call to consteval in immediate invocation context fails

[llvmbot]

Bugzilla Link	51840
Version	trunk
OS	Linux
Reporter	LLVM Bugzilla Contributor
CC	@dwblaikie,@zygoloid

Extended Description

Reduced from https://stackoverflow.com/questions/69166564

The following code fails on clang >= 11.0 (including trunk)

consteval int f(int) { return 1; }

template
consteval bool g(T a)
{
int n = f(a); // error here
return true;
}

static_assert(g(2));

---

with
error: call to consteval function 'f' is not a constant expression
note: read of non-const variable 'a' is not allowed in a constant expression

This seems incorrect. The evaluation of f is in an immediate context, i.e. inside g so f(a) should be a constant expression. Also, the error only appears if g is a template, and if the result of f(a) is stored in a local variable. GCC accepts the code.

[Izaron]

Thank you for the issue! I proposed a fix here - https://reviews.llvm.org/D119651

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[usx95]

Alternative proposal: https://reviews.llvm.org/D132659