See: https://godbolt.org/z/EY7f6M5bE
constexpr int foo(int (*bla)(void)) {
return bla();
}
static_assert(foo(nullptr) == 1);Output:
<source>:5:15: error: static assertion expression is not an integral constant expression
static_assert(foo(nullptr) == 1);
^~~~~~~~~~~~~~~~~
<source>:2:10: note: subexpression not valid in a constant expression
return bla();
^
<source>:5:15: note: in call to 'foo(nullptr)'
static_assert(foo(nullptr) == 1);
^
1 error generated."subexpression not valid in a constant expression" is a pretty poor diagnostic for this case. When passing a non-constexpr function to foo(), clang manages to print a proper diagnostic:
<source>:9:15: error: static assertion expression is not an integral constant expression
static_assert(foo(f) == 1);
^~~~~~~~~~~
<source>:6:10: note: non-constexpr function 'f' cannot be used in a constant expression
return bla();
^
<source>:9:15: note: in call to 'foo(&f)'
static_assert(foo(f) == 1);
^
<source>:2:5: note: declared here
int f() { return 1; }
^
See: https://godbolt.org/z/EY7f6M5bE
Output:
"subexpression not valid in a constant expression" is a pretty poor diagnostic for this case. When passing a non-constexpr function to
foo(), clang manages to print a proper diagnostic: