CTAD on variable with parenthesized initializer

[llvmbot]

Bugzilla Link	40465
Version	trunk
OS	Linux
Reporter	LLVM Bugzilla Contributor
CC	@zygoloid

Extended Description

template <typename = void>
struct Foo {};

int main() {
  Foo f; // everyone accepts
  Foo (g); // clang rejects
           // gcc accepts, edg accepts, msvc accepts
}

clang says:

error: cannot use parentheses when declaring variable with deduced class template specialization type

This seems to be a highly specific error message but I can't see why this would be disallowed. [dcl.type.class.deduct] seems to allow g, but again since the message is so specific, I don't know. The other three compilers accept the code.

[zygoloid]

This was ill-formed in p0091r3:

7.1.6.5 Deduced Class Template Types [deduced.class.type]

If a placeholder for a deduced class type appears as a decl-specifier in the decl-specifier-seq of a simple-declaration, the init-declarator of that declaration shall be of the form

declarator-id attribute-specifier-seq[opt] initializer

The restriction on the form of the declarator was accidentally removed in p0620r0; as such, the wording no longer has any restriction, and according to the wording

template<typename T> struct X { X(T); };
X *p = 0;

... is apparently valid and deduces the type of p as X<int> *.

That's clearly broken, so we ignore the relevant change from p0620r0. (Even before p0620r0, the other compilers didn't implement the restriction, so I think they just missed that subtlety of the rule.)

See http://lists.isocpp.org/core/2017/09/2890.php and core issue 2376.

[cor3ntin]

The wording is there https://eel.is/c++draft/dcl.type.class.deduct

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (llvmbot)

| | | | --- | --- | | Bugzilla Link | [40465](https://llvm.org/bz40465) | | Version | trunk | | OS | Linux | | Reporter | LLVM Bugzilla Contributor | | CC | @zygoloid |

Extended Description

template <typename = void>
struct Foo {};

int main() {
  Foo f; // everyone accepts
  Foo (g); // clang rejects
           // gcc accepts, edg accepts, msvc accepts
}

clang says:

error: cannot use parentheses when declaring variable with deduced class template specialization type

This seems to be a highly specific error message but I can't see why this would be disallowed. [dcl.type.class.deduct] seems to allow g, but again since the message is so specific, I don't know. The other three compilers accept the code.

[zygoloid]

Now CWG 2376 has been resolved, clang is wrong to reject. The new rule is "the declared type of the variable shall be cv T", which does allow parentheses.