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Clang issue in CTAD #62925

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walge-laine opened this issue May 24, 2023 · 3 comments · Fixed by #94752
Closed

Clang issue in CTAD #62925

walge-laine opened this issue May 24, 2023 · 3 comments · Fixed by #94752
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@yuanfang-chen
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c++20 clang:frontend Language frontend issues, e.g. anything involving "Sema" confirmed Verified by a second party

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@walge-laine
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Clang fails to compile cppreference example code. See Compiler Explorer.
@EugeneZelenko EugeneZelenko added c++20 clang:frontend Language frontend issues, e.g. anything involving "Sema" and removed new issue labels May 24, 2023
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llvmbot commented May 24, 2023

@llvm/issue-subscribers-c-20

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llvmbot commented May 24, 2023

@llvm/issue-subscribers-clang-frontend

@shafik
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shafik commented May 24, 2023 •
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Code in question:

#include <map>

int main() {
   std::map m1 = {std::pair{"foo", 2}, {"bar", 3}}; // guide #2
   std::map m2(m1.begin(), m1.end()); // guide #1
}

Diagnostic:

<source>:4:13: error: no viable constructor or deduction guide for deduction of template arguments of 'map'
   std::map m1 = {std::pair{"foo", 2}, {"bar", 3}}; // guide #2
            ^
/opt/compiler-explorer/gcc-12.2.0/lib/gcc/x86_64-linux-gnu/12.2.0/../../../../include/c++/12.2.0/bits/stl_map.h:204:7: note: candidate template ignored: couldn't infer template argument '_Key'
      map(const _Compare& __comp,
      ^
/opt/compiler-explorer/gcc-12.2.0/lib/gcc/x86_64-linux-gnu/12.2.0/../../../../include/c++/12.2.0/bits/stl_map.h:283:2: note: candidate template ignored: couldn't infer template argument '_Key'
        map(_InputIterator __first, _InputIterator __last)
        ^
/opt/compiler-explorer/gcc-12.2.0/lib/gcc/x86_64-linux-gnu/12.2.0/../../../../include/c++/12.2.0/bits/stl_map.h:238:7: note: candidate template ignored: could not match 'initializer_list' against 'pair'
      map(initializer_list<value_type> __l,
      ^
/opt/compiler-explorer/gcc-12.2.0/lib/gcc/x86_64-linux-gnu/12.2.0/../../../../include/c++/12.2.0/bits/stl_map.h:250:7: note: candidate template ignored: could not match 'map' against 'pair'
      map(const map& __m, const __type_identity_t<allocator_type>& __a)
      ^
/opt/compiler-explorer/gcc-12.2.0/lib/gcc/x86_64-linux-gnu/12.2.0/../../../../include/c++/12.2.0/bits/stl_map.h:254:7: note: candidate template ignored: could not match 'map' against 'pair'
      map(map&& __m, const __type_identity_t<allocator_type>& __a)
      ^
/opt/compiler-explorer/gcc-12.2.0/lib/gcc/x86_64-linux-gnu/12.2.0/../../../../include/c++/12.2.0/bits/stl_map.h:260:7: note: candidate template ignored: could not match 'initializer_list' against 'pair'
      map(initializer_list<value_type> __l, const allocator_type& __a)
      ^
/opt/compiler-explorer/gcc-12.2.0/lib/gcc/x86_64-linux-gnu/12.2.0/../../../../include/c++/12.2.0/bits/stl_map.h:1472:5: note: candidate template ignored: substitution failure [with _InputIterator = std::pair<const char *, int>]: no member named 'value_type' in 'std::iterator_traits<std::pair<const char *, int>>'
    map(_InputIterator, _InputIterator,
    ^
/opt/compiler-explorer/gcc-12.2.0/lib/gcc/x86_64-linux-gnu/12.2.0/../../../../include/c++/12.2.0/bits/stl_map.h:1481:5: note: candidate template ignored: could not match 'initializer_list' against 'pair'
    map(initializer_list<pair<_Key, _Tp>>,
    ^
/opt/compiler-explorer/gcc-12.2.0/lib/gcc/x86_64-linux-gnu/12.2.0/../../../../include/c++/12.2.0/bits/stl_map.h:1494:5: note: candidate template ignored: could not match 'initializer_list' against 'pair'
    map(initializer_list<pair<_Key, _Tp>>, _Allocator)
    ^
/opt/compiler-explorer/gcc-12.2.0/lib/gcc/x86_64-linux-gnu/12.2.0/../../../../include/c++/12.2.0/bits/stl_map.h:217:7: note: candidate function template not viable: requires 1 argument, but 2 were provided
      map(const map&) = default;
      ^
/opt/compiler-explorer/gcc-12.2.0/lib/gcc/x86_64-linux-gnu/12.2.0/../../../../include/c++/12.2.0/bits/stl_map.h:225:7: note: candidate function template not viable: requires 1 argument, but 2 were provided
      map(map&&) = default;
      ^
/opt/compiler-explorer/gcc-12.2.0/lib/gcc/x86_64-linux-gnu/12.2.0/../../../../include/c++/12.2.0/bits/stl_map.h:246:7: note: candidate function template not viable: requires single argument '__a', but 2 arguments were provided
      map(const allocator_type& __a)
      ^
/opt/compiler-explorer/gcc-12.2.0/lib/gcc/x86_64-linux-gnu/12.2.0/../../../../include/c++/12.2.0/bits/stl_map.h:100:11: note: candidate function template not viable: requires 1 argument, but 2 were provided
    class map
          ^
/opt/compiler-explorer/gcc-12.2.0/lib/gcc/x86_64-linux-gnu/12.2.0/../../../../include/c++/12.2.0/bits/stl_map.h:266:2: note: candidate function template not viable: requires 3 arguments, but 2 were provided
        map(_InputIterator __first, _InputIterator __last,
        ^
/opt/compiler-explorer/gcc-12.2.0/lib/gcc/x86_64-linux-gnu/12.2.0/../../../../include/c++/12.2.0/bits/stl_map.h:1488:5: note: candidate function template not viable: requires 3 arguments, but 2 were provided
    map(_InputIterator, _InputIterator, _Allocator)
    ^
/opt/compiler-explorer/gcc-12.2.0/lib/gcc/x86_64-linux-gnu/12.2.0/../../../../include/c++/12.2.0/bits/stl_map.h:195:7: note: candidate function template not viable: requires 0 arguments, but 2 were provided
      map() = default;
      ^
/opt/compiler-explorer/gcc-12.2.0/lib/gcc/x86_64-linux-gnu/12.2.0/../../../../include/c++/12.2.0/bits/stl_map.h:300:2: note: candidate function template not viable: requires at least 3 arguments, but 2 were provided
        map(_InputIterator __first, _InputIterator __last,
        ^
1 error generated.
Compiler returned: 1

