Fix CS0419 to display fully qualified type names for ambiguous cref symbols

src/Compilers/CSharp/Portable/Binder/Binder_Crefs.cs

@@ -908,7 +908,9 @@ private ImmutableArray<Symbol> ProcessParameterlessCrefMemberLookupResults(
                 {
                     CrefSyntax crefSyntax = GetRootCrefSyntax(memberSyntax);
                     int otherIndex = symbolIndex == 0 ? 1 : 0;
-                    diagnostics.Add(ErrorCode.WRN_AmbiguousXMLReference, crefSyntax.Location, crefSyntax.ToString(), symbol, symbols[otherIndex]);
+                    diagnostics.Add(ErrorCode.WRN_AmbiguousXMLReference, crefSyntax.Location, crefSyntax.ToString(),
+                        new FormattedSymbol(symbol, SymbolDisplayFormat.CSharpErrorMessageFormat),
+                        new FormattedSymbol(symbols[otherIndex], SymbolDisplayFormat.CSharpErrorMessageFormat));
 
                     ambiguityWinner = ConstructWithCrefTypeParameters(arity, typeArgumentListSyntax, symbol);
                     return symbols.SelectAsArray(sym => ConstructWithCrefTypeParameters(arity, typeArgumentListSyntax, sym));

src/Compilers/CSharp/Test/Symbol/DocumentationComments/CrefTests.cs

@@ -7029,5 +7029,78 @@ record CacheContext(string String)" + terminator;
             var symbol = model.GetSymbolInfo(crefSyntaxes.Single()).Symbol;
             Assert.Equal(SymbolKind.Property, symbol.Kind);
         }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/4031")]
+        public void AmbiguousReferenceInDifferentNamespaces()
+        {
+            var source = """
+                namespace System
+                {
+                    class TypeA
+                    {
+                    }
+                }
+
+                namespace System.Goo
+                {
+                    class TypeA
+                    {
+                    }
+                }
+
+                namespace A
+                {
+                    using System;
+                    using System.Goo;
+
+                    /// <summary>
+                    ///     <see cref="TypeA"/>
+                    /// </summary>
+                    class Bar
+                    {
+                    }
+                }
+                """;
+            CreateCompilationWithMscorlib40AndDocumentationComments(source).VerifyDiagnostics(
+                // (21,24): warning CS0419: Ambiguous reference in cref attribute: 'TypeA'. Assuming 'System.Goo.TypeA', but could have also matched other overloads including 'System.TypeA'.
+                //         <see cref="TypeA"/>
+                Diagnostic(ErrorCode.WRN_AmbiguousXMLReference, "TypeA").WithArguments("TypeA", "System.Goo.TypeA", "System.TypeA").WithLocation(21, 24));
+        }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/4031")]
+        public void AmbiguousReferenceInDifferentNamespaces_WithParameters()
+        {
+            var source = """
+                namespace System
+                {
+                    class TypeA
+                    {
+                    }
+                }
+
+                namespace System.Goo
+                {
+                    class TypeA
+                    {
+                    }
+                }
+
+                namespace A
+                {
+                    /// <summary>
+                    ///     <see cref="M{T}"/>
+                    /// </summary>
+                    class Bar
+                    {
+                        void M<T>(System.TypeA a) { }
+                        void M<T>(System.Goo.TypeA a) { }
+                    }
+                }
+                """;
+            CreateCompilationWithMscorlib40AndDocumentationComments(source).VerifyDiagnostics(
+                // (18,24): warning CS0419: Ambiguous reference in cref attribute: 'M{T}'. Assuming 'A.Bar.M<T>(System.TypeA)', but could have also matched other overloads including 'A.Bar.M<T>(System.Goo.TypeA)'.
+                //     ///     <see cref="M{T}"/>
+                Diagnostic(ErrorCode.WRN_AmbiguousXMLReference, "M{T}").WithArguments("M{T}", "A.Bar.M<T>(System.TypeA)", "A.Bar.M<T>(System.Goo.TypeA)").WithLocation(18, 24));
+        }
     }
 }