What tokens are permitted as arguments to #include?

Question

What sort of tokens are required to be allowed by the standard in includes? E.g., are spaces in file names allowed?

Answer 1

From cppreference on Source file inclusion

Any preprocessing tokens (macro constants or expressions) are permitted as arguments to #include and __has_include (since C++17) as long as they expand to a sequence of characters surrounded by < > or "".

Then in Explanation

Searches for the file in implementation-defined manner. The intent of this syntax is to search for the files that are not controlled by the implementation.

Furthermore, the c++20 final working draft 5.8 Header names [lex.header] and
ISO/IEC 9899:1999 6.4.7 Header names except newline, > and ".

header-name:
    < h-char-sequence >
    " q-char-sequence "
h-char-sequence :
    h-char
    h-char-sequence h-char
h-char:
    any member of the source character set except new-line and >
q-char-sequence :
    q-char
    q-char-sequence q-char
q-char:
    any member of the source character set except new-line and "