Tuple: Silence modernize-pass-by-value

[ax3l]

Summary

clang-tidy's modernize-pass-by-value fires on detail::gpu_tuple_element's constructor and asks for the argument to be taken by value and moved. Following that advice would be a pessimization here. This adds a NOLINTNEXTLINE with the reasoning written down, because it is not obvious from the code.

Why by value is wrong here

The forwarding constructor immediately below already takes everything that can be deduced — lvalues and rvalues of T alike. The const-reference overload is reached only by arguments it cannot deduce, a braced initializer list above all:

struct P { int a, b; };

P p{1,2};
E<P> a(p);        // forwarding constructor
E<P> b(P{3,4});   // forwarding constructor
E<P> c({5,6});    // this one -- U cannot be deduced from a braced-init-list

(verified by instrumenting the two constructors and running the three cases)

So taking it by value would add a move for exactly the callers this overload exists to serve, and save a copy for nobody, since rvalues never reach it. Removing the overload instead is not an option either — the braced-init case would stop compiling.

Why it is showing up now

Nothing changed in AMReX_Tuple.H. The SIMD CI job runs clang-tidy over ccache misses, so which headers get linted depends on what was recompiled. A change to a widely included header — #5644 touches AMReX_Math.H — pulls AMReX_Tuple.H into the linted set and the finding appears. It is pre-existing on development and unrelated to that work, which is why it is split out here: this is a two-line comment change that can be reviewed on its own, and #5644 can rebase on it afterwards.

Testing

clang-tidy-21 --config-file=.clang-tidy --header-filter='.*AMReX.*'

over Tests/SIMD/main.cpp and Tests/Particles/ParticleReduceSIMD/main.cpp: modernize-pass-by-value findings across all AMReX headers go from 1 to 0, and this was the only one. AMReX_Tuple.H still compiles and behaves the same (amrex::makeTuple with scalar and aggregate members).

🤖 Generated with Claude Code

[ax3l]

Open to either compact or explained version:

Suggested change

	// This overload is what catches arguments the forwarding constructor below
	// cannot deduce, a braced initializer list above all. Everything else, both
	// lvalues and rvalues of T, already goes there. Taking this one by value
	// would therefore add a move for exactly those callers while saving a copy
	// for nobody.
	// NOLINTNEXTLINE(modernize-pass-by-value)
	// NOLINTNEXTLINE(modernize-pass-by-value)