cmd/go: detect case-insensitive import path collision

We already detect this collision when both imports are used
anywhere in a single program. Also detect it when they are in
different targets being processed together.

Fixes #20264.

Change-Id: I5d3c822aae136053fbcb5ed167e1d67f9b847a0f
Reviewed-on: https://go-review.googlesource.com/46424
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/go_test.go

@@ -2040,8 +2040,10 @@ func TestCaseCollisions(t *testing.T) {
 		)`)
 	tg.tempFile("src/example/a/pkg/pkg.go", `package pkg`)
 	tg.tempFile("src/example/a/Pkg/pkg.go", `package pkg`)
-	tg.runFail("list", "example/a")
-	tg.grepStderr("case-insensitive import collision", "go list example/a did not report import collision")
+	tg.run("list", "-json", "example/a")
+	tg.grepStdout("case-insensitive import collision", "go list -json example/a did not report import collision")
+	tg.runFail("build", "example/a")
+	tg.grepStderr("case-insensitive import collision", "go build example/a did not report import collision")
 	tg.tempFile("src/example/b/file.go", `package b`)
 	tg.tempFile("src/example/b/FILE.go", `package b`)
 	f, err := os.Open(tg.path("src/example/b"))
@@ -2059,6 +2061,13 @@ func TestCaseCollisions(t *testing.T) {
 	}
 	tg.runFail(args...)
 	tg.grepStderr("case-insensitive file name collision", "go list example/b did not report file name collision")
+
+	tg.runFail("list", "example/a/pkg", "example/a/Pkg")
+	tg.grepStderr("case-insensitive import collision", "go list example/a/pkg example/a/Pkg did not report import collision")
+	tg.run("list", "-json", "-e", "example/a/pkg", "example/a/Pkg")
+	tg.grepStdout("case-insensitive import collision", "go list -json -e example/a/pkg example/a/Pkg did not report import collision")
+	tg.runFail("build", "example/a/pkg", "example/a/Pkg")
+	tg.grepStderr("case-insensitive import collision", "go build example/a/pkg example/a/Pkg did not report import collision")
 }
 
 // Issue 8181.

src/cmd/go/internal/load/pkg.go

@@ -828,6 +828,8 @@ var cgoSyscallExclude = map[string]bool{
 	"runtime/msan": true,
 }
 
+var foldPath = make(map[string]string)
+
 // load populates p using information from bp, err, which should
 // be the result of calling build.Context.Import.
 func (p *Package) load(stk *ImportStack, bp *build.Package, err error) *Package {
@@ -1109,17 +1111,16 @@ func (p *Package) load(stk *ImportStack, bp *build.Package, err error) *Package
 		return p
 	}
 
-	// In the absence of errors lower in the dependency tree,
-	// check for case-insensitive collisions of import paths.
-	if len(p.DepsErrors) == 0 {
-		dep1, dep2 := str.FoldDup(p.Deps)
-		if dep1 != "" {
-			p.Error = &PackageError{
-				ImportStack: stk.Copy(),
-				Err:         fmt.Sprintf("case-insensitive import collision: %q and %q", dep1, dep2),
-			}
-			return p
+	// Check for case-insensitive collisions of import paths.
+	fold := str.ToFold(p.ImportPath)
+	if other := foldPath[fold]; other == "" {
+		foldPath[fold] = p.ImportPath
+	} else if other != p.ImportPath {
+		p.Error = &PackageError{
+			ImportStack: stk.Copy(),
+			Err:         fmt.Sprintf("case-insensitive import collision: %q and %q", p.ImportPath, other),
 		}
+		return p
 	}
 
 	if p.BinaryOnly {

src/cmd/go/internal/str/str.go

@@ -29,13 +29,13 @@ func StringList(args ...interface{}) []string {
 	return x
 }
 
-// toFold returns a string with the property that
-//	strings.EqualFold(s, t) iff toFold(s) == toFold(t)
+// ToFold returns a string with the property that
+//	strings.EqualFold(s, t) iff ToFold(s) == ToFold(t)
 // This lets us test a large set of strings for fold-equivalent
 // duplicates without making a quadratic number of calls
 // to EqualFold. Note that strings.ToUpper and strings.ToLower
 // do not have the desired property in some corner cases.
-func toFold(s string) string {
+func ToFold(s string) string {
 	// Fast path: all ASCII, no upper case.
 	// Most paths look like this already.
 	for i := 0; i < len(s); i++ {
@@ -74,7 +74,7 @@ Slow:
 func FoldDup(list []string) (string, string) {
 	clash := map[string]string{}
 	for _, s := range list {
-		fold := toFold(s)
+		fold := ToFold(s)
 		if t := clash[fold]; t != "" {
 			if s > t {
 				s, t = t, s