Adapted code to newest gsl::span

Quite trivial fix but I do not understand why we need it. Maybe @graven can comment ?

As an hint, here is the relevant change in the gsl::span code :

-    constexpr span(pointer ptr, size_type count) noexcept : storage_(ptr, count)
+    template <std::size_t MyExtent = Extent, std::enable_if_t<MyExtent != gsl::dynamic_extent, int> = 0>
+    constexpr explicit span(pointer ptr, size_type count) noexcept : storage_(ptr, count)
     {
-        if (Extent != dynamic_extent) Expects(count == Extent);
+        Expects(count == Extent);
     }

I suppose the templating breaks the usage of initializer list ?

added lhcb-lcg-dev4 label

@cattanem for info

added backport run2 label

added lhcb-lcg-dev3 label

Does this work also with the LCG_97a version of gsl::span?

It is the addition of explicit to the constructor that forces the change -- see here. There has been a proposal to make return { ... } implicitly be considered as being explicit (i.e. allow it invoke constructors labelled as explicit), but that didn't make it....

Oh I did not notice the explicit ! Indeed.

An to answer @cattanem, yes this is backward compatible. It's basically the same code as before, just more verbose.

This reminds me of something else -- for the new GSL, they made it 'tricky' to remove the bounds checking, which actually uses non-zero resources. So in STLExtensions.h I worked around it so that in non-debug mode the bounds checking is still removed. But due to the dependencies, the code in LHCbMath directly uses GSL::span instead of through STLExtensions.h. So the little pre-amble that does some preprocessor magic to remove the bounds checking has to be copied into Similarity.h. I'll make a separate MR for that....

see !2696 (merged)

• [2020-08-14 00:06] Validation started with lhcb-lcg-dev4#1373
• [2020-08-14 00:16] Validation started with lhcb-lcg-dev3#1358

This fixed the errors in Similarity, but there are a couple of similar ones elsewhere. See https://lhcb-nightlies.web.cern.ch/logs/build/nightly/lhcb-lcg-dev3/1358/x86_64-centos7-gcc9-opt/LHCb/

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• 77d0eb48 - Adapted Similarity code to newest gsl::span

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changed title from Adapted Similarity code to newest gsl::span to Adapted code to newest gsl::span

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• cf846b61 - Adapted Similarity code to newest gsl::span

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