[Sema] Don't crash on array types with incomplete element type whose bound results in overflow

[majnemer]

Summary

We can't be sure that an array is too large without knowing how large
array elements are.

[majnemer]

Ping.

[rsmith]

It's awkward to be rewriting chunks of Sema::BuildArrayType here. Would it be possible to factor out the "check for negative bound and array too large, and build a constant array type" part and use it in both places? The callers of TryToFixInvalidVariablyModifiedType all apparently want to diagnose those failure conditions in the same way (except that CheckVariableDeclarationType apparently chooses to ignore these bits and give a worse diagnostic instead.

lib/Sema/SemaDecl.cpp

if (const ParenType* PTy = dyn_cast<ParenType>(Ty)) {
     FixedType = Context.getParenType(FixedType);
     return Qs.apply(Context, FixedType);
   }
 
   const VariableArrayType* VLATy = dyn_cast<VariableArrayType>(T);
   if (!VLATy)
     return QualType();
   // FIXME: We should probably handle this case
-  if (VLATy->getElementType()->isVariablyModifiedType())
+  if (VLATy->getElementType()->isVariablyModifiedType() ||
+      VLATy->getElementType()->isIncompleteType())
     return QualType();
 
   llvm::APSInt Res;
   if (!VLATy->getSizeExpr() ||
       !VLATy->getSizeExpr()->EvaluateAsInt(Res, Context))
     return QualType();
 
   // Check whether the array size is negative.

test/CXX/dcl.dcl/dcl.spec/dcl.typedef/p2-0x.cpp

namespace VariableLengthArrays {
   int n = 32;
   using T = int[n]; // expected-error {{variable length array declaration not allowed at file scope}}
 
   const int m = 42;
   using U = int[m]; // expected-note {{previous definition}}
   using U = int[42]; // ok
   using U = int; // expected-error {{type alias redefinition with different types ('int' vs 'int [42]')}}
 
+  using V = struct A[  // expected-error {{variable length array declaration not allowed at file scope}}
+      (1 << 30) << 2]; // expected-warning {{requires 34 bits to represent}}
+
   void f() {
     int n = 42;
     goto foo; // expected-error {{cannot jump}}
     using T = int[n]; // expected-note {{bypasses initialization of VLA type alias}}
   foo: ;
   }
 }