Note both gcc/MSVC accept: https://godbolt.org/z/6v4fvabb5

@shafik shafik added the confirmed Verified by a second party label May 24, 2023
@yuanfang-chen yuanfang-chen self-assigned this Jul 5, 2023
@spaits spaits mentioned this issue Jun 7, 2024
Merged
@spaits spaits closed this as completed in f80bd9b Jun 18, 2024
AlexisPerry pushed a commit to llvm-project-tlp/llvm-project that referenced this issue Jul 9, 2024
@spaits @AlexisPerry
[Sema][CTAD] Allow user defined conversion for copy-list-initializati…
816c92b 
…on (llvm#94752)

Fixes llvm#62925.

The following code:
```cpp
#include <map>

int main() {
   std::map m1 = {std::pair{"foo", 2}, {"bar", 3}}; // guide llvm#2
   std::map m2(m1.begin(), m1.end()); // guide llvm#1
}
```
Is rejected by clang, but accepted by both gcc and msvc:
https://godbolt.org/z/6v4fvabb5 .

So basically CTAD with copy-list-initialization is rejected.

Note that this exact code is also used in a cppreference article:
https://en.cppreference.com/w/cpp/container/map/deduction_guides

I checked the C++11 and C++20 standard drafts to see whether suppressing
user conversion is the correct thing to do for user conversions. Based
on the standard I don't think that it is correct.

```
13.3.1.4 Copy-initialization of class by user-defined conversion [over.match.copy]
Under the conditions specified in 8.5, as part of a copy-initialization of an object of class type, a user-defined
conversion can be invoked to convert an initializer expression to the type of the object being initialized.
Overload resolution is used to select the user-defined conversion to be invoked
```
So we could use user defined conversions according to the standard.

```
If a narrowing conversion is required to initialize any of the elements, the
program is ill-formed.
```
We should not do narrowing.

```
In copy-list-initialization, if an explicit constructor is chosen, the initialization is ill-formed.
```
We should not use explicit constructors.
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@yuanfang-chen yuanfang-chen

Labels
c++20 clang:frontend Language frontend issues, e.g. anything involving "Sema" confirmed Verified by a second party
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C++ 20 in Clang
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[Sema][CTAD] Allow user defined conversion for copy-list-initialization spaits/llvm-project
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@yuanfang-chen @shafik @EugeneZelenko @llvmbot @walge-laine