[clang] [Clang] Prevent Null Pointer Dereference in in sema::​isNormalAssignmentOperator() (PR #115880)

[via cfe-commits]

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@@ -482,8 +482,8 @@ static bool isNormalAssignmentOperator(const FunctionDecl *FD) {
     if (RetT->isLValueReferenceType()) {
       ASTContext &Ctx = FD->getASTContext();
       QualType LHST;
-      auto *MD = dyn_cast<CXXMethodDecl>(FD);
-      if (MD && MD->isCXXInstanceMember())
+      auto *MD = cast<CXXMethodDecl>(FD);
+      if (MD->isCXXInstanceMember())
         LHST = Ctx.getLValueReferenceType(MD->getFunctionObjectParameterType());
       else
         LHST = MD->getParamDecl(0)->getType();
----------------
Sirraide wrote:

```suggestion
      else
        LHST = FD->getParamDecl(0)->getType();
```
I’m pretty sure this is a typo and is actually supposed to be this. I don’t think a `cast` instead of `dyn_cast` is right here because this is also a valid assignment operator, but it isn’t a `CXXMethodDecl`:
```c++
struct S {};
void operator|=(S, S) {}
```

@cor3ntin’s question as to whether this handles explicit object parameters correctly is also something we should investigate, but that seems like a separate issue.

https://github.com/llvm/llvm-project/pull/115880