}
-			a := &Action{Mode: "go build"}
-			for _, p := range pkgs {
-				if p.Name != "main" {
-					continue
-				}
+	if cfg.BuildO == "" {
+		cfg.BuildO = "."
+	}
+	buildToDir := func() bool {
+		fi, err := os.Stat(cfg.BuildO)
+		switch {
+		case err == nil && fi.IsDir():
+			return true
+		case strings.HasSuffix(cfg.BuildO, "/"):
+			return true
+		case strings.HasSuffix(cfg.BuildO, string(os.PathSeparator)):
+			return true
+		}
+
+		return false
+	}
+	buildDiscard := func() bool {
+		return cfg.BuildO == os.DevNull
+	}
 
-				p.Target = filepath.Join(cfg.BuildO, p.DefaultExecName())
-				p.Target += cfg.ExeSuffix
-				p.Stale = true
-				p.StaleReason = "build -o flag in use"
-				a.Deps = append(a.Deps, b.AutoAction(ModeInstall, depMode, p))
+	a := &Action{Mode: "go build"}
+	switch {
+	case buildDiscard():
+		// Don't write anything at all.
+		for _, p := range pkgs {
+			if p.Name != "main" {
+				continue
 			}
-			if len(a.Deps) == 0 {
-				base.Fatalf("go build: no main packages to build")
+
+			a.Deps = append(a.Deps, b.AutoAction(ModeBuild, depMode, p))
+		}
+	case buildToDir():
+		// Write all main packages to the specified directory.
+		for _, p := range pkgs {
+			if p.Name != "main" {
+				continue
 			}
-			b.Do(ctx, a)
-			return
+
+			p.Target = filepath.Join(cfg.BuildO, p.DefaultExecName())
+			p.Target += cfg.ExeSuffix
+			p.Stale = true
+			p.StaleReason = "build -o flag in use"
+
+			a.Deps = append(a.Deps, b.AutoAction(ModeInstall, depMode, p))
 		}
+	default:
+		// Require only a single package be built.
 		if len(pkgs) > 1 {
 			base.Fatalf("go build: cannot write multiple packages to non-directory %s", cfg.BuildO)
 		} else if len(pkgs) == 0 {
 			base.Fatalf("no packages to build")
 		}
+
 		p := pkgs[0]
 		p.Target = cfg.BuildO
 		p.Stale = true // must build - not up to date
 		p.StaleReason = "build -o flag in use"
+
 		a := b.AutoAction(ModeInstall, depMode, p)
 		b.Do(ctx, a)
+
 		return
 	}
 
-	a := &Action{Mode: "go build"}
+	// Build and discard all the non main packages.
 	for _, p := range pkgs {
+		if p.Name == "main" {
+			continue
+		}
+
 		a.Deps = append(a.Deps, b.AutoAction(ModeBuild, depMode, p))
 	}
 	if cfg.BuildBuildmode == "shared" {
 		a = b.buildmodeShared(ModeBuild, depMode, args, pkgs, a)
 	}
+
 	b.Do(ctx, a)
 }
 

See golang/go#37378