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Author: sdkrystian

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -4687,6 +4687,25 @@ bool Sema::InstantiateDefaultArgument(SourceLocation CallLoc, FunctionDecl *FD,
   NamedDecl *Pattern = FD;
   std::optional<ArrayRef<TemplateArgument>> Innermost;
 
+  // C++ [dcl.fct.default]p4
+  //   For non-template functions, default arguments can be added in later
+  //   declarations of a function that inhabit the same scope.
+  //
+  // C++ [dcl.fct.default]p6
+  //   Except for member functions of templated classes, the default arguments
+  //   in a member function definition that appears outside of the class
+  //   definition are added to the set of default arguments provided by the
+  //   member function declaration in the class definition; the program is
+  //   ill-formed if a default constructor, copy or move constructor, or copy
+  //   or move assignment operator is so declared. Default arguments for a
+  //   member function of a templated class shall be specified on the initial
+  //   declaration of the member function within the templated class.
+  //
+  // We need to collect the template arguments from the context of the function
+  // where the default argument was defined. For a specialization of a function
+  // template explicitly specialized for an implicit instantiation of a class
+  // template, that context is the (implicitly instantiated) declaration in the
+  // definition of the class template specialization.
   if (FD->isCXXClassMember() &&
       !isGenericLambdaCallOperatorOrStaticInvokerSpecialization(FD)) {
     if (FunctionTemplateDecl *FTD = FD->getPrimaryTemplate()) {