https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119591
Bug ID: 119591
Summary: Concept constraint expression normalization uses too
much memory
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: marco.rubini08 at gmail dot com
Target Milestone: ---
Consider this code
```
template <int X>
concept C1 = (X, true);
template<int X>
concept C2 = C1<X + X - X*2 + X - X + 1>;
template<int X>
concept C3 = C2<X + X - X*2 + X - X + 1>;
template<int X>
concept C4 = C3<X + X - X*2 + X - X + 1>;
template<int X>
concept C5 = C4<X + X - X*2 + X - X + 1>;
template<int X>
concept C6 = C5<X + X - X*2 + X - X + 1>;
template<int X>
concept C7 = C6<X + X - X*2 + X - X + 1>;
template<int X>
concept C8 = C7<X + X - X*2 + X - X + 1>;
template<int X>
concept C9 = C8<X + X - X*2 + X - X + 1>;
template<int X>
concept C10 = C9<X + X - X*2 + X - X + 1>;
template<int X>
concept C11 = C10<X + X - X*2 + X - X + 1>;
static_assert(C11<0>);
```
GCC uses ~2GB of memory to compile.
I think there is an exponential blowup issue in the size of atomic constraints.
If the bug is confirmed, I think it is possible to compress the constraint
expressions that are formed during normalization, to decrease their memory
footprint (but not their "real" size), and could start working on a patch.
