cmd: fail go mod vendor on case-insensitive import collisions

The existing implementation of go mod vendor allows having
case-insensitive imports, which will anyway fail during go build.
This improvement validates such collisions during go mod vendor command.

Fixes golang#38571

src/cmd/go/internal/modcmd/vendor.go

@@ -19,6 +19,7 @@ import (
 	"cmd/go/internal/cfg"
 	"cmd/go/internal/imports"
 	"cmd/go/internal/modload"
+	"cmd/go/internal/str"
 
 	"golang.org/x/mod/module"
 	"golang.org/x/mod/semver"
@@ -64,6 +65,8 @@ func runVendor(ctx context.Context, cmd *base.Command, args []string) {
 	}
 	_, pkgs := modload.LoadPackages(ctx, loadOpts, "all")
 
+	checkCaseInsensitivePathsDuplications(modload.LoadBuildList())
+
 	vdir := filepath.Join(modload.ModRoot(), "vendor")
 	if err := os.RemoveAll(vdir); err != nil {
 		base.Fatalf("go mod vendor: %v", err)
@@ -298,3 +301,19 @@ func copyDir(dst, src string, match func(dir string, info os.FileInfo) bool) {
 		}
 	}
 }
+
+// checkCaseInsensitivePathsDuplications verifies that given paths have
+// no case-insensitive duplications.
+//
+// (See https://golang.org/issue/38571).
+func checkCaseInsensitivePathsDuplications(mods []module.Version) {
+	paths := make(map[string]string, len(mods))
+	for _, mod := range mods {
+		fold := str.ToFold(mod.Path)
+		if other := paths[fold]; other == "" {
+			paths[fold] = mod.Path
+		} else if other != mod.Path {
+			base.Fatalf("go mod vendor: case-insensitive module path collision: %s and %s", other, mod.Path)
+		}
+	}
+}

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

@@ -30,7 +30,7 @@ func StringList(args ...interface{}) []string {
 }
 
 // ToFold returns a string with the property that
-//	strings.EqualFold(s, t) iff ToFold(s) == ToFold(t)
+// strings.EqualFold(s, t) if 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

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

@@ -0,0 +1,33 @@
+# Tests issue #38571
+# Tests that go mod vendor fails on case-insensitive import collision.
+
+env GO111MODULE=on
+
+! go mod vendor
+stderr 'case-insensitive module path collision'
+
+-- go.mod --
+module m
+
+go 1.14
+
+require (
+	example.com/foo v0.1.0
+	example.com/Foo v0.1.0
+)
+
+replace (
+	example.com/foo => ./foo
+	example.com/Foo => ./!foo
+)
+-- foo/go.mod --
+module example.com/foo
+
+-- foo/foo.go --
+package foo
+
+-- !foo/go.mod --
+module example.com/Foo
+
+-- !foo/foo.go --
+package Foo