`cfg_select!` in expression context interacts surprisingly with block expressions

I had code similar to the following:

return cfg_select! {
    feature = "check_required" => Err(()),
    _ => {
        warn!("Did not check.");
        Ok(())
    }
};

I expected this to expand to a block expression, somewhat in line with the behavior one is used to from match or various select! macros for sockets and channels.
Instead, the second branch is expanded to the contents of the curly braces rather than a block expression, triggering semicolon_in_expressions_from_macros.
This feels especially unexpected since the usage without any curly braces is also permitted. One could therefore reasonably expect that the curly braces are parsed as a block expression rather than as part of the macro invocation.

Meta

rustc --version --verbose:

rustc 1.98.1 (48a229cea 2026-09-01)
binary: rustc
commit-hash: 48a229ceaefd4985c50990b14116b6d856af0985
commit-date: 2026-09-01
host: x86_64-unknown-linux-gnu
release: 1.98.1
LLVM version: 22.1.8

Start by reproducing the cfg_select! example from the issue with the reported rustc version and inspect its macro expansion and diagnostic. Trace how the branch containing curly braces is interpreted, then establish the expected expansion and add or update coverage so the behavior is clearly validated.