a variably-modified type should not be referenceable from a local class scope

[zygoloid]

Various lines of this testcase cause Clang to crash:

void f(int n) {
  using T = int[n];
  struct A {
    using U = T;
    void f() { U u; }
    void g() { T t; }
  };
  A::U u;
  A a;
  a.f();
  a.g();
}

Most of this is pretty clearly unsupportable: in particular, the calls to A::f and A::g don't provide the value of the VLA bound, so those functions shouldn't be able to use the type. Probably the best rule here is that a variably-modified type is simply not referenceable at all from within a local class. (Note that we do allow this from within a local lambda, and have special logic to capture the type.)

[llvmbot]

@llvm/issue-subscribers-c-1

[xgupta]

if we try to fix the first issue where struct "A" uses the type alias "U" as a non-static data member which is defined as "T" which is a variable length array type. then test case would be

void f(int n) {
  using T = int[n];
  struct A {
    using U = T;
  };
  A::U u;
}

I think there should be a check implemented in lib/Sema/SemaDecl.cpp to disallow non-static data member if the type is isVariablyModifiedType().

[shafik]

gcc gives the following diagnostic: https://godbolt.org/z/qvdErjf6j

<source>: In function 'void f(int)':
<source>:4:11: error: data member may not have variably modified type 'T' {aka 'int [n]'}
    4 |     using U = T;
      |           ^
Compiler returned: 1

I think that is the approach we should take.

CC @AaronBallman