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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - ad hoc requires-expression being evaluated too eagerly"
href="https://bugs.llvm.org/show_bug.cgi?id=50137">50137</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>ad hoc requires-expression being evaluated too eagerly
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>unspecified
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>C++2a
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>pilarlatiesa@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>blitzrakete@gmail.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
</td>
</tr></table>
<p>
<div>
<pre>The following program prints: "Overridden", "Fallback":
```cpp
#include <iostream>
#include <type_traits>
template<typename T>
class Base
{
private:
static constexpr bool IsOverridden = requires(T const x) { x.template
f<std::true_type{}>(); };
public:
template<std::false_type = std::false_type{}> requires IsOverridden
void f() const
{ static_cast<T const *>(this)->f(); }
template<std::false_type = std::false_type{}>
void f() const
{ std::cout << "Fallback" << std::endl; }
};
class A : public Base<A>
{
public:
template<std::true_type = std::true_type{}>
void f() const
{ std::cout << "Overridden" << std::endl; }
};
class B : public Base<B> {};
template<typename T>
void f(Base<T> const &x)
{ x.f(); }
int main()
{
A const a;
B const b;
f(a);
f(b);
return 0;
}
```
However, if we remove the bool constant and use the requires-expression
directly in the requires-clause, it prints "Fallback", "Fallback":
```cpp
#include <iostream>
#include <type_traits>
template<typename T>
class Base
{
public:
template<std::false_type = std::false_type{}>
requires requires(T const x) { x.template f<std::true_type{}>(); };
void f() const
{ static_cast<T const *>(this)->f(); }
template<std::false_type = std::false_type{}>
void f() const
{ std::cout << "Fallback" << std::endl; }
};
class A : public Base<A>
{
public:
template<std::true_type = std::true_type{}>
void f() const
{ std::cout << "Overridden" << std::endl; }
};
class B : public Base<B> {};
template<typename T>
void f(Base<T> const &x)
{ x.f(); }
int main()
{
A const a;
B const b;
f(a);
f(b);
return 0;
}
```
GCC consistently prints "Overridden", "Fallback" in both cases, which I believe
is what makes sense. Please let me know if it's a bug, and otherwise I'll
report it to GCC.
Thanks
Pili</pre>
</div>
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