Multiple conversions to the same type are ambiguous but for the

purpose of overload resolution is to be treated as a uner-defined
conversion.

llvm-svn: 83004

clang/lib/Sema/SemaOverload.cpp

@@ -1511,8 +1511,6 @@ Sema::OverloadingResult Sema::IsUserDefinedConversion(
       return OR_Deleted;
 
     case OR_Ambiguous:
-      // FIXME: See C++ [over.best.ics]p10 for the handling of
-      // ambiguous conversion sequences.
       return OR_Ambiguous;
     }
 
@@ -2236,9 +2234,21 @@ Sema::AddOverloadCandidate(FunctionDecl *Function,
                                 /*InOverloadResolution=*/true);
       if (Candidate.Conversions[ArgIdx].ConversionKind
             == ImplicitConversionSequence::BadConversion) {
+      // 13.3.3.1-p10 If several different sequences of conversions exist that 
+      // each convert the argument to the parameter type, the implicit conversion 
+      // sequence associated with the parameter is defined to be the unique conversion 
+      // sequence designated the ambiguous conversion sequence. For the purpose of 
+      // ranking implicit conversion sequences as described in 13.3.3.2, the ambiguous 
+      // conversion sequence is treated as a user-defined sequence that is 
+      // indistinguishable from any other user-defined conversion sequence
+        if (Candidate.Conversions[ArgIdx].ConversionFunctionSet.size() > 0)
+          Candidate.Conversions[ArgIdx].ConversionKind =
+            ImplicitConversionSequence::UserDefinedConversion;
+        else {
           Candidate.Viable = false;
           break;
         }
+      }
     } else {
       // (C++ 13.3.2p2): For the purposes of overload resolution, any
       // argument for which there is no corresponding parameter is

clang/test/SemaCXX/ambig-user-defined-conversions.cpp

@@ -1,10 +1,11 @@
 // RUN: clang-cc -fsyntax-only -verify %s
 
+// Test1
 struct BASE { 
-  operator int &(); // expected-note 4 {{candidate function}}
+  operator int &(); // expected-note {{candidate function}}
 }; 
 struct BASE1 { 
-  operator int &(); // expected-note 4 {{candidate function}}
+  operator int &(); // expected-note {{candidate function}}
 }; 
 
 struct B : public BASE, BASE1 { 
@@ -14,13 +15,38 @@ struct B : public BASE, BASE1 {
 extern B f(); 
 
 B b1;
-void func(const int ci, const char cc); // expected-note {{function not viable because of ambiguity in conversion of argument 1}}
-void func(const char ci, const B b); // expected-note {{function not viable because of ambiguity in conversion of argument 1}}
-void func(const B b, const int ci); // expected-note {{function not viable because of ambiguity in conversion of argument 2}}
+void func(const int ci, const char cc); // expected-note {{candidate function}}
+void func(const char ci, const B b); // expected-note {{candidate function}}
+void func(const B b, const int ci); // expected-note {{candidate function}}
 
-
-const int main() {
-  func(b1, f()); // expected-error {{no matching function for call to 'func'}}
+const int Test1() {
+  func(b1, f()); // expected-error {{call to 'func' is ambiguous}}
   return f(); // expected-error {{conversion from 'struct B' to 'int const' is ambiguous}}
 }
 
+
+// Test2
+struct E;
+struct A { 
+  A (E&); 
+};
+
+struct E { 
+  operator A (); 
+};
+
+struct C { 
+  C (E&);  
+};
+
+void f1(A);	// expected-note {{candidate function}}
+void f1(C);	// expected-note {{candidate function}}
+
+void Test2()
+{
+	E b;
+	f1(b);  // expected-error {{call to 'f1' is ambiguous}}	
+                // ambiguous because b -> C via constructor and
+                // b → A via constructor or conversion function.
+}
+