Question: why aren’t decomposition declarations be allowed to be constexpr? (apart from “because the Standard says so”).
There is no other reason. The standard says in [dcl.dcl] p8:
The decl-specifier-seq shall contain only the type-specifier
auto(7.1.7.4) and cv-qualifiers.
That means it can’t be declared with constexpr.
This was the subject of a National Body comment on the C++17 CD, see US-95 in P0488R0:
Comment: There is no obvious reason why decomposition
declarations cannot be declared as static,
thread_local, or constexpr.
Proposed change: Allow constexpr, static, and thread_local to the
permitted set of decl-specifiers.
Comments GB 16 and GB 17 are also related.
These comment were rejected for C++17 after review by the Evolution Working Group at the Nov 2016 meeting. It was unclear what some storage classes would mean on a structured binding declaration, and exactly how to change the specification to allow constexpr (simply allowing it in the grammar wouldn’t say what it means). A paper exploring the design space was requested. It should be possible to change this in future without breaking any code, but there wasn’t time to do it for C++17.