[Bug c++/84843] [8 Regression] C++ ICE on builtin redefinition since r258391

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84843

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
I think if we have a definition we need to use newdecl ...

Now consider

extern "C" int __atomic_compare_exchange (int x, int y);

void bar() { __atomic_compare_exchange (1, 2); }

extern "C" int __atomic_compare_exchange (int x, int y)
{
  return x + y;
}

with -fpermissive and things get even more complicated.

> ./cc1plus -quiet t.ii  -fpermissive
t.ii:1:16: warning: new declaration ‘int __atomic_compare_exchange(int, int)’
ambiguates built-in declaration ‘bool __atomic_compare_exchange(long unsigned
int, volatile void*, void*, void*, int, int)’ [-fpermissive]
 extern "C" int __atomic_compare_exchange (int x, int y);
                ^~~~~~~~~~~~~~~~~~~~~~~~~
t.ii:1:16: note: ignoring the ‘int __atomic_compare_exchange(int, int)’
declaration
t.ii: In function ‘void bar()’:
t.ii:3:45: error: incorrect number of arguments to function
‘__atomic_compare_exchange’
 void bar() { __atomic_compare_exchange (1, 2); }
                                             ^
t.ii: At global scope:
t.ii:5:16: warning: new declaration ‘int __atomic_compare_exchange(int, int)’
ambiguates built-in declaration ‘bool __atomic_compare_exchange(long unsigned
int, volatile void*, void*, void*, int, int)’ [-fpermissive]
 extern "C" int __atomic_compare_exchange (int x, int y)
                ^~~~~~~~~~~~~~~~~~~~~~~~~
t.ii:5:16: note: ignoring the ‘int __atomic_compare_exchange(int, int)’
declaration
t.ii: In function ‘bool __atomic_compare_exchange(long unsigned int, volatile
void*, void*, void*, int, int)’:
t.ii:5:55: internal compiler error: in start_preparsed_function, at
cp/decl.c:15060
 extern "C" int __atomic_compare_exchange (int x, int y)
                                                       ^
0x93685e start_preparsed_function(tree_node*, tree_node*, int)
...