Add regression test (#47851)

src/Compilers/CSharp/Test/Semantic/Semantics/NullableReferenceTypesTests.cs

@@ -113,6 +113,40 @@ public void M<T>(bool condition, T t)
             comp.VerifyDiagnostics();
         }
 
+        [Fact, WorkItem(46461, "https://github.com/dotnet/roslyn/issues/46461")]
+        public void DefaultLiteralInConditional_02()
+        {
+            var comp = CreateNullableCompilation(@"
+using System;
+
+public class C
+{
+    public void M()
+    {
+        M1(true, b => b ? """" : default);
+        M1<bool, string?>(true, b => b ? """" : default);
+        M1<bool, string>(true, b => b ? """" : default); // 1
+
+        M1(true, b => b ? """" : null);
+        M1<bool, string?>(true, b => b ? """" : null);
+        M1<bool, string>(true, b => b ? """" : null); // 2
+    }
+
+    public void M1<T,U>(T t, Func<T, U> fn)
+    {
+        fn(t);
+    }
+}");
+
+            comp.VerifyDiagnostics(
+                // (10,37): warning CS8603: Possible null reference return.
+                //         M1<bool, string>(true, b => b ? "" : default); // 1
+                Diagnostic(ErrorCode.WRN_NullReferenceReturn, @"b ? """" : default", isSuppressed: false).WithLocation(10, 37),
+                // (14,37): warning CS8603: Possible null reference return.
+                //         M1<bool, string>(true, b => b ? "" : null); // 2
+                Diagnostic(ErrorCode.WRN_NullReferenceReturn, @"b ? """" : null", isSuppressed: false).WithLocation(14, 37));
+        }
+
         [Fact, WorkItem(33982, "https://github.com/dotnet/roslyn/issues/33982")]
         public void AssigningNullToRefLocalIsSafetyWarning()
         {