cmd/go: do not write output when -o is not specified, but folder with same name exists

Fixes #31296

Change-Id: Ib8850fe22749ca0bf268614ba045ffe3fc68f5cc
Reviewed-on: https://go-review.googlesource.com/c/go/+/171057
Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/work/build.go

@@ -283,6 +283,8 @@ func runBuild(cmd *base.Command, args []string) {
 
 	pkgs := load.PackagesForBuild(args)
 
+	explicitO := len(cfg.BuildO) > 0
+
 	if len(pkgs) == 1 && pkgs[0].Name == "main" && cfg.BuildO == "" {
 		cfg.BuildO = load.DefaultExecName(pkgs[0].ImportPath)
 		cfg.BuildO += cfg.ExeSuffix
@@ -320,6 +322,9 @@ func runBuild(cmd *base.Command, args []string) {
 		// 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() {
+			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" {

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

@@ -7,6 +7,9 @@ go build -o $WORK/bin ./cmd/c1 ./cmd/c2
 ! go build -o $WORK/bin ./pkg1 ./pkg1
 stderr 'no main packages'
 
+! go build ./cmd/c1
+stderr 'already exists and is a directory'
+
 -- go.mod --
 module exmod
 
@@ -25,3 +28,6 @@ package pkg1
 
 -- pkg2/pkg2.go --
 package pkg2
+
+-- c1$exe/keep.txt --
+Create c1 directory.