The spec does not define equality for tuples or structs.

[mihaibudiu]

I think we should extend the spec rather than forbid these operations.

[jamescoole-cisco]

Makes sense to me. We may also want to consider defining it for headers.

[jamescoole-cisco]

The language for structs includes a requirement that their fields be recursively comparable

Two structs can be compared for equality (==) or inequality (!=) only if they have the same type and all of their fields can be recursively compared for equality.

but this language is missing for headers

Two headers can be compared for equality (==) or inequality (!=) only if they have the same type.

However, varbits can appear in headers but don't appear to be directly comparable

p4-spec/p4-16/spec/P4-16-spec.mdk

Lines 2968 to 2981 in 54e425a

	Variable-length bit-strings support a limited set of operations:
	- Parser extraction into a variable-sized bit-string using the
	two-argument `extract` method of a `packet_in` extern object
	(see Section [#sec-packet-lookahead]). This operation sets the
	dynamic width of the field.
	- Assignment to another variable-sized bit-string. The target of the
	assignment must have the same static width as the source. When
	executed, the assignment sets the dynamic width of the target to the
	dynamic width of the source.
	- The `emit` method of a `packet_out` extern object, which
	inserts a variable-sized bit-string with a known dynamic width into
	the packet being constructed (see Section [#sec-deparse]).

It seems like we should allow comparing headers that contain varbits, but if we're going to allow that we should probably also define == and != for varbits so they can be directly compared as well.