[Bug c++/86485] New: [ 7 regression] "anonymous" maybe-uninitialized false positive with ternary operator

[thomas.o...@pdv-fs.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86485

            Bug ID: 86485
           Summary: [ 7 regression] "anonymous" maybe-uninitialized false
                    positive with ternary operator
           Product: gcc
           Version: 7.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: thomas.o...@pdv-fs.de
  Target Milestone: ---

Created attachment 44382
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=44382&action=edit
Minimal example for maybe-uninitialized false positive regression

The following minimal code example triggers the maybe-uninitialized warning on
gcc 8.1 and 7.3, but not on 5.3 or 6.2. Only shows up when optimizations are
enabled, and removing seemingly unrelated code "fixes" it again.



// compile with g++ 7.3 or 8.1 and optimizations enabled (not 5.3 or 6.2)
// g++ -O1 -c maybe_uninitialized_false_positive.cpp -o /dev/null
-Werror=maybe-uninitialized
// minimal working example created with creduce

struct empty_but_important {};
struct in_ternary_op {
  in_ternary_op() noexcept {}
  empty_but_important e; // REMOVE (or replace with int)
                         // and the error disappears
};

struct M {
  M(in_ternary_op) noexcept;
};

struct MustInheritBase {
  virtual M get_a(bool b) noexcept;
};

struct ZZ : MustInheritBase {
  M get_a(bool b) noexcept { // b = true; // ADD and the error disappears
    return M( b ? in_ternary_op{} : in_ternary_op{} );
//         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// error: '<anonymous>' may be used uninitialized in 
//        this function [-Werror=maybe-uninitialized]
// --------------------------------------------------
  }
};

ZZ zz_fn() { // REMOVE and the error disappears
  ZZ z;
  return z;
}