[lex.phases] replace term 'input file' with 'source file' in phase 1

[AlisdairM]

The only part of the standard that refers to input files is phase 1 of the phases of translation. Everywhere else, we use the term source file, that has a clear definition in the preceding [lex.separate] clarifying that we do not necessarily mean text files in a hosted filing system.

[jensmaurer]

CWG and/or SG16 spent quite some time massaging [lex.phases] p1 into its present form (for the UTF-8 source support); I won't touch that wording editorially.

Maybe "input file" is the subject of phase 1, and "source file" is what's handled in phases 2 and 3. And it's intentional that we only ever talk about the raw sources of translation in phase 1 and nowhere else.

[AlisdairM]

I am very happy to defer this to Core, but at the moment "input file" is not a defined term in the standard, and occurs in only this one paragraph. We are very careful to define "source file" as not necessarily implying a file in a filing system, but have no such notion of what "input file" should mean. I believe we need clarity here.

[jensmaurer]

Where in [lex.separate] do we expressly say we don't imply a "file"? I think we just do so by omission, and "input file" is no better or worse here.

[AlisdairM]

I stand corrected. The only reference I can find to "not necessarily a file" is a footnote in [headers], 16.4.2.3, and that applies to headers, not to source files.

[jensmaurer]

Can we please not do this change, then?

[AlisdairM]

I would still like the discussion in Core, because the term "input file" is not defined, unlike the term "source file". If you would prefer to create a core issue and close this PR, I would be content.

[AlisdairM]

Late comment: I actually just ran across the note that says source files need not be represented as physical files --- it is a note in phase 7 of translation, so not normative but clarifying what must otherwise be inferred.