cmd/go: do not allow main module to replace itself

Change-Id: I6d94f00e1c607581ba32253ba00fe4110e2bb5b8

Author: GrigoriyMikhalkin

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

@@ -1012,6 +1012,17 @@ func indexModFile(data []byte, modFile *modfile.File, needsFix bool) *modFileInd
 		if prev, dup := i.replace[r.Old]; dup && prev != r.New {
 			base.Fatalf("go: conflicting replacements for %v:\n\t%v\n\t%v", r.Old, prev, r.New)
 		}
+
+		if r.Old.Path == modFile.Module.Mod.Path && r.Old.Version == "" {
+			base.Fatalf("go: %s:%d: the main module (%s) cannot be replaced",
+				modFile.Syntax.Name, r.Syntax.Start.Line, modFile.Module.Mod.Path)
+		}
+
+		if modfile.IsDirectoryPath(r.New.Path) && HasModRoot() && filepath.Join(ModRoot(), r.New.Path) == ModRoot() {
+			base.Fatalf("go: %s:%d: the main module root directory (%s) cannot be a replacement for another module",
+				modFile.Syntax.Name, r.Syntax.Start.Line, modFile.Module.Mod.Path)
+		}
+
 		i.replace[r.Old] = r.New
 	}
 

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

@@ -0,0 +1,38 @@
+env GO111MODULE=on
+[short] skip
+
+cd fail1
+! go list all
+stderr 'go: \$WORK\/gopath\/src\/fail1\/go\.mod:3: the main module root directory \(example\.com\/m\) cannot be a replacement for another module'
+
+cd ../fail2
+! go list all
+stderr 'go: \$WORK\/gopath\/src\/fail2\/go\.mod:3: the main module \(example\.com\/m\) cannot be replaced'
+
+# Different module version can be replaced
+cd ../success
+go list all
+stdout 'example.com/m'
+
+-- fail1/go.mod --
+module example.com/m
+
+replace example.com/m2 => ./.
+
+-- fail2/go.mod --
+module example.com/m
+
+replace example.com/m => example.com/m v1.1.1
+
+-- success/go.mod --
+module example.com/m
+
+replace example.com/m v1.1.1 => example.com/m v1.1.2
+
+-- fork/go.mod --
+module example.com/m
+
+-- success/m.go --
+package main
+
+func main() {}