ignore fixed-field check if inside nameof binder (#13941)

* ignore fixed-field check if inside nameof binder

* redo is nameof-binder strategy

* pick a good name for the test

* use existing method for determining if we are inside of a nameof node

* fix nameof(outer field) crash

* minor style tweaks

* allow nulls in VisitLValue

* harden vb VisitLValue/VisitRValue

* Revert "harden vb VisitLValue/VisitRValue"

This reverts commit edeaddd00ce310b0408b5fc6609b15f17b674654.

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

@@ -5664,7 +5664,7 @@ private static void CombineExtensionMethodArguments(BoundExpression receiver, An
                 hasError = this.CheckInstanceOrStatic(node, receiver, fieldSymbol, ref resultKind, diagnostics);
             }
 
-            if (!hasError && fieldSymbol.IsFixed)
+            if (!hasError && fieldSymbol.IsFixed && EnclosingNameofArgument == null)
             {
                 TypeSymbol receiverType = receiver.Type;
                 hasError =

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

@@ -501,7 +501,7 @@ protected void SetUnreachable()
         protected void VisitLvalue(BoundExpression node)
         {
             if (_trackRegions && node == this.firstInRegion && this.regionPlace == RegionPlace.Before) EnterRegion();
-            switch (node.Kind)
+            switch (node?.Kind)
             {
                 case BoundKind.Parameter:
                     VisitLvalueParameter((BoundParameter)node);

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

@@ -1209,7 +1209,7 @@ class Other {
 }
 ";
             var compilation = CreateCompilationWithMscorlib45(source, null, new CSharpCompilationOptions(OutputKind.ConsoleApplication).WithAllowUnsafe(true));
-            CompileAndVerify(compilation, expectedOutput: 
+            CompileAndVerify(compilation, expectedOutput:
                 "MessageType x MessageType").VerifyDiagnostics();
         }
 
@@ -1258,5 +1258,73 @@ class Other {
                 //         return nameof(myStruct.MessageType);
                 Diagnostic(ErrorCode.ERR_BadArgType, "myStruct.MessageType").WithArguments("1", "char*", "char[]").WithLocation(26, 23));
         }
+
+
+        [Fact, WorkItem(12696, "https://github.com/dotnet/roslyn/issues/12696")]
+        public void FixedFieldAccessInsideNameOf()
+        {
+            var source =
+@"
+using System;
+
+struct MyType
+{
+  public static string a = nameof(MyType.normalField);
+  public static string b = nameof(MyType.fixedField);
+  public static string c = nameof(fixedField);
+
+  public int normalField;
+  public unsafe fixed short fixedField[6];
+
+  public MyType(int i) {
+      this.normalField = i;
+  }
+}
+
+class EntryPoint
+{
+    public static void Main(string[] args)
+    {
+        Console.Write(MyType.a + "" "");
+        Console.Write(MyType.b + "" "");
+        Console.Write(MyType.c);
+    }
+}
+";
+            var compilation = CreateCompilationWithMscorlib45(source, null, new CSharpCompilationOptions(OutputKind.ConsoleApplication).WithAllowUnsafe(true));
+            CompileAndVerify(compilation, expectedOutput: "normalField fixedField fixedField").VerifyDiagnostics();
+        }
+
+        [Fact, WorkItem(12696, "https://github.com/dotnet/roslyn/issues/12696")]
+        public void FixedFieldAccessFromInnerClass()
+        {
+            var source =
+@"
+using System;
+
+public struct MyType
+{
+  public static class Inner
+  {
+     public static string a = nameof(normalField);
+     public static string b = nameof(fixedField);
+  }
+
+  public int normalField;
+  public unsafe fixed short fixedField[6];
+}
+
+class EntryPoint
+{
+    public static void Main(string[] args)
+    {
+        Console.Write(MyType.Inner.a + "" "");
+        Console.Write(MyType.Inner.b);
+    }
+}
+";
+            var compilation = CreateCompilationWithMscorlib45(source, null, new CSharpCompilationOptions(OutputKind.ConsoleApplication).WithAllowUnsafe(true));
+            CompileAndVerify(compilation, expectedOutput: "normalField fixedField").VerifyDiagnostics();
+        }
     }
 }