Class template argument deduction fails in case of function type argument

[llvmbot]

Bugzilla Link	52368
Version	trunk
OS	Windows NT
Reporter	LLVM Bugzilla Contributor
CC	@DougGregor,@zygoloid

Extended Description

In the following program Clang is unable to perform class template argument deduction:

template<typename T>
struct A{
    A(T f()){ f(); }
};

int foo() { return 1; }

int main() {
    [[maybe_unused]] A y = foo; // Clang error
}

GCC accepts it successfully, demo: https://gcc.godbolt.org/z/PKe6T5aW5

Related discussion: https://stackoverflow.com/q/69778510/7325599

[Endilll]

Clang behaves as expected if user-provided deduction guide written after http://eel.is/c++draft/over.match.class.deduct#1.1 is present: https://godbolt.org/z/3o367Wz9x

[llvmbot]

@llvm/issue-subscribers-c-17

Author: None (llvmbot)

| | | | --- | --- | | Bugzilla Link | [52368](https://llvm.org/bz52368) | | Version | trunk | | OS | Windows NT | | Reporter | LLVM Bugzilla Contributor | | CC | @DougGregor,@zygoloid |

Extended Description

In the following program Clang is unable to perform class template argument deduction:

template<typename T>
struct A{
    A(T f()){ f(); }
};

int foo() { return 1; }

int main() {
    [[maybe_unused]] A y = foo; // Clang error
}

GCC accepts it successfully, demo: https://gcc.godbolt.org/z/PKe6T5aW5

Related discussion: https://stackoverflow.com/q/69778510/7325599

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (llvmbot)

| | | | --- | --- | | Bugzilla Link | [52368](https://llvm.org/bz52368) | | Version | trunk | | OS | Windows NT | | Reporter | LLVM Bugzilla Contributor | | CC | @DougGregor,@zygoloid |

Extended Description

In the following program Clang is unable to perform class template argument deduction:

template<typename T>
struct A{
    A(T f()){ f(); }
};

int foo() { return 1; }

int main() {
    [[maybe_unused]] A y = foo; // Clang error
}

GCC accepts it successfully, demo: https://gcc.godbolt.org/z/PKe6T5aW5

Related discussion: https://stackoverflow.com/q/69778510/7325599

[Endilll]

CC @erichkeane

[hokein]

From the diagnostic message:

:3:5: note: candidate template ignored: could not match 'T ()' against 'int (*)()'

and the AST for the implicitly-generated deduction guide from the explicit constructor:

-FunctionTemplateDecl 0xc82ed58 <line:1:1, line:3:20> col:5 implicit <deduction guide for A>
| |-TemplateTypeParmDecl 0xc80f688 <line:1:10, col:19> col:19 referenced typename depth 0 index 0 T
| `-CXXDeductionGuideDecl 0xc82ec90 <line:3:5, col:20> col:5 implicit <deduction guide for A> 'auto (T (*)()) -> A<T>'
|   `-ParmVarDecl 0xc8103c0 <col:7, col:11> col:9 f 'T ()'

The type for the ParaVarDecl is T (), which doesn't seem correct, it should be a function pointer. We probably miss the decay case when transforming the constructor decl.