https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107360
--- Comment #4 from Alexey Izbyshev <izbyshev at ispras dot ru> ---
(In reply to Martin Uecker from comment #3)
> I think there there are cases were variably modified
> return types are allowed in ISO C:
>
> void f(int n, double (*(bar(void)))[n])
> {
> double (*p)[n] = bar();
> }
Yes, I should have been more precise. Per C11 6.7.6.2:
> If an identifier is declared as having a variably modified type, it shall be
> an ordinary identifier (as defined in 6.2.3), have no linkage, and have
> either block scope or function prototype scope.
So, since my original example is in C++ and involves return type deduction,
what I really wanted to say is that it's not allowed to *define* a function
with a variably modified return type in ISO C (hence it seems fine to not
support it in GCC's VLA extension for C++ too and consider such code invalid).
GCC does allow to define a nested function with a variably modified return type
though, and, perhaps surprisingly, a test case similar to my original C++ one
is compiled successfully:
extern int n;
int f() {
int a[n];
int (*g())[n] {
return &a;
};
return sizeof *g();
}
But what would seem to be a C++ equivalent crashes GCC:
extern int n;
int f() {
int a[n];
auto g = [&]() {
return &a;
};
return sizeof *g();
}
during RTL pass: expand
<source>: In function 'int f()':
<source>:6:13: internal compiler error: in expand_expr_real_1, at expr.cc:10586
6 | return &a;
| ^
...
And for this one it's less clear to me whether GCC wants to consider it
invalid.
But without return type deduction GCC refuses to compile:
extern int n;
int f() {
typedef int t[n];
t a;
auto g = [&]() -> t* {
return &a;
};
return sizeof *g();
}
<source>: In function 'int f()':
<source>:6:12: error: data member may not have variably modified type 'int
(*())[n]'
6 | auto g = [&]() -> t* {
| ^
<source>:6:12: error: declaration of 'operator()' as non-function
Compiler returned: 1
