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Fedr opened this issue Jan 8, 2024 · 5 comments
Closed

Class template partial specialization with concept-auto non-type parameter is not supported #77377

Fedr opened this issue Jan 8, 2024 · 5 comments
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clang:frontend Language frontend issues, e.g. anything involving "Sema" concepts C++20 concepts confirmed Verified by a second party

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@Fedr
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Fedr commented Jan 8, 2024

In this program

#include <concepts>

template <auto>
struct A;

template <std::same_as<int> auto p>
struct A<p> {};

A<0> a;

A<auto> is a class template with non-type parameter, and A<std::same_as<int> auto> is its specialization for int parameters. The program is accepted by GCC and MSVC, but Clang complains:

error: class template partial specialization is not more specialized than the primary template [-Winvalid-partial-specialization]

Online demo: https://godbolt.org/z/ee4TcPY5K
@EugeneZelenko EugeneZelenko added clang:frontend Language frontend issues, e.g. anything involving "Sema" and removed new issue labels Jan 8, 2024
@llvmbot
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llvmbot commented Jan 8, 2024

@llvm/issue-subscribers-clang-frontend

Author: None (Fedr)

In this program ``` #include <concepts>

template <auto>
struct A;

template <std::same_as<int> auto p>
struct A<p> {};

A<0> a;

`A&lt;auto&gt;` is a class template with non-type parameter, and `A&lt;std::same_as&lt;int&gt; auto&gt;` is its specialization for `int` parameters. The program is accepted by GCC and MSVC, but Clang complains:

error: class template partial specialization is not more specialized than the primary template [-Winvalid-partial-specialization]

Online demo: https://godbolt.org/z/ee4TcPY5K
</details>

@cor3ntin cor3ntin added concepts C++20 concepts confirmed Verified by a second party labels Jan 10, 2024
@cor3ntin
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Interestingly, it does work with a requires clause
https://godbolt.org/z/bYM43r4Yr

@erichkeane

@zyn0217
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zyn0217 commented Jan 11, 2024

We have to compare the template parameter equivalence before going through the constraint check, per [temp.func.order]p6.2.2:

Otherwise, if the corresponding template-parameters of the template-parameter-lists are not equivalent [temp.over.link]... neither template is more specialized than the other.

And the definition for 'equivalent' is at [temp.over.link]p6.3:

Two template-parameters are equivalent under the following conditions:

if they declare non-type template parameters, they have equivalent types ignoring the use of type-constraints for placeholder types

And this is implemented by bc62fb9.

Interestingly, it does work with a requires clause
https://godbolt.org/z/bYM43r4Yr

I think this could also explain why using a requires expression does work -- the parameters are considered equivalent that way.

(I'm not 100% sure the standard is intended to reject such cases. CC @zygoloid for some opinions.)

@zygoloid
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And the definition for 'equivalent' is at [temp.over.link]p6.3:

Two template-parameters are equivalent under the following conditions:
if they declare non-type template parameters, they have equivalent types ignoring the use of type-constraints for placeholder types

And this is implemented by bc62fb9.

In this case, the template parameters are equivalent, so we should continue to [temp.func.order]/6.4 which orders the function templates based on their constraints.

zyn0217 added a commit to zyn0217/llvm-project that referenced this issue Jan 19, 2024
@zyn0217
[Clang] llvmGH-77377
98988ea
vsaulue added a commit to vsaulue/Gustave that referenced this issue Apr 24, 2024
@vsaulue
cUnitOf: rewrite to workaround clang-17 bug.
04e3680 
See llvm/llvm-project#77377
@jcsxky jcsxky mentioned this issue May 11, 2024
Closed
@sdkrystian sdkrystian mentioned this issue May 16, 2024
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sdkrystian added a commit that referenced this issue May 16, 2024
@sdkrystian
[Clang][Sema] ASTContext::getUnconstrainedType propagates dependence (#…
da928c6 
…92425)

When the argument passed to `ASTContext::getUnconstrainedType` is an
unconstrained `AutoType`, will return the argument unchanged. However,
when called with a constrained `AutoType`, an unconstrained,
non-dependent `AutoType` will be returned even if the argument was
dependent. Consider the following:
```
template<typename T>
concept C = sizeof(T) == sizeof(int);

template<auto N>
struct A;

template<C auto N>
struct A<N>; // error: class template partial specialization is not more specialized than the primary template
```
When comparing the template parameters for equivalence,
`ASTContext::getUnconstrainedType` is used to remove the constraints per
[temp.over.link] p6 sentence 2. For the template
parameter `N` of the class template, it returns a dependent `AutoType`.
For the template parameter `N` of the class template partial
specialization, it returns a non-dependent `AutoType`. We subsequently
compare the adjusted types and find they are not equivalent, thus we
consider the partial specialization to not be more specialized than the
primary template per [temp.func.order] p6.2.2.

This patch changes `ASTContext::getUnconstrainedType` such that the
dependence of a constrained `AutoType` will propagate to the returned
unconstrained `AutoType`. This causes the above example to be correctly
accepted, fixing #77377.
@sdkrystian
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Closed by #92425

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clang:frontend Language frontend issues, e.g. anything involving "Sema" concepts C++20 concepts confirmed Verified by a second party
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[clang] visit constraint of NTTP jcsxky/llvm-project
7 participants
@zygoloid @cor3ntin @Fedr @EugeneZelenko @sdkrystian @llvmbot @zyn0217