[clang] crash infinite recursion template template parameter default argument

[mizvekov]

Example: https://godbolt.org/z/5eKPfxK85

template<class T, template<class> class TT>     struct A;
template<class T, template<class> class TT = A> struct A {};
A<int> a;

This exhausts the stack before we hit the instantiation depth implementation limit:

<source>:2:46: warning: stack nearly exhausted; compilation time may suffer, and crashes due to stack overflow are likely [-Wstack-exhausted]
    2 | template<class T, template<class> class TT = A> struct A {};
      |                                              ^
Program terminated with signal: SIGSEGV
Compiler returned: 139

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Matheus Izvekov (mizvekov)

Example: https://godbolt.org/z/5eKPfxK85 ```C++ template<class T, template<class> class TT> struct A; template<class T, template<class> class TT = A> struct A {}; A<int> a; ```

This exhausts the stack before we hit the instantiation depth implementation limit:

<source>:2:46: warning: stack nearly exhausted; compilation time may suffer, and crashes due to stack overflow are likely [-Wstack-exhausted]
    2 | template<class T, template<class> class TT = A> struct A {};
      |                                              ^
Program terminated with signal: SIGSEGV
Compiler returned: 139

[llvmbot]

@llvm/issue-subscribers-bug

Author: Matheus Izvekov (mizvekov)

Example: https://godbolt.org/z/5eKPfxK85 ```C++ template<class T, template<class> class TT> struct A; template<class T, template<class> class TT = A> struct A {}; A<int> a; ```

This exhausts the stack before we hit the instantiation depth implementation limit:

<source>:2:46: warning: stack nearly exhausted; compilation time may suffer, and crashes due to stack overflow are likely [-Wstack-exhausted]
    2 | template<class T, template<class> class TT = A> struct A {};
      |                                              ^
Program terminated with signal: SIGSEGV
Compiler returned: 139

[shafik]

Goes back to clang-19: https://godbolt.org/z/1E9sdh6M9

[mizvekov]

It goes all the way back to clang-6: https://godbolt.org/z/Wh7v7aYGE
Before clang-19, you needed -frelaxed-template-template-args in order for the code to be accepted.
In clang-19 we made that flag the default.