https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91990
Bug ID: 91990
Summary: Too slow compilation of recursively-nested template
class with two instances of its template parent
Product: gcc
Version: 9.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: b7.10110111 at gmail dot com
Target Milestone: ---
Consider the following code:
```
template<int i> class A
{
typedef A<i-1> B;
B x, y;
};
template<> class A<0> { char m; };
int main()
{
A<LEVEL> a;
}
```
Depending on the value of `LEVEL`, g++ compilation takes exponential time. But
if you replace `x, y` with `x[2]`, compilation will be in constant (negligibly
small) time. I've tested this on g++ 6.5.0, 8.3.0 and 9.1.0, and in all these
versions the problem of slow compilation reproduces.
For comparison, clang++ 6.0 compiles both versions (with `x, y` and `x[2]`) in
negligibly small time regardless of `LEVEL` (tested up to `LEVEL=906`, on 907
it crashes).
