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[temp.deduct.call] Clear up wording regarding function parameter packs appearing in non-deduced contexts #3900

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[temp.deduct.call] Clear up wording regarding function parameter packs appearing in non-deduced contexts #3900

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4 changes: 2 additions & 2 deletions source/templates.tex
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Original file line number Diff line number Diff line change
Expand Up @@ -7881,8 +7881,8 @@
Each deduction deduces template arguments for subsequent positions in
the template parameter packs expanded by the function parameter pack.
When a function parameter pack appears in a non-deduced
context\iref{temp.deduct.type}, the type of that pack is
never deduced.
context\iref{temp.deduct.type}, the template parameter pack(s)
expanded by that pack are never deduced, even if they appear in a deduced context elsewhere.
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The new wording seems confusing: packs are expanded by pack expansions, not by packs. Replacing "that pack" with "that function parameter pack" would help. Also, "(s)" -> "s", please.

Is it clear that the "even if" part is the intent? This seems ambiguous.

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... not only ambiguous, but inconsistent with unanimous implementation practice: https://godbolt.org/z/78TjoY

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IMO, it seems the current wording is suitable for either case.

If a template parameter is used only in non-deduced contexts and is not explicitly specified, template argument deduction fails.

When a function parameter pack appears in a non-deduced context ([temp.deduct.type]), the type of that pack is never deduced.

  1. Template parameter that appears in non-deduced context does not participate in template argument deduction
  2. These template parameters that only appear in non-deduced context should be explicitly specified their template arguments, otherwise, the deduction fails.

In addition, except that [temp.deduct.call#1] explicitly says that

When a function parameter pack appears in a non-deduced context ([temp.deduct.type]), the type of that pack is never deduced.

It seems that the other kinds of argument deduction do not strong this concept, that is

any template parameter that appears in a non-deduced context is never deduced

\begin{example}
\begin{codeblock}
template<class ... Types> void f(Types& ...);
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