Fix DoNotOptimize compile failure on types with const members

[devtejasx]

Fixes #1997

Problem

GCC and Clang both reject objects of class types with const data members as asm output operands — such objects cannot be stored to:

gcc:   error: read-only reference 'value' used as 'asm' output
clang: error: invalid lvalue in asm output

Since the const& overload of DoNotOptimize is deprecated, there was no usable spelling at all for these types when reached through a deduced non-const reference ([](auto& v) { DoNotOptimize(v); }), as reported in #1997.

Fix

Add an internal::IsAsmOutputOperand trait and route non-output-capable types to an input-only constraint with a memory clobber. Such objects cannot be legally written through anyway, so no optimization barrier is lost relative to the read-write constraint.

There is no portable trait for "contains a const member", so the trait approximates it as "not assignable" (is_array || is_copy_assignable || is_move_assignable):

• types with const members → correctly take the fallback (they'd fail to compile as outputs)
• reference-member types (e.g. the existing BitRef test type) are actually valid asm outputs but are indistinguishable by trait — they conservatively take the same fallback, which still compiles and keeps the clobber
• arrays are not assignable but are valid memory outputs, so they're accepted unconditionally

Both the clang/ICC branch and the GCC ≥ 5 branch get the dispatch; the GCC branch keeps its existing register-size/trivially-copyable split in the fallbacks ("r,m" vs "m", mirroring the deprecated const-ref overloads). MSVC needs no change (no inline asm).

Testing

• The reproducer from [BUG] DoNotOptimize on values with const members again #1997 now compiles with GCC 15 and Clang 21 (it fails with both on current main)
• donotoptimize_test.cc extended with const-member types: register-sized and larger, as lvalues, rvalues, and through a deduced non-const reference; builds and runs clean with -Wall -Wextra on GCC and Clang, -O1 and -O3

Disclosure

Per AGENTS.md: this PR was developed with AI assistance (Claude Code). The change and analysis have been reviewed and are owned by me.

[dmah42]

just going to put this here first: https://github.com/google/benchmark/blob/main/AGENTS.md

please explain why this is such a big problem that we need such complex code to be introduced to the library.

[devtejasx]

Disclosure, per AGENTS.md: this PR was developed with AI assistance (Claude Code). I've reviewed the change and the analysis, and I stand behind both — happy to argue any part of it on the merits.

On the substance — why this is a real problem:

DoNotOptimize currently cannot be used at all on a type with a const data member. The non-const overloads fail to compile (GCC: "read-only reference used as 'asm' output", Clang: "invalid lvalue in asm output"), and the const& overload is deprecated, so there is no non-warning spelling left. The failure mode bites hardest in generic code — the reproducer in #1997 is a lambda taking auto&, where the caller doesn't control the type being benchmarked.

Why the patch looks the way it does: the GCC branch already dispatches on trivially-copyable × register-size (8 overloads before this change). The patch adds one boolean trait to that existing matrix, and the fallbacks reuse the exact constraints of the existing deprecated const& overloads — no new mechanism.

If you'd prefer a smaller surface, two options I'd be fine with:

1. collapse the fallbacks to a single "m"-input overload per value category (register-sized const-member types lose the "r" alternative — negligible in practice), or
2. reject this class of types with a static_assert and a clear message instead of supporting them — smaller patch, but leaves [BUG] DoNotOptimize on values with const members again #1997 unfixed for generic code.

Happy to rework in either direction.