[Bug c++/94265] New: wrong warning "duplicated 'if' condition"

[f.heckenbach@fh-soft.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94265

            Bug ID: 94265
           Summary: wrong warning "duplicated 'if' condition"
           Product: gcc
           Version: 9.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: f.heckenbach@fh-soft.de
  Target Milestone: ---

% cat t.cpp
#include <tuple>

struct S { int a, b; };

int main ()
{
  if (auto [a,b] = S { 0, 1 }; a) { }
  else if (std::tie (a, b) = std::tuple (2, 3); a) { }
}

% g++ -std=c++17 -Wduplicated-cond t.cpp
t.cpp: In function 'int main()':
t.cpp:8:8: warning: duplicated 'if' condition [-Wduplicated-cond]
    8 |   else if (std::tie (a, b) = std::tuple (2, 3); a) { }
      |        ^~
t.cpp:7:3: note: previously used here
    7 |   if (auto [a,b] = S { 0, 1 }; a) { }
      |   ^~

Though the condition "a" looks the same in both "if" statements, the value of a
can change in between, and in fact does in this example (the second "if" branch
is actually taken).

Unless the "tie" assignment to "auto []" pseudo-variables is for some reason
incorrect, but then of course gcc should mention that.