Avoid AmbigMember in lookup of interfaces with nullability differences only (#49338)

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

@@ -1058,12 +1058,21 @@ static void addAllInterfaces(NamedTypeSymbol @interface, HashSet<NamedTypeSymbol
             if (interfaces.Length > 0)
             {
                 var tmp = LookupResult.GetInstance();
-                foreach (TypeSymbol baseInterface in interfaces)
+                HashSet<NamedTypeSymbol> seenInterfaces = null;
+                if (interfaces.Length > 1)
+                {
+                    seenInterfaces = new HashSet<NamedTypeSymbol>(Symbols.SymbolEqualityComparer.IgnoringNullable);
+                }
+
+                foreach (NamedTypeSymbol baseInterface in interfaces)
+                {
+                    if (seenInterfaces is null || seenInterfaces.Add(baseInterface))
                     {
                         LookupMembersWithoutInheritance(tmp, baseInterface, name, arity, options, originalBinder, accessThroughType, diagnose, ref useSiteDiagnostics, basesBeingResolved);
                         MergeHidingLookupResults(current, tmp, basesBeingResolved, ref useSiteDiagnostics);
                         tmp.Clear();
                     }
+                }
                 tmp.Free();
             }
         }

src/Compilers/CSharp/Test/Symbol/Symbols/DefaultInterfaceImplementationTests.cs

@@ -57708,13 +57708,7 @@ interface D : A.E
             compilation1.VerifyDiagnostics(
                 // (4,11): warning CS8645: 'C<object>' is already listed in the interface list on type 'A' with different nullability of reference types.
                 // interface A : B, C<object>
-                Diagnostic(ErrorCode.WRN_DuplicateInterfaceWithNullabilityMismatchInBaseList, "A").WithArguments("C<object>", "A").WithLocation(4, 11),
-                // (18,17): error CS0104: 'E' is an ambiguous reference between 'C<object?>.E' and 'C<object>.E'
-                // interface D : A.E
-                Diagnostic(ErrorCode.ERR_AmbigContext, "E").WithArguments("E", "C<object?>.E", "C<object>.E").WithLocation(18, 17),
-                // (20,7): error CS0104: 'E' is an ambiguous reference between 'C<object?>.E' and 'C<object>.E'
-                //     A.E Test();
-                Diagnostic(ErrorCode.ERR_AmbigContext, "E").WithArguments("E", "C<object?>.E", "C<object>.E").WithLocation(20, 7)
+                Diagnostic(ErrorCode.WRN_DuplicateInterfaceWithNullabilityMismatchInBaseList, "A").WithArguments("C<object>", "A").WithLocation(4, 11)
                 );
         }
 

src/Compilers/Test/Utilities/CSharp/SymbolUtilities.cs

@@ -118,9 +118,10 @@ public static string[] ToTestDisplayStrings(this IEnumerable<ISymbol> symbols)
             return symbols.Select(s => s.ToTestDisplayString()).ToArray();
         }
 
-        public static string[] ToTestDisplayStrings(this IEnumerable<Symbol> symbols)
+        public static string[] ToTestDisplayStrings(this IEnumerable<Symbol> symbols, SymbolDisplayFormat format = null)
         {
-            return symbols.Select(s => s.ToTestDisplayString()).ToArray();
+            format ??= SymbolDisplayFormat.TestFormat;
+            return symbols.Select(s => s.ToDisplayString(format)).ToArray();
         }
 
         public static string ToTestDisplayString(this ISymbol symbol, bool includeNonNullable)