https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105110
Bug ID: 105110
Summary: NTTP type deduction fails when dependent of previous
NTTPs
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: j.galecki11 at gmail dot com
Target Milestone: ---
Consider the following minimal example:
```
template <int>
struct S {};
template <auto>
struct V {};
template <int I, S<I> s>
void foo(V<s>) {}
void bar() {
constexpr S<1> s;
foo(V<s>{});
}
```
CE link for the reader's convenience: https://godbolt.org/z/1cYK9s1Ta
GCC fails to compile this (consistent across versions), complaining about
cv-qualifier mismatch, which seems wholly irrelevant. Clang compiles without
warnings.
The problem is that when instantiating a function parametrized with a NTTP, GCC
fails to deduce the type of the NTTP parameter based on the other function
parameters. This causes problems with passing around constexpr values, as shown
in the following second example: https://godbolt.org/z/3Tnjx1Wxb (again, Clang
compiles and executes as intended).
As I cannot quite figure out whether the standard requires the deduction
described above to be possible, I see 2 possibilities:
1) This is simply a bug in GCC.
2) GCC is correct in refusing to compile the provided examples (Clang is
wrong). In this case, the emitted error should probably better describe the
problem.
