I had a Swift method with a parameter called module. I noticed that the [Project]-Swift.h autogenerated header renamed that parameter to module_ in the Objective-C signature for that method. After some more investigation, it looks like this is happening for C++ keywords:

@objcMembers class SomeClass: NSObject {
    func someMethod(init: String) {}

    // ↓ These will append an underscore in the header file.
    func someMethod(template: String) {}
    func someMethod(consteval: String) {}
}

becomes:

SWIFT_CLASS("_TtC9iOSTester9SomeClass")
@interface SomeClass : NSObject
- (void)someMethodWithInit:(NSString * _Nonnull)init;
- (void)someMethodWithTemplate:(NSString * _Nonnull)template_;
- (void)someMethodWithConsteval:(NSString * _Nonnull)consteval_;
- (nonnull instancetype)init OBJC_DESIGNATED_INITIALIZER;
@end

Is this documented behaviour of the compiler or just some implementation detail? Why would it be necessary?