https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67018
Bug ID: 67018
Summary: [c++-concepts] Failure to partially order function
templates by constraints
Product: gcc
Version: c++-concepts
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: Casey at Carter dot net
Target Milestone: ---
Created attachment 36058
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=36058&action=edit
Preprocessed test case
Compiling this program - preprocessed test case attached - with r226205:
#include <stl2/iterator.hpp>
template<stl2::InputIterator>
constexpr bool f() { return false; } // Line 4
template<stl2::ForwardIterator>
constexpr bool f() { return true; } // Line 6
int main() {
static_assert(f<int*>()); // Line 9
}
results in the error:
~/concept-gcc/bin/g++ -std=gnu++1z -I ~/cmcstl2/include -I
~/cmcstl2/meta/include simplefoo.cpp -c
simplefoo.cpp: In function ‘int main()’:
simplefoo.cpp:9:25: error: call of overloaded ‘f()’ is ambiguous
static_assert(f<int*>());
^
simplefoo.cpp:4:16: note: candidate: constexpr bool f() [with
<template-parameter-1-1> = int*]
constexpr bool f() { return false; }
^
simplefoo.cpp:6:16: note: candidate: constexpr bool f() [with
<template-parameter-1-1> = int*]
constexpr bool f() { return true; }
Given that the ForwardIterator concept directly refines InputIterator:
template <class I>
concept bool ForwardIterator() {
return InputIterator<I>() &&
Incrementable<I>() &&
DerivedFrom<IteratorCategory<I>, forward_iterator_tag>();
}
the constraints of the ForwardIterator overload of f subsume those of the
InputIterator overload and the two templates should be ordered appropriately.
In the definition of WeakInputIterator, which InputIterator refines:
template <class I>
concept bool WeakInputIterator() {
return WeakIterator<I>() &&
Readable<I>() &&
requires (I& i, const I& ci) {
typename IteratorCategory<I>;
DerivedFrom<IteratorCategory<I>, weak_input_iterator_tag>();
requires Readable<decltype(i++)>();
requires Same<ValueType<I>, ValueType<decltype(i++)>>();
{ *ci } -> const ValueType<I>&;
};
}
template <class I>
concept bool InputIterator() {
return WeakInputIterator<I>() &&
Iterator<I>() &&
DerivedFrom<IteratorCategory<I>, input_iterator_tag>();
}
commenting out the line
requires Readable<decltype(i++)>();
makes the test compile and behave as expected.
