[teacup50]

However:

- Almost nobody actually used C prior to 1978.

- Most C compilers have supported both current and previous versions of the C standard via -std flags.

- Other languages with breaking changes, like Java, are similar. Java has always supported specifying the source language level via -src.

Swift is unique in breaking old code completely, without recourse other than semi-functional code rewriting tools.

[mikeash]

Almost nobody actually used Swift prior to mid-2014. Did any 1978 C compilers offer both K&R and prior as options?

IMO the example of Java is a great illustration of why Swift shouldn't be tied to source compatibility too soon. You don't want to be stuck supporting your 1.0 syntax until the end of time, or you'll end up like Java!

[teacup50]

Used everywhere, with decades old libraries that still function fine?