X3: compilation errors upgrading from 1.60.0 from 1.61.0

When I upgraded from Boost 1.60.0 to 1.61.0, a compilation error was encountered coming from `spirit/home/x3/operator/detail/sequence.hpp`.

I've created a minimum repro here: https://gist.github.com/peterhuene/2ce5e403bf313ffc3f9caaaac8461bef

This repro mimics custom parsers I have to adapt Spirit.Lex to work with X3 (this was a port from an existing Spirit.Qi parser and the lexer does some complicated work, such as lexing out-of-order tokens like heredocs). Namely, there is a "raw" parser whose job it is to consume a matching token; this parser uses `x3::unused_type` as the attribute type.

When this "unused_type attributed" parser is used in a sequence, as in the repro, the deduced attribute type for the LHS of the sequence operator appears to be incorrect in 1.61.0 and this causes a compilation error when assigning the attribute's value.

The repro (and more importantly, my real parser) compiles without error and produces the expected result in 1.60.0.

I've tracked the problem to this commit: https://github.com/boostorg/spirit/commit/a8e391bd99dddb3f9ece84bdb1bb9236b0a37cf7. Reverting the commit locally eliminates the compilation error.

Is there something that needs to change in my implementation to make this work for 1.61.0 or is this a bug?

Thanks!

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