[clang] [Sema] Fix parameter index checks on explicit object member functions (PR #165586)

[via cfe-commits]

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@@ -328,7 +328,8 @@ class ParamIdx {
       : Idx(Idx), HasThis(false), IsValid(true) {
     assert(Idx >= 1 && "Idx must be one-origin");
     if (const auto *FD = dyn_cast<FunctionDecl>(D))
-      HasThis = FD->isCXXInstanceMember();
+      HasThis = FD->isCXXInstanceMember() &&
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Sirraide wrote:

Instead of this, just `dyn_cast` to `CXXMethodDecl` instead and call `isImplicitObjectMemberFunction()`

https://github.com/llvm/llvm-project/pull/165586