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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 - Class template argument deduction (CTAD) not working in module"
href="https://bugs.llvm.org/show_bug.cgi?id=45292">45292</a>
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<th>Summary</th>
<td>Class template argument deduction (CTAD) not working in module
</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>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>C++2a
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>rbock@eudoxos.de
</td>
</tr>
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<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>
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<pre>Given the following module
// ----------------------------------
export module mod;
export template<typename T>
struct something
{
constexpr something(T){}
};
export template<typename T>
constexpr auto make_something(T t)
{
return something{t}; // uses CTAD
}
// ----------------------------------
Compiled with clang: clang++ -std=c++20 -stdlib=libc++ -fmodules -c -Xclang
-emit-module-interface -o mod.pcm mod.cpp
And the following application code:
// ----------------------------------
import mod;
int main()
{
constexpr auto x = make_something(7);
}
// ----------------------------------
Compiled with: clang++ -std=c++20 -stdlib=libc++ -fmodules -fimplicit-modules
-fimplicit-module-maps -fprebuilt-module-path=. main.cpp
The latter fails with the following error message:
// ----------------------------------
In file included from main.cpp:1:
mod.cpp:13:12: error: no viable constructor or deduction guide for deduction of
template arguments of 'something'
return something{t}; // uses CTAD
^
main.cpp:5:28: note: in instantiation of function template specialization
'make_something<int>' requested here
constexpr auto x = make_something(7);
^
1 error generated.
// ----------------------------------
Of course, I can change the module code and specify the the template parameter.
But this seems to be a bug, see also discussion here:
<a href="https://stackoverflow.com/questions/60827958/should-class-template-argument-deduction-ctad-work-inside-modules">https://stackoverflow.com/questions/60827958/should-class-template-argument-deduction-ctad-work-inside-modules</a></pre>
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