P4012R0 — value-preserving consteval broadcast to simd::vec

P4012R0 — value-preserving consteval broadcast to simd::vec (6 items) LEWG, LWG

Matthias Kretz

This paper proposes adding a consteval broadcast constructor overload to std::simd's basic_vec that enables value-preserving implicit conversions from arithmetic types like int, restoring behavior from the Parallelism 2 TS that was lost in the C++26 Committee Draft. The consteval overload is constrained to types that are not already value-preserving-convertible to the target type, and uses constexpr exceptions to make value-changing conversions ill-formed at compile time. A secondary alternative of simply removing explicit conversion on broadcast is also presented as a fallback to preserve design space for C++29.

Section 10.4, p. 9 — The consteval constructor declaration in the class synopsis ([simd.overview]) is missing a terminating semicolon; all other declarations in the same synopsis have one. [1]
Section 10.5, p. 10 — The consteval constructor declaration heading in the specification ([simd.ctor]) is missing a terminating semicolon. [3] [2]
Section 10.5, p. 10 — Stable name "[simd.ctore]" does not exist; should be "[simd.ctor]". [1] [3]
Section 11.3, p. 11 — Stable name "[simd.ctore]" does not exist; should be "[simd.ctor]". [2] [4]
Section 5 (Motivation), p. 4 — "instantations" is misspelled; should be "instantiations". [5]
Section 5 (Motivation), p. 3 — "it's" (contraction) used where possessive "its" is required. [6]

References — Anthropic Citations API

[1]
"P4012R0 11 Wording for removing explicit conversion on broadcast 10.5 modify [simd.ctor] In [simd.ctore], change: [simd.ctor] template U> constexpr..."

[1]
"P4012R0 A really_convertible_to definition 11.3 modify [simd.ctor] In [simd.ctore], change: [simd.ctor] template constexpr explicit(see below) basic_vec(U&& value) noexcept; 5 Let..."

[2]
"P4012R0 11 Wording for removing explicit conversion on broadcast 10.5 modify [simd.ctor] In [simd.ctore], change: [simd.ctor] template U> constexpr..."

[2]
"P4012R0 A really_convertible_to definition 11.3 modify [simd.ctor] In [simd.ctore], change: [simd.ctor] template constexpr explicit(see below) basic_vec(U&& value) noexcept; 5 Let..."

[3]
"template U> consteval basic_vec(U&& x) template"

[4]
"template U> consteval basic_vec(U&& x) -?- Preconditions: The value of x is equal to the value of x after conversion to value_type. "

[5]
"However, V(0x5EAF00D) is shorter and needs fewer template instantations."

[6]
"a basic_vec broadcast constructor (with it's helper types/concepts to determine whether the specialization is allowed)"

Summary: P4012R0 proposes making the broadcast constructor of simd::basic_vec a consteval function so that narrowing or non-value-preserving conversions from scalar to vector element type are diagnosed at compile time rather than silently truncating. It also removes the explicit qualifier from the broadcast constructor for consteval-safe argument types. Six defects were found in the proposed wording and motivational text.

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