Misleading error message in constexpr function when variable is uninitialized

[Kered13]

Bugzilla Link	52194
Version	trunk
OS	Linux
CC	@Kered13,@zygoloid

Extended Description

Minimal example:

constexpr int func(int a)
{
	int sum;
	sum += a;
	return sum;
}
constexpr int val = func(4);

When I compile this on Godbolt using the "x86-64 clang (trunk)" branch and the -std=c++20 flag I get the following error message:

<source>:7:15: error: constexpr variable 'val' must be initialized by a constant expression
constexpr int val = func(4);
              ^     ~~~~~~~
<source>:4:6: note: subexpression not valid in a constant expression
        sum += a;
            ^
<source>:7:21: note: in call to 'func(4)'
constexpr int val = func(4);
                    ^
1 error generated.

This is misleading because the actual problem is that sum was not initialized. If sum is initialized with sum=0 then there is no error. This problem exists for other modify and assign operators as well. Using separate assignment and equality operators does not have this problem. For example:

constexpr int func(int a)
{
	int sum;
	sum = sum + a;
	return sum;
}
constexpr int val = func(4);

Give this error message:

<source>:7:15: error: constexpr variable 'val' must be initialized by a constant expression
constexpr int val = func(4);
              ^     ~~~~~~~
<source>:4:8: note: read of uninitialized object is not allowed in a constant expression
        sum = sum + a;
              ^
<source>:7:21: note: in call to 'func(4)'
constexpr int val = func(4);
                    ^
1 error generated.

Which correctly identifies that the problem is that sum is uninitialized.

[llvmbot]

@llvm/issue-subscribers-c-2b

[llvmbot]

@llvm/issue-subscribers-c-11

[AaronBallman]

I think this actually goes back farther than C++20. Even in C++11 the diagnostic doesn't clearly tell you that sum is the issue. You will get warnings about extensions allowing uninitialized variables in a constexpr function, but those extension warnings are not uncommon to disable for code bases not intending to port to newer language standards.