Implement CWG 909 #3
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when reference binding to temporary https://stackoverflow.com/q/66816741/567292
(C-style cast notation), per [expr.cast].
ecatmur
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Jul 17, 2022
This patch makes us avoid substituting into the TEMPLATE_PARM_CONSTRAINTS of each template parameter except as necessary for declaration matching, like we already do for the other constituent constraints of a declaration. This patch also improves the CA104 implementation of explicit specialization matching of a constrained function template inside a class template, by considering the function's combined constraints instead of just its trailing constraints. This allows us to correctly handle the first three explicit specializations in concepts-spec2.C below, but because we compare the constraints as a whole, it means we incorrectly accept the fourth explicit specialization which writes #3's constraints in a different way. For complete correctness here, determine_specialization should use tsubst_each_template_parm_constraints and template_parameter_heads_equivalent_p. PR c++/100374 gcc/cp/ChangeLog: * pt.cc (determine_specialization): Compare overall constraints not just the trailing constraints. (tsubst_each_template_parm_constraints): Define. (tsubst_friend_function): Use it. (tsubst_friend_class): Use it. (tsubst_template_parm): Don't substitute TEMPLATE_PARM_CONSTRAINTS. gcc/testsuite/ChangeLog: * g++.dg/cpp2a/concepts-spec2.C: New test. * g++.dg/cpp2a/concepts-template-parm11.C: New test.
ecatmur
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that referenced
this pull request
May 17, 2023
I noticed that for member class templates of a class template we were
unnecessarily substituting both the template and its type. Avoiding that
duplication speeds compilation of this silly testcase from ~12s to ~9s on my
laptop. It's unlikely to make a difference on any real code, but the
simplification is also nice.
We still need to clear CLASSTYPE_USE_TEMPLATE on the partial instantiation
of the template class, but it makes more sense to do that in
tsubst_template_decl anyway.
#define NC(X) \
template <class U> struct X##1; \
template <class U> struct X##2; \
template <class U> struct X##3; \
template <class U> struct X##4; \
template <class U> struct X##5; \
template <class U> struct X##6;
#define NC2(X) NC(X##a) NC(X##b) NC(X##c) NC(X##d) NC(X##e) NC(X##f)
#define NC3(X) NC2(X##A) NC2(X##B) NC2(X##C) NC2(X##D) NC2(X##E)
template <int I> struct A
{
NC3(am)
};
template <class...Ts> void sink(Ts...);
template <int...Is> void g()
{
sink(A<Is>()...);
}
template <int I> void f()
{
g<__integer_pack(I)...>();
}
int main()
{
f<1000>();
}
gcc/cp/ChangeLog:
* pt.cc (instantiate_class_template): Skip the RECORD_TYPE
of a class template.
(tsubst_template_decl): Clear CLASSTYPE_USE_TEMPLATE.
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Apply C-style cast rules for user-defined conversions; add const qualification to static_cast stage of C-style cast; when reference binding to temporary
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_closed.html#909
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77465
https://stackoverflow.com/q/66816741/567292