Unspecified behaviour during token pasting

Is the following program fragment ill-formed?

#define hash_hash %: ## % ## :
#define ignore(x)
ignore(hash_hash)

The standard says "The order of evaluation of ## operators is
unspecified". It also says "If the result is not a valid preprocessing
token, the behavior is undefined". Therefore, it appears that it's
unspecified whether the above example has defined behavior.

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