US055 09.02.3.5 [dcl.fct].18 Parameter with placeholder-type-specifier and default argument is valid but useless

[wg21bot]

Defaulting an argument with a placeholder-type-specifier in an abbreviated function template is valid but useless. Calling the function without providing values for the default parameters will result in a deduction error.

void foo(std::integral auto i = 0) {} 

foo();  // COMPILE FAILURE
foo(1); // Ok

This is also a problem for generic lambdas:

auto foo = [] (auto i = 0) {};

foo();  // COMPILE FAILURE
foo(1); // Ok

Proposed change:
This problem could potentially be resolved if the invented template parameter was assigned an appropriate default value, e.g. decltype() of the provided default argument, although this could not work if the default argument references other parameters in the function.

[jfbastien]

EWG Monday: http://wiki.edg.com/bin/view/Wg21belfast/US055

Does EWG want to consider US055 for C++20

F	A
3	17

No consensus for change.

[jensmaurer]

Rejected. There was no consensus to adopt this change.