Framework: Add "develop-to-master promotion version" macro

[mhoemmen]

Add a macro somewhere in Trilinos, with an integer that gets incremented on each develop-to-master promotion.

@trilinos/framework

Current Behavior

I can't write code that distinguishes between different Trilinos develop-to-master promotions.

Motivation and Context

Last commit to Trilinos 12.0 was in 2015. Last minor version release (12.12) was in July 2017. This suggests that major version releases may never happen, and that develop-to-master promotions are now the de facto minor version releases. Thus, Trilinos major version numbers are no longer useful for tracking backwards compatibility issues. #2290 (where a harmless change announced literally 3 years ago may break applications that forward-declare templated classes) highlights the value of being able to distinguish between Trilinos develop-to-master promotions.

This issue hinders Tpetra FY18 goals. @kddevin @trilinos/tpetra

[mhoemmen]

@rrdrake and @jwillenbring expressed interest in this today.

[barche]

Interested too, after also hitting a compile error after #2290

[mhoemmen]

@barche

1. FYI forward declarations of templated classes don't have a way to specify default template parameter values
2. Please don't ever specify template parameters that have default values, unless you intend to use a nondefault value for them

[barche]

I hit this when mapping Tpetra types to Julia, specifically here: https://github.com/barche/Trilinos.jl/blob/master/deps/src/trilinoswrap/tpetra.cpp#L213 Here, T is e.g. a Tpetra::CrsGraph. I would have expected that the false would not need to be specified since it is the default, but if I don't put it explicitly the compiler says there are not enough template arguments.

FWIW, I'm glad this parameter is removed (it will simplify my code!), but in general I think it's a good idea to have a macro to detect breaking changes, so it's easier to support multiple versions. It's not really a pressing issue for me, since the Julia wrappers are still in heavy development and I can just use git master on all my machines.

[jwillenbring]

@trilinos/framework let’s discuss this at the Monday standup.

[prwolfe]

2 thoughts - shouldn't the code be embedded in a version - i.e. would not need to discriminate between versions, and what about git describe? If we are doing a single merge commit per promotion it will give us the count you are after. If we doing full history imports it won't.

[mhoemmen]

@barche wrote:

but if I don't put it explicitly the compiler says there are not enough template arguments.

Just curious -- what compiler are you using? I've only ever seen this when people do their own forward declarations of Tpetra classes.

I have some type aliases in the works that should make it easier to do backwards compatibility for Tpetra classes' template parameters.

[barche]

After realizing I forgot to remove the bool inthe template template parameter spec, the error I get on GCC 7.3.1 is:

expected a template of type ‘template<class, class, class> class T’, got ‘template<class LO, class GO, class N, bool isClassic> class Tpetra::CrsGraph’

Clang gives a similar error, it seems that default arguments are not taken into account when matching template template parameters.

[mhoemmen]

@barche Template parameters that are constexpr values (instead of types) are weird. They don't count as part of a parameter pack, for example. This is one of my motivations for getting rid of this ;-) .

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