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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 - Invalid code generation and ICE when template references local static constexpr variable by pointer"
href="https://bugs.llvm.org/show_bug.cgi?id=39916">39916</a>
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<th>Summary</th>
<td>Invalid code generation and ICE when template references local static constexpr variable by pointer
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>release blocker
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>C++'17
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>andyg1001@hotmail.co.uk
</td>
</tr>
<tr>
<th>CC</th>
<td>blitzrakete@gmail.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<p>
<div>
<pre>The following minimised code fragment generates incorrect code in all clang
version from 4.0 (oldest I have) up to trunk (tested using godbolt and -O2
-std=c++1z):
template <const int* X>
struct S
{
static int Value() { return *X; }
};
template <typename T>
int fn()
{
static constexpr int value = 10;
return S<&value>::Value();
}
int test()
{
return fn<int>(); // will return 0 not 10
}
There are ways that the code can be made to produce correct values:
1. Move the 'static constexpr int value = 10' line up to the global scope
2. Remove the template from 'int fn()' to make it a plain function
3. Change the template parameter in struct S from 'const int* X' to 'const
int& X'
So, for example, the following compiles correctly...
template <const int& X>
struct S
{
static int Value() { return X; }
};
template <typename T>
int fn()
{
static constexpr int value = 10;
return S<value>::Value();
}
int test()
{
return fn<int>(); // will return 0 not 10
}
The following, slightly more complex example, crashes the compiler:
template <const int* X>
struct S
{
static int Value() { return *X; }
};
template <typename T>
int fn()
{
const int c = ([]() {
static constexpr int value = 10;
return S<&value>::Value();
})();
return c;
}
int test()
{
return fn<int>();
}
Again, any one of the three "solutions" above solves the problem. So the issue
seems to be in the specific use of pointers in the template for S when called
from another templated function.
However, while we're here, this also crashes the compiler:
template <const int& X>
struct S
{
static int Value() { return X; }
};
template <typename T>
int fn()
{
const int c = ([]() {
static int value = 10; // not const or constexpr!
return S<value>::Value();
})();
return c;
}
int test()
{
return fn<int>();
}
But utilising a second "solution" from above means this is ok again:
template <const int& X>
struct S
{
static int Value() { return X; }
};
int fn()
{
const int c = ([]() {
static int value = 10;
return S<value>::Value();
})();
return c;
}
int test()
{
return fn();
}</pre>
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