With this code:

int main() { struct A { void foo(auto x) {} // auto not permitted }; } 

And clang-18,19,20 or trunk and -std=c++20 or -std=c++23:

error: templates cannot be declared inside of a local class

No line number, no filename and refer to template.

With -std=c++17:

test.cpp:5:14: error: 'auto' not allowed in function prototype 5 | void foo(auto x) {} | ^~~~ 

With gcc (same message for C++17 or C++20):

test.cpp: In function ‘int main()’: test.cpp:5:14: error: ‘auto’ parameter not permitted in this context 5 | void foo(auto x) {} | ^~~~ 

https://gcc.godbolt.org/z/5YGdh6Pnb