Yep, that's why it's a breaking change (or why breaking changes are bad). On the other hand, if they're using the C# 5 compiler to output C# 4 projects then maybe there will be a new warning (which, of course, is another breaking change if you treat warnings as errors).
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The bigger issue will be the confusion when people who are used to the C# 5 semantics work on a C# 4 project.
Yep, that's why it's a breaking change (or why breaking changes are bad). On the other hand, if they're using the C# 5 compiler to output C# 4 projects then maybe there will be a new warning (which, of course, is another breaking change if you treat warnings as errors).