[llvm-bugs] [Bug 50038] New: Miscompilation of for-loop with a declaration condition initialized with ternary operator

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=50038

            Bug ID: 50038
           Summary: Miscompilation of for-loop with a declaration
                    condition initialized with ternary operator
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: egor.suvorov at gmail.com
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org,
                    neeilans at live.com, richard-llvm at metafoo.co.uk

Consider the following code:

#include <iostream>
int main()
{
  for (int i = 0; int x = i < 2 ? 1 : 0; i++) {
      std::cout << x;
  }
}

Its expected output is 11: there are two iterations with i=0 (x=1) and i=1
(x=1), and then the final iterations with i=2 (x=0) stops the loop.

However, clang miscompiles it to produce an infinite series of 000000...

It also gives following warnings:

<source>:5:20: warning: variable 'x' is uninitialized when used here
[-Wuninitialized]
      std::cout << x;
                   ^
<source>:4:19: note: variable 'x' is declared here
  for (int i = 0; int x = i < 2 ? 1 : 0; i++) {

The behavior traces back as far as clang 8.0.0, see Godbolt:
https://godbolt.org/z/jMb37G7nq

If I replace `i < 2 ? 1 : 0` with `2 - i`, the output is `21` as expected.

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