do not allow main module to replace itself

Change-Id: I6d94f00e1c607581ba32253ba00fe4110e2bb5b8

Author: GrigoriyMikhalkin

src/cmd/go/internal/modload/init.go

@@ -426,6 +426,7 @@ func InitMod() {
 		legacyModInit()
 	}
 
+	checkMainModuleReplaces()
 	modFileToBuildList()
 	setDefaultBuildMod()
 	if cfg.BuildMod == "vendor" {
@@ -555,6 +556,29 @@ func setDefaultBuildMod() {
 	}
 }
 
+// checkMainModuleReplaces checks that modFile is not trying to replace
+// main module with the same version of itself
+func checkMainModuleReplaces() {
+	for _, r := range modFile.Replace {
+		isMainModule := r.Old.Path == modFile.Module.Mod.Path
+		isSameVersion := r.Old.Path == r.New.Path && r.Old.Version == r.New.Version
+
+		if isMainModule {
+			if isSameVersion {
+				base.Fatalf("go: replacing main module with the same version of itself is forbidden")
+			}
+
+			if modfile.IsDirectoryPath(r.New.Path) {
+				newModFilePath, _ := filepath.Abs(r.New.Path)
+
+				if newModFilePath == ModRoot() {
+					base.Fatalf("go: replace directive points to current main module directory")
+				}
+			}
+		}
+	}
+}
+
 // checkVendorConsistency verifies that the vendor/modules.txt file matches (if
 // go 1.14) or at least does not contradict (go 1.13 or earlier) the
 // requirements and replacements listed in the main module's go.mod file.

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

@@ -0,0 +1,40 @@
+env GO111MODULE=on
+[short] skip
+
+# Replace directive that points to current main module directory isn't allowed
+cd fail1
+! go list all
+stderr 'go: replace directive points to current main module directory'
+
+# Replace directive that attempts to replace main module with the same version of itself isn't allowed
+cd ../fail2
+! go list all
+stderr 'go: replacing main module with the same version of itself is forbidden'
+
+# Replace directive that resolves to another directory with main module is allowed
+cd ../success
+go list all
+stdout 'example.com/m'
+
+-- fail1/go.mod --
+module example.com/m
+
+replace example.com/m => ./.
+
+-- fail2/go.mod --
+module example.com/m
+
+replace example.com/m v1.1.1 => example.com/m v1.1.1
+
+-- success/go.mod --
+module example.com/m
+
+replace example.com/m => ../fork
+
+-- fork/go.mod --
+module example.com/m
+
+-- success/m.go --
+package main
+
+func main() {}