web2c: \input braced filename
4.5.2 '\input' braced filename: '\input{some name}'
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As of Web2c 2020, '\input' filenames in TeX engines (this does not apply
in Metafont and MetaPost) can also be specified within a TeX group,
typically curly braces. For example:
\input{filename with spaces}
As always with TeX, the brace characters are not hardwired; what
counts is the category code: the first token after the '\input' must be
of catcode 1 (begin group), and it is matched with the next character of
catcode 2 (end group).
Within the group-delimited filename, braces are treated as normal
characters.
As with all forms of filenames, following spaces are ignored (after
the end group), and double quote ('"') characters are ignored within the
filename.