-Wvla warning twice for VM type at sizeof

[Origami404]

Reproduce code:

int func(int n) {
    return sizeof(int [n]);
}

godbolt here.

Clang issue a VLA usage twice instead of once.

I am currently working on VLA and VM type checking in clang and I am going to solve this bug by myself. I guess this may be caused by evaling runtime sizeof expressions required to call Sema::BuildArrayType twice.

[Origami404]

I guess this may be caused by evaling runtime sizeof expressions required to call Sema::BuildArrayType twice.

I verified my guess by setting a breakpoint at the beginning of Sema::BuildArrayType. This helper function is called first at parsing declaration, and second by Sema::TransformToPotentiallyEvaluated to transform the array declarator into a potentially evaluated expression (TreeTransform<Derived>::RebuildArrayType is what actually called it). This diag logic is implemented in Sema::BuildArrayType, so when it is called twice, the diag is trigged twice.

I am afraid that putting all the logic of VLA-related warnings within just Sema::BuildArrayType is impossible, due to some upper information being needed to decide whether a VLA type is actually creating a VLA instead of just being a part of a VM-type. New flags of VLA checking are discussed at #57098 , requiring more semantics info in VLA-related checking.

I am currently thinking of a way to propper implement the new flag and fix this issue. If anyone has ideas on the implementing details, please at me :).