[headers] Is a header not necessarily a source file?

[cor3ntin]

In [headers] (technically library wording), we have

Each element of the C++ standard library is declared or defined (as appropriate) in a header (A header is not necessarily a source file)

Presumably, the intent here is to permit an implementation to make a set of declaration available when incanting #include <vector> through magic rather than by following steps 1-4 of translations

However, both [cpp.include] and [lex.phases] seem to apply to "headers" and seem to contradict the note in [headers]

A #include preprocessing directive causes the named header or source file to be processed from phase 1 through phase 4, recursively.

In [cpp.include], we also say "If the *header identified by the denotes an importable header [...]", and here "header" need not be a standard library header (as there can be non-standard importable headers).

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We have talked about the lack of clarity in this area before, but I could not find an issue for it.

I think a solution would be on the lines of

• Any source file whose content was included to the current processing unit through an #include directive is a header
• Both source files and headers behave the same through phases 1-4
• In [headers], rename header to standard library header
• We should remove the possibility that standard library headers are not source files as I'm not aware of anyone using that capability, nor can i imagine anyone wanting to ever do something like that.

[jensmaurer]

What's the observable differences for those approaches?

Since headers are supplied by the implementation, I think the as-if rule says you can't detect any difference.

[frederick-vs-ja]

IMO there's no actual defect here. But it seems to me that the suggestions can simplify and clarify the wording without changing normative requirements.

[cpplearner]

An implementation may use a Pre-Compiled Header (.pch file) when a header is requested. However, name lookup requires the point of declaration to be well-defined, which requires the declaration to exist as-if in a source file.

[frederick-vs-ja]

An implementation may use a Pre-Compiled Header (.pch file) when a header is requested.

Per [lex.phase] p1.1, IIUC, we can treat decoding PCH as an implementation-defined manner of mapping characters from the input file. So it's perhaps meaningless to allow a header not to be a source file.