fix: parse explicit short flag values

README.md

@@ -115,7 +115,11 @@ flag.VarP(&flagVal, "varname", "v", "help message")
 ```
 
 Shorthand letters can be used with single dashes on the command line.
-Boolean shorthand flags can be combined with other shorthand flags.
+Boolean shorthand flags can be combined with other shorthand flags. A shorthand
+with `NoOptDefVal` behaves the same way inside a cluster: for example, if `-a`
+has a no-option default, `-abc` uses that default for `-a` and continues with
+`-b` and `-c`. To pass an explicit value to such a shorthand, use `-a=value`
+or `-a value`.
 
 The default set of command-line flags is controlled by
 top-level functions.  The FlagSet type allows one to define

flag.go

@@ -1184,17 +1184,17 @@ func (f *FlagSet) parseSingleShortArg(shorthands string, args []string, fn parse
 		// '-f=arg'
 		value = shorthands[2:]
 		outShorts = ""
-	} else if flag.NoOptDefVal != "" {
-		// '-f' (arg was optional)
-		value = flag.NoOptDefVal
-	} else if len(shorthands) > 1 {
+	} else if len(shorthands) > 1 && flag.NoOptDefVal == "" {
 		// '-farg'
 		value = shorthands[1:]
 		outShorts = ""
-	} else if len(args) > 0 {
-		// '-f arg'
+	} else if len(shorthands) == 1 && len(args) > 0 && !strings.HasPrefix(args[0], "-") && !isNoOptBoolValue(flag.Value) {
+		// '-f arg' for non-boolean optional-value flags
 		value = args[0]
 		outArgs = args[1:]
+	} else if flag.NoOptDefVal != "" {
+		// '-f' (arg was optional), including '-fb' shorthand clusters
+		value = flag.NoOptDefVal
 	} else {
 		// '-f' (arg was required)
 		err = f.fail(&ValueRequiredError{

flag_test.go

@@ -1626,3 +1626,33 @@ func TestVisitFlagOrder(t *testing.T) {
 		i++
 	})
 }
+
+func TestShortFlagNoOptDefValWithExplicitValue(t *testing.T) {
+	f := NewFlagSet("test", ContinueOnError)
+	value := f.StringP("value", "v", "initial", "value")
+	f.Lookup("value").NoOptDefVal = "default"
+
+	if err := f.Parse([]string{"-v", "explicit"}); err != nil {
+		t.Fatal(err)
+	}
+	if got, want := *value, "explicit"; got != want {
+		t.Fatalf("-v explicit = %q, want %q", got, want)
+	}
+}
+
+func TestShortFlagNoOptDefValInClusterKeepsCurrentSemantics(t *testing.T) {
+	f := NewFlagSet("test", ContinueOnError)
+	value := f.StringP("value", "v", "initial", "value")
+	f.Lookup("value").NoOptDefVal = "default"
+	b := f.BoolP("bool", "b", false, "bool")
+
+	if err := f.Parse([]string{"-vb"}); err != nil {
+		t.Fatal(err)
+	}
+	if got, want := *value, "default"; got != want {
+		t.Fatalf("-vb value = %q, want %q", got, want)
+	}
+	if !*b {
+		t.Fatal("-vb did not parse -b after -v used its NoOptDefVal")
+	}
+}