Crash when passing a lambda to function in decltype, or no viable conversion from lambda to itself #111283
Description
The following code sometimes segfaults the compiler, and sometimes gives a nonsensical error.
v1:
void foo(auto &&) {}
auto bar(auto &&a) -> decltype (foo([]( auto && ... ){})) {}
struct A {};
int main()
{
bar(A());
}v2:
namespace N
{
void foo(auto &&) {}
auto bar(auto &&a) -> decltype (foo([]( auto && ... ){})) {}
struct A {};
}
int main()
{
bar(N::A());
}Here's v1 segfaulting Clang 19: https://gcc.godbolt.org/z/1Ycq96r6b
And v2 emits this error:
<source>:10:5: error: no matching function for call to 'bar'
10 | bar(N::A());
| ^~~
<source>:4:10: note: candidate template ignored: substitution failure [with a:auto = N::A]: no viable conversion from '(lambda at <source>:4:41)' to 'N::(lambda at <source>:4:41)'
4 | auto bar(auto &&a) -> decltype (foo([]( auto && ... ){})) {}
| ^
There's some variance here. My local Clang 19 (as opposed to godbolt) segfaults on v2 instead, and emits this error on v1.
Clang 18 and trunk on godbolt emits this error instead of segfaulting on both v1 and v2.