[Issue 12074] Add BlockStatement to TemplateSingleArgument

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=12074

--- Comment #4 from Vladimir Panteleev <dlang-bugzilla at thecybershadow.net> ---
That is a semantic issue and completely orthogonal to this syntactic one.
`call!({ return 0; })();` has the same issue as your example.

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