[llvm-bugs] [Bug 52194] New: Misleading error message in constexpr function when variable is uninitialized

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=52194

            Bug ID: 52194
           Summary: Misleading error message in constexpr function when
                    variable is uninitialized
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++2b
          Assignee: unassignedclangbugs at nondot.org
          Reporter: derekbrowncmu at gmail.com
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

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.

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