https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102728
Bug ID: 102728
Summary: requires statement fails to recognize lambda in
unevaluated context
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: nickhuang99 at hotmail dot com
Target Milestone: ---
Consider that this snippet of code gives error of no matching specialization
which is not rooted from lambda in function parameter, instead it is rooted
from "requires" statement violation of lambda in unevaluated-context. The
warning about no-return-statement in lambda within requirement exposes this is
"requires" issue.
#include <concepts>
template<class T>
concept IsLambda=requires(T t){
{t}->std::convertible_to<decltype(+[]{})>;
};
template<IsLambda T> // OK if replace IsLambda with "class"
void foo(T t){ t();}
template<>
void foo<decltype(+[]{})>(decltype(+[]{}) t){
t();
}
Also, a proof that this is not lambda-in-function-parameter issue is that you
can replace template parameter "IsLambda" in template function declaration with
"class" and GCC works fine.
Both clang and MSVC++ accepts above code. (https://www.godbolt.org/z/xhW7Tofao)
<source>: In substitution of 'template<class T> requires IsLambda<T> void
foo(T) [with T = void (*)()]':
<source>:12:44: required from here
<source>:5:40: warning: no return statement in function returning non-void
[-Wreturn-type]
5 | {t}->std::convertible_to<decltype(+[]{})>;
| ^~~~
<source>:12:6: error: template-id 'foo<void (*)()>' for 'void foo(void (*)())'
does not match any template declaration
12 | void foo<decltype(+[]{})>(decltype(+[]{}) t){
| ^~~~~~~~~~~~~~~~~~~~
<source>:9:6: note: candidate is: 'template<class T> requires IsLambda<T>
void foo(T)'
9 | void foo(T t){ t();}
| ^~~
