GSL violates SF.12

[beinhaerter]

SF.12: Prefer the quoted form of #include for files relative to the including file and the angle bracket form everywhere else

[...] the guidance is to use the quoted form for including files that exist at a relative path to the file containing the #include statement (from within the same component or project) and to use the angle bracket form everywhere else, where possible. [...]

The GSL includes its own files as #include <gsl/xxx>. It would be better to use #include "xxx" as SF.12 proposes.

If you agree to SF.12, I would be ready to prepare a PR.

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The way it currently is, results in problems in our projects. Following is some background.

We have a build environment which handles all projects and the include pathes. For example

Project not under my control:

project1/include/...
project1/src/project1.cpp

Project under my control:

project2/include/project2/project2.h
project2/src/project2.cpp

Project under my control:

project3/include/project3/project3.h
project3/src/project3.cpp

For project3 the build environment (not under my control) uses gcc with -Iproject1/include -Iproject2/include.
project3.cpp uses #include <project2/project2.h>.

In this structure we want to integrate GSL. GSL cannot be a made global project, so I must make it a subfolder in a project under my control (project2). I want to add a new folder project2/include/project2/gsl with all the GSL headers like

project2/include/project2/gsl/narrow
project2/include/project2/gsl/util

The GSL folder is added under project2/include/project2/ and not under project2/include/ so that it cannot collide if for example project1's author decides to add the gsl folder as project1/include/gsl.

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In this setup, to use GSL, project2 needs to write #include <project2/gsl/narrow>.

This fails because gsl/narrow does #include <gsl/util> which fails. It would however work if GSL used #include "util" as SF.12 suggests.

The way we currently solve it, is to apply the changes that I ask for, in our local copy of the GSL.

[dmitrykobets-msft]

Hi @beinhaerter, thanks for the clear explanation of this issue. Your proposal seems good to me, I'd be happy to take a look at your PR.