https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92197
Bug ID: 92197
Summary: String Concatenation, Commutativity, and Side-Effect
Inducing Functions
Product: gcc
Version: 9.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: LouisJenkinsCS at hotmail dot com
Target Milestone: ---
In GCC version 9.2.1 (and some previous versions as far back as 7.4.0), this
program results in different behavior than what is expected...
```cpp
#include <string>
#include <iostream>
char n = '0';
std::string g(void) {
n++;
return std::string("g") + n;
}
std::string f(void) {
n++;
return std::string("f") + n;
}
int main(void) {
std::string temp = "";
temp += "(" + g() + "," + f() + ")";
std::cout << temp << std::endl;
}
```
In GCC, it produces `(g2,f1)`, which implies that `g()` was called before
`f()`; in Clang version 8.0.0 _and_ 10.0.1, it produces the desired output of
`(g1,f2)`. This subtle bug can result in some _very_ bad behavior; for context,
I am a TA in a class and a student's submission just looped infinitely without
any reasonable cause. The assignment was on implementing a recursive descent
parser, and the student used a global variable similar to what can be seen
here, and it resulted in it resolving this rule out of order...
SL -> S, SL
Where it originally was to call 'S' and then 'SL', it instead calls 'SL' (which
infinitely recurses) and then 'S'. I'm assuming this is not an intended effect.
