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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - unused member variable causes code to compile for member to function for undefined function"
href="https://bugs.llvm.org/show_bug.cgi?id=44175">44175</a>
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<th>Summary</th>
<td>unused member variable causes code to compile for member to function for undefined function
</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++17
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>marcpawl@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><a href="https://godbolt.org/z/VygQuN">https://godbolt.org/z/VygQuN</a>
If the class is not instantiated and the variable is not static then the
reference to a non-existing method is not reported as an error.
#include <type_traits>
#include <cstdio>
struct Foo {
virtual void hello(int) = 0;
};
struct Bar : public Foo {
void hello(int) override {};
};
struct Bad { };
template <typename T>
struct is_Foo {
using hello_fn_t = void (T::*)(int);
void (T::*hello)(int) = &T::hello;
constexpr static bool value=true;
};
template <typename T, typename U=is_Foo<T>>
void say(T& t)
{
static_assert(U::value, "not a Foo");
puts("hello\n");
//U u; // Line 22
//t.hello(4); // Line 23
}
int main()
{
//Bar b;;
//Foo& f = b;
//say(b);
//say(f);
Bad bad;
say(bad);
}
====
Found while exploring a GCC bug
to gcc-bugzilla-account-request
<a href="https://godbolt.org/z/SFZmZJ">https://godbolt.org/z/SFZmZJ</a>
In clang you get
no member names 'hello' in 'Bad'
which is the expected result.
In all GCC versions using --std=c++17 from 5.2 onwards the code compiles.
#include <type_traits>
#include <cstdio>
struct Foo {
virtual void hello(int) = 0;
};
struct Bar : public Foo {
void hello(int) override {};
};
struct Bad { };
template <typename T>
struct is_Foo {
using hello_fn_t = void (T::*)(int);
constexpr static void (T::*hello)(int) = &T::hello;
static constexpr bool value=true;
};
template <typename T>
auto say(T& t) -> std::enable_if_t<is_Foo<T>::value, void>{
//U u; // Line 22
//t.hello(4); // Line 23
puts("hello\n");
}
int main()
{
//Bar b;;
//Foo& f = b;
//say(b);
//say(f);
Bad bad;
say(bad);
}
If you change say to
template <typename T, typename U=is_Foo<T>>
void say(T& t)
{
U u; // Line 22
//t.hello(4); // Line 23
}
you get the same errors in clang, but no errors in gcc.</pre>
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