Consider adding `_Pragma` operator

[gklimowicz]

In C 2023 §6.10.10, the _Pragma operator is defined.

_Pragma ( string-literal ) "destringizes" the string literal an processes that string as if it were a #pragma, deleting the original four tokens.

[kc9jud]

Interesting... I somehow always assumed that it went #pragma -> _Pragma rather than the other way around.

I'm strongly for adding _Pragma.

[bonachea]

I'm also in favor of adding _Pragma.

I somehow always assumed that it went #pragma -> _Pragma rather than the other way around.

It is neither. The actual C spec-ese: (emphasis added)

A unary operator expression of the form:
_Pragma ( string-literal )
is processed as follows: The string literal is destringized by deleting any encoding prefix, deleting
the leading and trailing double-quotes, replacing each escape sequence " by a double-quote, and
replacing each escape sequence \ by a single backslash. The resulting sequence of characters
is processed through translation phase 3 to produce preprocessing tokens that are executed as if
they were the pp-tokens in a pragma directive. The original four preprocessing tokens in the unary
operator expression are removed.

In particular, it does NOT say that _Pragma is literally replaced by #pragma, it only says the argument pp-tokens are processed in the same (processor-specific) way as if they had been arguments to #pragma. This subtlety matters because _Pragma is a unary operator that appears on source lines, and source lines cannot change into directive lines during preprocess.

[kc9jud]

From yesterday's discussion, we decided to defer this to a separate paper from the initial specifications paper.