[clang] [clang] Bugfix for choosing the more specialized overload (PR #83279)

[Botond István Horváth via cfe-commits]

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@@ -5548,13 +5504,100 @@ static bool isAtLeastAsSpecializedAs(Sema &S,
 FunctionTemplateDecl *Sema::getMoreSpecializedTemplate(
     FunctionTemplateDecl *FT1, FunctionTemplateDecl *FT2, SourceLocation Loc,
     TemplatePartialOrderingContext TPOC, unsigned NumCallArguments1,
-    unsigned NumCallArguments2, bool Reversed) {
+    unsigned NumCallArguments2, QualType RawObjType1, QualType RawObjType2,
+    bool Reversed) {
+  SmallVector<QualType, 4> Args1;
+  SmallVector<QualType, 4> Args2;
+  const FunctionDecl *FD1 = FT1->getTemplatedDecl();
+  const FunctionDecl *FD2 = FT2->getTemplatedDecl();
+  bool shouldConvert1 = false;
+  bool shouldConvert2 = false;
+  QualType ObjType1;
+  QualType ObjType2;
+  if (TPOC == TPOC_Call) {
+    const FunctionProtoType *Proto1 =
+        FD1->getType()->getAs<FunctionProtoType>();
+    const FunctionProtoType *Proto2 =
+        FD2->getType()->getAs<FunctionProtoType>();
+
+    //   - In the context of a function call, the function parameter types are
+    //     used.
+    const CXXMethodDecl *Method1 = dyn_cast<CXXMethodDecl>(FD1);
+    const CXXMethodDecl *Method2 = dyn_cast<CXXMethodDecl>(FD2);
+
+    if (getLangOpts().CPlusPlus20) {
+      // C++20 [temp.func.order]p3
+      //   [...] Each function template M that is a member function is
+      //   considered to have a new first parameter of type
+      //   X(M), described below, inserted in its function parameter list.
+      //
+      // Note that we interpret "that is a member function" as
+      // "that is a member function with no expicit object argument".
+      // Otherwise the ordering rules for methods with expicit objet arguments
+      // against anything else make no sense.
+      shouldConvert1 = Method1 && !Method1->isExplicitObjectMemberFunction();
+      shouldConvert2 = Method2 && !Method2->isExplicitObjectMemberFunction();
+    } else {
+      // C++11 [temp.func.order]p3:
+      //   [...] If only one of the function templates is a non-static
+      //   member, that function template is considered to have a new
+      //   first parameter inserted in its function parameter list.
+      //
+      // Note that we interpret this to mean "if one of the function
+      // templates is a non-static member and the other is a non-member";
+      // otherwise, the ordering rules for static functions against non-static
+      // functions don't make any sense.
+      //
+      // C++98/03 doesn't have this provision but we've extended DR532 to cover
+      // it as wording was broken prior to it.
+      shouldConvert1 =
+          !Method2 && Method1 && Method1->isImplicitObjectMemberFunction();
+      shouldConvert2 =
+          !Method1 && Method2 && Method2->isImplicitObjectMemberFunction();
+    }
+    if (shouldConvert1) {
+      bool isR2 =
+          getLangOpts().CPlusPlus20 &&
+          (shouldConvert1
+               ? Method2->getRefQualifier() == RQ_RValue
+               : Proto2->param_type_begin()[0]->isRValueReferenceType());
+      // Compare 'this' from Method1 against first parameter from Method2.
+      ObjType1 = GetImplicitObjectParameterType(this->Context, Method1,
+                                                RawObjType1, isR2);
+      Args1.push_back(ObjType1);
+    }
+    if (shouldConvert2) {
+      bool isR1 =
+          getLangOpts().CPlusPlus20 &&
+          (shouldConvert2
+               ? Method1->getRefQualifier() == RQ_RValue
+               : Proto1->param_type_begin()[0]->isRValueReferenceType());
+      // Compare 'this' from Method2 against first parameter from Method1.
+      ObjType2 = GetImplicitObjectParameterType(this->Context, Method2,
+                                                RawObjType2, isR1);
+      Args2.push_back(ObjType2);
+    }
+    unsigned NumComparedArguments = NumCallArguments1 + shouldConvert1;
 
-  bool Better1 = isAtLeastAsSpecializedAs(*this, Loc, FT1, FT2, TPOC,
-                                          NumCallArguments1, Reversed);
-  bool Better2 = isAtLeastAsSpecializedAs(*this, Loc, FT2, FT1, TPOC,
-                                          NumCallArguments2, Reversed);
+    Args1.insert(Args1.end(), Proto1->param_type_begin(),
+                 Proto1->param_type_end());
+    Args2.insert(Args2.end(), Proto2->param_type_begin(),
+                 Proto2->param_type_end());
 
+    // C++ [temp.func.order]p5:
+    //   The presence of unused ellipsis and default arguments has no effect on
+    //   the partial ordering of function templates.
+    if (Args1.size() > NumComparedArguments)
----------------
HoBoIs wrote:

How can I make it less confusing? Should I put in a comment explaining that in `Args1` and `Args2` after the first `NumComparedArguments` elements there can only be unused ellipsis and default arguments? (It already states that  unused ellipsis and default arguments has no effect on the partial ordering of function templates. Just above the if)

https://github.com/llvm/llvm-project/pull/83279