[Issue 19928] disallow modification of immutable in constructor after calling base ctor

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19928

Steven Schveighoffer <schveiguy at yahoo.com> changed:

           What    |Removed                     |Added
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                 OS|Mac OS X                    |All

--- Comment #6 from Steven Schveighoffer <schveiguy at yahoo.com> ---
The reason you can modify immutable in a constructor is because nothing else
has access to it. As soon as you give access to it elsewhere, it can cause a
problem. Andrei is correct, calling any function which allows access to the
immutable provides a vector of breaking immutability.

(In reply to RazvanN from comment #1)
> I don't think this issue is valid. If we disallow modification of immutable
> fields after a base class ctor is called then it will be impossible to
> initialize that field after a super call, which in my opinion is
> unacceptable behavior.

It's tricky, and definitely difficult to deal with. It would disallow certain
solutions that seem otherwise valid. But that might be the cost of having a
correct immutable implementation.

I will note that Swift enforces all members are initialized (look for 2-phase
initialization) before calling a super ctor:
https://docs.swift.org/swift-book/LanguageGuide/Initialization.html

Java is the opposite, it requires you call any super ctor FIRST before
initializing members:
https://stackoverflow.com/questions/15682457/initialize-field-before-super-constructor-runs

But they also don't have a concept of immutable which is implicitly sharable.

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