问题
There is a very neat property of constant expressions in C++: their evaluation cannot have undefined behavior (7.7.4.7):
An expression e is a core constant expression unless the evaluation of e, following the rules of the abstract machine ([intro.execution]), would evaluate one of the following:
...
an operation that would have undefined behavior as specified in [intro] through [cpp] of this document [ Note: including, for example, signed integer overflow ([expr.prop]), certain pointer arithmetic ([expr.add]), division by zero, or certain shift operations — end note ] ;
Trying to store the value of 13!
in a constexpr int
indeed yields a nice compile error:
constexpr int f(int n)
{
int r = n--;
for (; n > 1; --n) r *= n;
return r;
}
int main()
{
constexpr int x = f(13);
return x;
}
Output:
9:19: error: constexpr variable 'x' must be initialized by a constant expression
constexpr int x = f(13);
^ ~~~~~
4:26: note: value 3113510400 is outside the range of representable values of type 'int'
for (; n > 1; --n) r *= n;
^
9:23: note: in call to 'f(3)'
constexpr int x = f(13);
^
1 error generated.
(BTW why does the error say "call to 'f(3)'", while it is a call to f(13)?..)
Then, I remove constexpr
from x
, but make f
a consteval
. According to the docs:
consteval - specifies that a function is an immediate function, that is, every call to the function must produce a compile-time constant
I do expect that such a program would again cause a compile error. But instead, the program compiles and runs with UB.
Why is that?
UPD: Commenters suggested that this is a compiler bug. I reported it: https://bugs.llvm.org/show_bug.cgi?id=43714
回答1:
This is a compiler bug. Or, to be more precise, this is an "underimplemented" feature (see the comment in bugzilla):
Yup - seems consteval isn't implemented yet, according to: https://clang.llvm.org/cxx_status.html
(the keyword's probably been added but not the actual implementation support)
来源:https://stackoverflow.com/questions/58454657/why-does-a-consteval-function-allow-undefined-behavior