[release/dev17.14] Fix nullable crash for field keyword in partial property

src/Compilers/CSharp/Portable/Symbols/Synthesized/SynthesizedBackingFieldSymbol.cs

@@ -170,11 +170,11 @@ private NullableAnnotation ComputeInferredNullableAnnotation()
                 return NullableAnnotation.Annotated;
             }
 
+            getAccessor = (SourcePropertyAccessorSymbol?)getAccessor.PartialImplementationPart ?? getAccessor;
             // If the get accessor is auto-implemented, the property is not null-resilient.
             if (getAccessor.IsAutoPropertyAccessor)
                 return NullableAnnotation.NotAnnotated;
 
-            getAccessor = (SourcePropertyAccessorSymbol?)getAccessor.PartialImplementationPart ?? getAccessor;
             var binder = getAccessor.TryGetBodyBinder() ?? throw ExceptionUtilities.UnexpectedValue(getAccessor);
             var boundGetAccessor = binder.BindMethodBody(getAccessor.SyntaxNode, BindingDiagnosticBag.Discarded);
 

src/Compilers/CSharp/Test/Emit3/FieldKeywordTests.cs

@@ -12638,5 +12638,77 @@ public void M()
                 //         Prop.ToString(); // unexpected warning
                 Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "Prop").WithLocation(14, 9));
         }
+
+        [Theory, WorkItem("https://github.com/dotnet/roslyn/issues/78592")]
+        [InlineData(""" = "a";""", "")]
+        [InlineData("", """ = "a";""")]
+        public void PartialProperty_AutoImplGetter_PropertyInitializer(string defInitializer, string implInitializer)
+        {
+            var source = $$"""
+                #nullable enable
+
+                partial class C
+                {
+                    public partial string Prop { get; set; }{{defInitializer}}
+                }
+
+                partial class C
+                {
+                    public partial string Prop { get; set => Set(ref field, value); }{{implInitializer}}
+
+                    private void Set(ref string dest, string value)
+                    {
+                        dest = value;
+                    }
+                }
+                """;
+            var comp = CreateCompilation(source);
+            comp.VerifyEmitDiagnostics();
+        }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/78592")]
+        public void Repro_78592()
+        {
+            var source1 = """
+                #nullable enable
+
+                using System.Collections.Generic;
+                using System.Runtime.CompilerServices;
+
+                namespace TestLibrary
+                {
+                    public partial class Class1
+                    {
+                        public partial int P1 { get; set; } = -1;
+
+                        protected virtual bool SetProperty<T>(ref T storage, T value, [CallerMemberName] string? propertyName = null)
+                        {
+                            if (EqualityComparer<T>.Default.Equals(storage, value))
+                            {
+                                return false;
+                            }
+
+                            storage = value;
+
+                            return true;
+                        }
+                    }
+
+                }
+                """;
+
+            var source2 = """
+                namespace TestLibrary
+                {
+                    public partial class Class1
+                    {
+                        public partial int P1 { get; set => SetProperty(ref field, value); }
+                    }
+                }
+                """;
+
+            var comp = CreateCompilation([source1, source2]);
+            comp.VerifyEmitDiagnostics();
+        }
     }
 }