[clang] [clang] Improve diagnostics with incompatible VLA types (PR #101261)

[Aaron Ballman via cfe-commits]

================
@@ -16644,7 +16644,32 @@ bool Sema::DiagnoseAssignmentResult(AssignConvertType ConvTy,
     if (Action == AA_Passing_CFAudited) {
       DiagKind = diag::err_arc_typecheck_convert_incompatible_pointer;
     } else if (getLangOpts().CPlusPlus) {
-      DiagKind = diag::err_typecheck_convert_incompatible_pointer;
+      DiagKind = [this, &SrcType, &DstType] {
+        const VariableArrayType *SrcTypeVLA = nullptr, *DstTypeVLA = nullptr;
+        if (const PointerType *P = SrcType->getAs<PointerType>())
+          SrcTypeVLA = Context.getAsVariableArrayType(P->getPointeeType());
+        if (const PointerType *P = DstType->getAs<PointerType>())
+          DstTypeVLA = Context.getAsVariableArrayType(P->getPointeeType());
+
+        if (SrcTypeVLA == nullptr || DstTypeVLA == nullptr)
+          return diag::err_typecheck_convert_incompatible_pointer;
+
+        DeclRefExpr *SrcSizeExpr = nullptr, *DstSizeExpr = nullptr;
+        if (ImplicitCastExpr *I =
+                dyn_cast<ImplicitCastExpr>(SrcTypeVLA->getSizeExpr()))
+          SrcSizeExpr = dyn_cast<DeclRefExpr>(I->getSubExpr());
+        if (ImplicitCastExpr *I =
+                dyn_cast<ImplicitCastExpr>(DstTypeVLA->getSizeExpr()))
+          DstSizeExpr = dyn_cast<DeclRefExpr>(I->getSubExpr());
----------------
AaronBallman wrote:

For this, I would do:
```
auto GetSizeDeclRef = [](const VariableArrayType *VAT) -> const DeclRefExpr * {
  if (const Expr *Size = VAT->getSizeExpr())
    return dyn_cast<DeclRefExpr>(Size->IgnoreParenImpCasts())
  return nullptr;
};
DeclRefExpr *SrcSizeExpr = GetSizeDeclRef(SrcTypeVLA),
            *DestSizeExpr = GetSizeDeclRef(DestTypeVLA);
```
This will ignore more than just implicit cast expressions, but other types of implicit AST nodes (things like parentheses expressions, etc), so it will catch more situations.

https://github.com/llvm/llvm-project/pull/101261