https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116695
Bug ID: 116695
Summary: Undefined behaviour involving unsequenced side effects
on a memory location during constant evaluation should
not compile
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: pkeir at outlook dot com
Target Milestone: ---
The C++14 code below compiles, but should not, as the constant expression
contains undefined behaviour:
constexpr bool ub()
{
int i = 0;
i = i++ + i; // UB
return i==1;
}
constexpr bool b = ub();
A core constant expression should not contain an operation that would have
undefined or erroneous behaviour (https://eel.is/c++draft/expr.const#5.8).
The above code includes undefined behaviour as it contains side effects on a
memory location which is unsequenced relative to both another side effect on
the same memory location and a value computation using the value of the object
in the same memory location (http://eel.is/c++draft/intro.execution#10).
The compiler output can be viewed at https://godbolt.org/z/7vz4r5sf1
