cmd/go: better error for shadowed directories in GOPATH

Fixes #5774.

R=golang-dev, adg, r, bradfitz
CC=golang-dev
https://golang.org/cl/9164043

src/cmd/go/build.go

@@ -311,9 +311,13 @@ func runInstall(cmd *Command, args []string) {
 
 	for _, p := range pkgs {
 		if p.Target == "" && (!p.Standard || p.ImportPath != "unsafe") {
+			if p.ConflictDir != "" {
+				errorf("go install: no install location for %s: hidden by %s", p.Dir, p.ConflictDir)
+			} else {
 				errorf("go install: no install location for directory %s outside GOPATH", p.Dir)
 			}
 		}
+	}
 	exitIfErrors()
 
 	var b builder

src/cmd/go/pkg.go

@@ -34,6 +34,7 @@ type Package struct {
 	Standard    bool   `json:",omitempty"` // is this package part of the standard Go library?
 	Stale       bool   `json:",omitempty"` // would 'go install' do anything for this package?
 	Root        string `json:",omitempty"` // Go root or Go path dir containing this package
+	ConflictDir string `json:",omitempty"` // Dir is hidden by this other directory
 
 	// Source files
 	GoFiles        []string `json:",omitempty"` // .go source files (excluding CgoFiles, TestGoFiles, XTestGoFiles)
@@ -102,6 +103,7 @@ func (p *Package) copyBuild(pp *build.Package) {
 	p.Name = pp.Name
 	p.Doc = pp.Doc
 	p.Root = pp.Root
+	p.ConflictDir = pp.ConflictDir
 	// TODO? Target
 	p.Goroot = pp.Goroot
 	p.Standard = p.Goroot && p.ImportPath != "" && !strings.Contains(p.ImportPath, ".")

src/cmd/go/test.bash

@@ -439,6 +439,37 @@ TEST go get cover
 unset GOPATH
 rm -rf $d
 
+TEST shadowing logic
+export GOPATH=$(pwd)/testdata/shadow/root1:$(pwd)/testdata/shadow/root2
+
+# The math in root1 is not "math" because the standard math is.
+cdir=$(./testgo list -f '({{.ImportPath}}) ({{.ConflictDir}})' ./testdata/shadow/root1/src/math)
+if [ "$cdir" != "(_$(pwd)/testdata/shadow/root1/src/math) ($GOROOT/src/pkg/math)" ]; then
+	echo shadowed math is not shadowed: "$cdir"
+	ok=false
+fi
+
+# The foo in root1 is "foo".
+cdir=$(./testgo list -f '({{.ImportPath}}) ({{.ConflictDir}})' ./testdata/shadow/root1/src/foo)
+if [ "$cdir" != "(foo) ()" ]; then
+	echo unshadowed foo is shadowed: "$cdir"
+	ok=false
+fi
+
+# The foo in root2 is not "foo" because the foo in root1 got there first.
+cdir=$(./testgo list -f '({{.ImportPath}}) ({{.ConflictDir}})' ./testdata/shadow/root2/src/foo)
+if [ "$cdir" != "(_$(pwd)/testdata/shadow/root2/src/foo) ($(pwd)/testdata/shadow/root1/src/foo)" ]; then
+	echo shadowed foo is not shadowed: "$cdir"
+	ok=false
+fi
+
+# The error for go install should mention the conflicting directory.
+err=$(! ./testgo install ./testdata/shadow/root2/src/foo 2>&1)
+if [ "$err" != "go install: no install location for directory $(pwd)/testdata/shadow/root2/src/foo hidden by $(pwd)/testdata/shadow/root1/src/foo" ]; then
+	echo wrong shadowed install error: "$err"
+	ok=false
+fi
+
 # Only succeeds if source order is preserved.
 TEST source file name order preserved
 ./testgo test testdata/example[12]_test.go || ok=false

src/cmd/go/testdata/shadow/root1/src/foo/foo.go

+package foo

src/cmd/go/testdata/shadow/root1/src/math/math.go

+package math

src/cmd/go/testdata/shadow/root2/src/foo/foo.go

+package foo

src/pkg/go/build/build.go

@@ -350,6 +350,7 @@ type Package struct {
 	Goroot      bool     // package found in Go root
 	PkgObj      string   // installed .a file
 	AllTags     []string // tags that can influence file selection in this directory
+	ConflictDir string   // this directory shadows Dir in $GOPATH
 
 	// Source files
 	GoFiles        []string // .go source files (excluding CgoFiles, TestGoFiles, XTestGoFiles)
@@ -476,11 +477,13 @@ func (ctxt *Context) Import(path string, srcDir string, mode ImportMode) (*Packa
 				// else first.
 				if ctxt.GOROOT != "" {
 					if dir := ctxt.joinPath(ctxt.GOROOT, "src", "pkg", sub); ctxt.isDir(dir) {
+						p.ConflictDir = dir
 						goto Found
 					}
 				}
 				for _, earlyRoot := range all[:i] {
 					if dir := ctxt.joinPath(earlyRoot, "src", sub); ctxt.isDir(dir) {
+						p.ConflictDir = dir
 						goto Found
 					}
 				}