[clang] [LifetimeSafety] Merge lifetimebound attribute on implicit 'this' across method redeclarations (PR #172146)

[Gábor Horváth via cfe-commits]

================
@@ -4485,12 +4532,17 @@ bool Sema::MergeCompatibleFunctionDecls(FunctionDecl *New, FunctionDecl *Old,
   // Merge attributes from the parameters.  These can mismatch with K&R
   // declarations.
   if (New->getNumParams() == Old->getNumParams())
-      for (unsigned i = 0, e = New->getNumParams(); i != e; ++i) {
-        ParmVarDecl *NewParam = New->getParamDecl(i);
-        ParmVarDecl *OldParam = Old->getParamDecl(i);
-        mergeParamDeclAttributes(NewParam, OldParam, *this);
-        mergeParamDeclTypes(NewParam, OldParam, *this);
-      }
+    for (unsigned i = 0, e = New->getNumParams(); i != e; ++i) {
+      ParmVarDecl *NewParam = New->getParamDecl(i);
+      ParmVarDecl *OldParam = Old->getParamDecl(i);
+      mergeParamDeclAttributes(NewParam, OldParam, *this);
+      mergeParamDeclTypes(NewParam, OldParam, *this);
+    }
+
+  // Merge function type attributes (e.g., lifetimebound on implicit 'this').
+  if (auto *NewMethod = dyn_cast<CXXMethodDecl>(New))
+    if (auto *OldMethod = dyn_cast<CXXMethodDecl>(Old))
----------------
Xazax-hun wrote:

Is it possible for the old to not be a `CXXMethodDecl` when the new is already one? Should this be a `cast` instead?

https://github.com/llvm/llvm-project/pull/172146