For TypeFunction, nextOf() can return NULL if the function return type is meant to be inferred, and semantic() hasn't yet ben run on the function. After semantic(), it must no longer be NULL.
The D Programming Language's Unofficial Docs
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For TypeFunction, nextOf() can return NULL if the function return type is meant to be inferred, and semantic() hasn't yet ben run on the function. After semantic(), it must no longer be NULL.