Handle option void type

- There's been no explicit support having argument void type
  as you would not be able to use with annotation model but
  surely can be wrapped as Type with CommandRegistration.
- For now change CommandInfoModel so that it uses empty string
  which is i.e. expected in help options which are just
  used as flags.
- It's outside of this commit to change parser to fail if
  user gives an argument value for this type of options.
- Fixes #586

spring-shell-core/src/main/java/org/springframework/shell/command/CommandRegistration.java

@@ -1141,7 +1141,8 @@ public interface CommandRegistration {
 			if (helpOptionsSpec != null) {
 				String[] longNames = helpOptionsSpec.longNames != null ? helpOptionsSpec.longNames : null;
 				Character[] shortNames = helpOptionsSpec.shortNames != null ? helpOptionsSpec.shortNames : null;
-				options.add(CommandOption.of(longNames, shortNames, "help for " + command));
+				options.add(CommandOption.of(longNames, shortNames, "help for " + command,
+						ResolvableType.forType(void.class)));
 			}
 			return options;
 		}

spring-shell-samples/src/main/java/org/springframework/shell/samples/e2e/OptionTypeCommands.java

@@ -230,4 +230,26 @@ public class OptionTypeCommands extends BaseE2ECommands {
 				.and()
 			.build();
 	}
+
+	//
+	// Void
+	//
+
+	@Bean
+	public CommandRegistration optionTypeVoidRegistration(Supplier<CommandRegistration.Builder> builder) {
+		return builder.get()
+			.command(REG, "option-type-void")
+			.group(GROUP)
+			.withOption()
+				.longNames("arg1")
+				.type(void.class)
+				.and()
+			.withTarget()
+				.function(ctx -> {
+					return "Hello ";
+				})
+				.and()
+			.build();
+	}
+
 }

spring-shell-standard-commands/src/main/java/org/springframework/shell/standard/commands/CommandInfoModel.java

@@ -91,7 +91,17 @@ class CommandInfoModel {
 			return o.getLabel();
 		}
 		else {
-			return o.getType() == null ? "String" : ClassUtils.getShortName(o.getType().getRawClass());
+			if (o.getType() != null) {
+				if (ClassUtils.isAssignable(o.getType().getRawClass(), Void.class)) {
+					return "";
+				}
+				else {
+					return ClassUtils.getShortName(o.getType().getRawClass());
+				}
+			}
+			else {
+				return "String";
+			}
 		}
 	}
 

spring-shell-standard-commands/src/test/java/org/springframework/shell/standard/commands/CommandInfoModelTests.java

@@ -122,4 +122,37 @@ public class CommandInfoModelTests {
 		CommandInfoModel cim = CommandInfoModel.of("main1", r1);
 		assertThat(cim.getAliases()).containsExactly("alias1");
 	}
+
+	@Test
+	void voidTypeUsesEmptyStringAsName() {
+		CommandRegistration r1 = CommandRegistration.builder()
+			.command("main1")
+			.withOption()
+				.longNames("arg1")
+				.type(void.class)
+				.and()
+			.withTarget()
+				.consumer(ctx -> {})
+				.and()
+			.build();
+		CommandInfoModel cim1 = CommandInfoModel.of("main1", r1);
+		assertThat(cim1.getParameters()).hasSize(1);
+		assertThat(cim1.getParameters().get(0).getArguments()).containsExactly("--arg1");
+		assertThat(cim1.getParameters().get(0).getType()).isEmpty();
+
+		CommandRegistration r2 = CommandRegistration.builder()
+			.command("main1")
+			.withOption()
+				.longNames("arg1")
+				.type(Void.class)
+				.and()
+			.withTarget()
+				.consumer(ctx -> {})
+				.and()
+			.build();
+		CommandInfoModel cim2 = CommandInfoModel.of("main1", r2);
+		assertThat(cim2.getParameters()).hasSize(1);
+		assertThat(cim2.getParameters().get(0).getArguments()).containsExactly("--arg1");
+		assertThat(cim2.getParameters().get(0).getType()).isEmpty();
+	}
 }