Code using a lambda to capture a reference to a function rejected with `could not match (null template argument) against (null template argument)`

[abbeyj]

The following code is accepted by other compilers but rejected by clang. The error message is a bit weird.

template<typename T>
void g(const T& t) {
}

template<typename T>
void f(const T& t) {
  [t] { g(t); };
}

void h() {
}

int main() {
  f(h);
}

Output:

<source>:7:9: error: no matching function for call to 'g'
    7 |   [t] { g(t); };
      |         ^
<source>:7:7: note: while substituting into a lambda expression here
    7 |   [t] { g(t); };
      |       ^
<source>:14:3: note: in instantiation of function template specialization 'f<void ()>' requested here
   14 |   f(h);
      |   ^
<source>:2:6: note: candidate template ignored: could not match (null template argument) against (null template argument)
    2 | void g(const T& t) {
      |      ^

Changing the call g(t) to be either g<T>(t) or g<void()>(t) changes the error message to:

note: candidate function template not viable: 1st argument ('void (const()') would lose const qualifier

Note that the type mentioned here (void (const()) has unbalanced parentheses.

Workaround: change the lambda capture from [t] to [t=t].

This code was accepted by clang 3.0.0 but is rejected by all version from 3.1 to the latest trunk currently available on Godbolt.

Godbolt link: https://godbolt.org/z/d87rPjja4

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: James Abbatiello (abbeyj)

The following code is accepted by other compilers but rejected by clang. The error message is a bit weird.

template<typename T>
void g(const T& t) {
}

template<typename T>
void f(const T& t) {
  [t] { g(t); };
}

void h() {
}

int main() {
  f(h);
}

Output:

<source>:7:9: error: no matching function for call to 'g'
    7 |   [t] { g(t); };
      |         ^
<source>:7:7: note: while substituting into a lambda expression here
    7 |   [t] { g(t); };
      |       ^
<source>:14:3: note: in instantiation of function template specialization 'f<void ()>' requested here
   14 |   f(h);
      |   ^
<source>:2:6: note: candidate template ignored: could not match (null template argument) against (null template argument)
    2 | void g(const T& t) {
      |      ^

Changing the call g(t) to be either g<T>(t) or g<void()>(t) changes the error message to:

note: candidate function template not viable: 1st argument ('void (const()') would lose const qualifier

Note that the type mentioned here (void (const()) has unbalanced parentheses.

Workaround: change the lambda capture from [t] to [t=t].

This code was accepted by clang 3.0.0 but is rejected by all version from 3.1 to the latest trunk currently available on Godbolt.

Godbolt link: https://godbolt.org/z/d87rPjja4

[Sirraide]

Ok, yeah, there’s something very wrong here. I’ll see if I can take a look at this.

[Sirraide]

The AST for the type of t in g(t):

QualType 0x5585adce8131 'void (const(void)' const
`-SubstTemplateTypeParmType 0x5585adce8130 'void (void)' sugar typename depth 0 index 0 T
  |-FunctionTemplate 0x5585adccb838 'f'
  `-FunctionProtoType 0x5585adccbdf0 'void (void)' cdecl
    `-BuiltinType 0x5585adc7f2c0 'void'

[zyn0217]

@Sirraide You have any progress with it? Otherwise I think I have figured out why and it's seemingly not hard to fix.

[Sirraide]

You have any progress with it?

Ah, no, my bad, I somehow completely lost track of this issue..