cmd/go/internal/modload: don't infer a /v1 suffix module path

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

@@ -1710,6 +1710,15 @@ func findModulePath(dir string) (string, error) {
 				badPathErr = err
 				break
 			}
+			// Ensure the inferred path is valid.
+			if _, _, ok := module.SplitPathVersion(path); !ok {
+				if strings.HasPrefix(path, "gopkg.in/") {
+					badPathErr = errors.New("module paths beginning with gopkg.in/ must always have a major version suffix in the form of .vN")
+				} else {
+					badPathErr = errors.New("major version suffixes must be in the form of /vN and are only allowed for v2 or later")
+				}
+				break
+			}
 			return path, nil
 		}
 	}

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

@@ -8,6 +8,22 @@ stdout '^example.com$'
 go list
 stdout '^example.com$'
 
+# Reset $GOPATH
+env GOPATH=$WORK/gopath
+
+# 'go mod init' should not create a go.mod file in v0 or v1 directory.
+cd $GOPATH/src/example.com/m/v0
+! go mod init
+stderr '(?s)^go: cannot determine module path for source directory(.*)example.com/m/v0 \(bad module path inferred from directory in GOPATH: major version suffixes must be in the form of /vN and are only allowed for v2 or later\)(.*)Example usage:(.*)''go mod init example.com/m'' to initialize a v0 or v1 module(.*)''go mod init example.com/m/v2'' to initialize a v2 module(.*)Run ''go help mod init'' for more information.$'
+
+cd $GOPATH/src/example.com/m/v1
+! go mod init
+stderr '(?s)^go: cannot determine module path for source directory(.*)example.com/m/v1 \(bad module path inferred from directory in GOPATH: major version suffixes must be in the form of /vN and are only allowed for v2 or later\)(.*)Example usage:(.*)''go mod init example.com/m'' to initialize a v0 or v1 module(.*)''go mod init example.com/m/v2'' to initialize a v2 module(.*)Run ''go help mod init'' for more information.$'
+
+cd $GOPATH/src/example.com/m/v2
+go mod init
+stderr '^go: creating new go.mod: module example.com/m/v2$'
+
 -- go.mod --
 module example.com
 
@@ -19,3 +35,10 @@ func main() {}
 
 -- $WORK/invalid-gopath
 This is a text file, not a directory.
+
+-- example.com/m/v0/main.go --
+package main
+-- example.com/m/v1/main.go --
+package main
+-- example.com/m/v2/main.go --
+package main