Null pointer not accepted as non-type template argument

[llvmbot]

Bugzilla Link	9700
Resolution	FIXED
Resolved on	Apr 06, 2012 17:41
Version	unspecified
OS	MacOS X
Reporter	LLVM Bugzilla Contributor
CC	@DougGregor,@RealLitb

Extended Description

The following code:

template <char* Ptr>
struct A {};

char v = ' ';

int main(int, char**)
{
A<&v> a1; // Works
A<0> a2; // Doesn't
}

results in the following error message:

$ /opt/bin/clang -std=c++0x clang.cpp
clang.cpp:9:4: error: non-type template argument does not refer to any declaration
A<0> a2; // Doesn't
^
clang.cpp:1:17: note: template parameter is declared here
template <char* Ptr>
^
1 error generated.

even though according to 14.3.2p1, 0 should be accepted:

"A template-argument for a non-type, non-template template-parameter shall be one of [...] a constant expression that evaluates to a null pointer value"

My clang version:

$ /opt/bin/clang -v
clang version 3.0 (trunk 129269)
Target: x86_64-apple-darwin10
Thread model: posix

[llvmbot]

SemaTemplate.cpp, function CheckTemplateArgumentAddressOfObjectOrFunction: In C++0x mode, a check for an integral constant expression that evaluates to an integer of value zero or a value of type std::null_ptr_t should be added (as per 4.10p1).

[llvmbot]

SemaTemplate.cpp, function CheckTemplateArgumentAddressOfObjectOrFunction: [...]

Not really. A check similar to the required one is already present in Sema::CheckTemplateArgument and should be extended:

| // C++0x [temp.arg.nontype]p5 bullets 2, 4 and 6 permit conversion
| // from a template argument of type std::nullptr_t to a non-type
| // template parameter of type pointer to object, pointer to
| // function, or pointer-to-member, respectively.
| if (ArgType->isNullPtrType() &&
| (ParamType->isPointerType() || ParamType->isMemberPointerType())) {
| Converted = TemplateArgument((NamedDecl *)0);
| return Owned(Arg);
| }

Three things remain to be done:

• Also check for an integral constant expression of 0
• Only do these two checks if C++0x is enabled
• Adjust the comment, as the relevant paragraph is now the first, not the fifth,
  and as 0 is also accepted

[RealLitb]

The following code:

template <char* Ptr>
struct A {};

char v = ' ';

int main(int, char**)
{
A<&v> a1; // Works
A<0> a2; // Doesn't
}

results in the following error message:

$ /opt/bin/clang -std=c++0x clang.cpp
clang.cpp:9:4: error: non-type template argument does not refer to any
declaration
A<0> a2; // Doesn't
^
clang.cpp:1:17: note: template parameter is declared here
template <char* Ptr>
^
1 error generated.

even though according to 14.3.2p1, 0 should be accepted:

"A template-argument for a non-type, non-template template-parameter shall be
one of [...] a constant expression that evaluates to a null pointer value"

You are misreading this. "0" is not a null pointer value, but a null pointer constant. Hence, it is not allowed by that bullet. 0 is allowed by the first bullet, because it is an integer constant expression. The conversion of 0 to a non-type template parameter of pointer type is specifically not allowed by 14.3.2p5 first bullet, with an explicit note:

[ Note: In particular, neither the null pointer conversion for a zero-valued integral constant expression (4.10) nor the derived-to-base conversion (4.10) are applied. Although 0 is a valid template-argument for a non-type template-arameter of integral type, it is not a valid template-argument for a non-type template-parameter of pointer type. However, both (int*)0 and nullptr are valid template-arguments for a non-type template-parameter of type "pointer to int." - end note ]

The normative text of paragraph 14.3.2p1 does not allow "nullptr". I think that's a defect. But in any case, clang is correct to reject your testcase.

[llvmbot]

[...]

[14.3.2p5]
[ Note: In particular, neither the null pointer conversion for a zero-valued
integral constant expression (4.10) nor the derived-to-base conversion (4.10)
are applied. Although 0 is a valid template-argument for a non-type
template-arameter of integral type, it is not a valid template-argument for a
non-type template-parameter of pointer type.

D'oh!

However, both (int*)0 and nullptr
are valid template-arguments for a non-type template-parameter of type "pointer
to int." - end note ]

The normative text of paragraph 14.3.2p1 does not allow "nullptr". I think
that's a defect. But in any case, clang is correct to reject your testcase.

Indeed it is, you are right. On the other hand, clang also rejects

A<(char*)0> a2

which, according to the note above, should be valid.

(I'm also not sure if not mentioning nullptr in 14.3.2.p1 is a defect, as it is explicitely being allowed in the note above.)

[DougGregor]

*** Bug llvm/llvm-bugzilla-archive#10398 has been marked as a duplicate of this bug. ***

[DougGregor]

*** Bug llvm/llvm-bugzilla-archive#10396 has been marked as a duplicate of this bug. ***

[DougGregor]

Implemented in Clang r154219.