Fix nullability assertion failure with extension Add methods in collection expressions

src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs

@@ -3913,6 +3913,14 @@
 
                         var argIndex = initializer.AddMethod.IsExtensionMethod ? 1 : 0;
                         Debug.Assert(initializer.ArgsToParamsOpt.IsDefault);
+
+                        // For extension methods, mark the receiver argument with unknown nullability
+                        // since we don't analyze it (we only analyze the element argument).
+                        if (initializer.AddMethod.IsExtensionMethod)
+                        {
+                            SetUnknownResultNullability(initializer.Arguments[0]);
+                        }
+
                         var addArgument = initializer.Arguments[argIndex];
                         VisitRvalue(addArgument);
                         var addArgumentResult = _visitResult;

src/Compilers/CSharp/Test/Emit3/Semantics/ExtensionTests.cs

@@ -49191,6 +49191,36 @@ public class MyCollection<T> : IEnumerable<T>
         comp.VerifyEmitDiagnostics();
     }
 
+    [Fact]
+    public void Nullability_CollectionExpression_ExtensionAdd()
+    {
+        var src = """
+#nullable enable
+using System.Collections;
+using System.Collections.Generic;
+
+object? oNull = null;
+object oNotNull = new object();
+MyCollection c = [oNull, oNotNull];
+
+static class E
+{
+    public static void Add(this MyCollection c, object o) { }
+}
+
+public class MyCollection : IEnumerable<object>
+{
+    IEnumerator<object> IEnumerable<object>.GetEnumerator() => throw null!;
+    IEnumerator IEnumerable.GetEnumerator() => throw null!;
+}
+""";
+        var comp = CreateCompilation(src);
+        comp.VerifyEmitDiagnostics(
+            // (7,19): warning CS8604: Possible null reference argument for parameter 'o' in 'void E.Add(MyCollection c, object o)'.
+            // MyCollection c = [oNull, oNotNull];
+            Diagnostic(ErrorCode.WRN_NullReferenceArgument, "oNull").WithArguments("o", "void E.Add(MyCollection c, object o)").WithLocation(7, 19));
+    }
+
     [Fact]
     public void Nullability_ObjectInitializer_01()
     {