What was noalias and what ever happened to it?

noalias was another type qualifier, in the same syntactic class as const and volatile, which was intended to assert that an object was not pointed to (``aliased'') by other pointers. The primary application, which is an important one, would have been for the formal parameters of functions designed to perform computations on large arrays. A compiler cannot usually take advantage of vectorization or other parallelization hardware (on supercomputers which have it) unless it can ensure that the source and destination arrays do not overlap.
The noalias keyword was not backed up by any ``prior art,'' and it was introduced late in the review and approval process. It was surprisingly difficult to define precisely and explain coherently, and sparked widespread, acrimonious debate, including a scathing pan by Dennis Ritchie. It had far-ranging implications, particularly for several standard library interfaces, for which easy fixes were not readily apparent.
Because of the criticism and the difficulty of defining noalias well, the Committee declined to adopt it, in spite of its superficial attractions. (When writing a standard, features cannot be introduced halfway; their full integration, and all implications, must be understood.) The need for an explicit mechanism to support parallel implementation of non-overlapping operations remains unfilled (although some work is being done on the problem). .