[clang] [clang] Diagnose default arguments defined in different scopes (PR #124844)

[via cfe-commits]

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@@ -5810,6 +5810,62 @@ static bool isParenthetizedAndQualifiedAddressOfExpr(Expr *Fn) {
   return false;
 }
 
+/// @brief Checks that each default argument needed to make the call
+/// is defined only once, implementing [over.match.best]/4 rule.
+///
+/// @param FDecl Function declaration selected for the call
+/// @param NumArgs Number of argument explicitly specified in the call
+/// expression
+/// @param CallLoc Source location of the call expression
+static void checkDefaultArgumentsAcrossScopes(Sema &S, FunctionDecl *FDecl,
+                                              int NumArgs,
+                                              SourceLocation CallLoc) {
+  // [over.match.best]/4:
+  // If the best viable function resolves to a function
+  // for which multiple declarations were found,
+  // and if any two of these declarations inhabit different scopes
+  // and specify a default argument that made the function viable,
+  // the program is ill-formed.
+
+  // Calculate the range of parameters,
+  // default arguments of which made the candidate viable.
+  int FirstDefaultArgIndex = NumArgs;
+  int LastDefaultArgIndex = FDecl->getNumParams() - 1;
+
+  // For each such parameter, collect all redeclarations
+  // that have non-inherited default argument.
+  llvm::SmallDenseMap<int, llvm::TinyPtrVector<ParmVarDecl *>> ParamRedecls(
+      LastDefaultArgIndex - FirstDefaultArgIndex + 1);
+  for (FunctionDecl *Redecl : FDecl->redecls()) {
+    for (int i = FirstDefaultArgIndex; i <= LastDefaultArgIndex; ++i) {
+      ParmVarDecl *Param = Redecl->getParamDecl(i);
+      if (Param->hasDefaultArg() && !Param->hasInheritedDefaultArg())
+        ParamRedecls[i].push_back(Param);
+    }
+  }
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cor3ntin wrote:

If you iterate over `i` in the outer loop, you can forgo the map entirely and do it on one pass 

https://github.com/llvm/llvm-project/pull/124844