Minimum C++ standard: where should Boost.Graph draw the line?

[Becheler]

Story

BGL currently requires C++14 as its minimum standard. The discussion surfaced repeated requests for features from newer standards:

• C++17 std::execution policies for parallelism (mborland),
• C++20 concepts for better error messages (joaquintides, DmitriBogdanov),
• C++20 ranges for traversal (joaquintides, DmitriBogdanov)
• C++20 coroutines for concurrent algorithms (jeremy-murphy).

Each of these would meaningfully improve BGL's API and implementation, but each one raises the minimum bar for users.

The tension is real. Some BGL users are on older compilers by necessity: embedded systems, HPC clusters with frozen toolchains, corporate environments with slow upgrade cycles. Raising the minimum standard helps new contributors and improves the API, but potentially abandons existing users.

@jeremy-murphy also flagged reducing compiler warnings as a priority, many of which come from patterns that are deprecated or superseded in newer standards.

Dream solution

• BGL supports a minimum standard that is modern enough to express clean APIs ...
• ... without abandoning users who cannot upgrade.
• A clear policy exists for how and when the minimum standard can be raised.

Starting point

mborland's execution policy comment
joaquintides's concepts and ranges comment

Intermediary objective (45 min)

Read the starting points and discuss openly. Write down your findings on:

1. Which C++ features from C++17, C++20, and C++23 would most improve BGL if they were available unconditionally?
2. Who in the room or in the known user base would be broken by raising the minimum to C++17? To C++20?
3. Are there patterns from other Boost libraries that let a library support multiple standards gracefully, exposing richer APIs when newer standards are available?
4. What is the relationship between the minimum standard question and the boost::graph:: modern namespace question? Could the new namespace simply require C++20 while the legacy namespace stays at C++14?
5. What would it take to eliminate the current warning noise, and how much of that is standard-version dependent?

[pratzl]

Coroutines isn't very useful by itself in C++20. It's a building block for later features that are useful. A generator was added in C++23 that uses it.

[pratzl]

concepts aren't just about better error messages. They're useful to define constraints that are expected by the class/function. Their use also documents the requirements, reducing the need to "document" the requirements.