Merge pull request #21860 from VSadov/fix21858

Val-escape for a struct field should defer to _val_ escape of the receiver

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

@@ -1903,7 +1903,7 @@ internal static uint GetValEscape(BoundExpression expr, uint scopeOfTheContainin
                     Debug.Assert(!fieldSymbol.IsStatic && fieldSymbol.ContainingType.IsByRefLikeType);
 
                     // for ref-like fields defer to the receiver.
-                    return GetRefEscape(fieldAccess.ReceiverOpt, scopeOfTheContainingExpression);
+                    return GetValEscape(fieldAccess.ReceiverOpt, scopeOfTheContainingExpression);
 
                 case BoundKind.Call:
                     var call = (BoundCall)expr;

src/Compilers/CSharp/Portable/Symbols/Source/SourceLocalSymbol.cs

@@ -84,21 +84,9 @@ internal override SyntaxNode ScopeDesignatorOpt
             get { return _scopeBinder.ScopeDesignator; }
         }
 
-        internal override uint RefEscapeScope
-        {
-            get
-            {
-                return _refEscapeScope;
-            }
-        }
+        internal override uint RefEscapeScope => _refEscapeScope;
 
-        internal override uint ValEscapeScope
-        {
-            get
-            {
-                return _valEscapeScope;
-            }
-        }
+        internal override uint ValEscapeScope => _valEscapeScope;
 
         /// <summary>
         /// Binder that should be used to bind type syntax for the local.

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

@@ -1250,5 +1250,54 @@ static S1 MayWrap(ref int arg)
                 Diagnostic(ErrorCode.ERR_EscapeLocal, "sp1").WithArguments("sp1").WithLocation(21, 20)
                 );
         }
+
+        [WorkItem(21858, "https://github.com/dotnet/roslyn/issues/21858")]
+        [Fact()]
+        public void FieldOfRefLikeEscape()
+        {
+            var text = @"
+    class Program
+    {
+        static void Main()
+        {
+        }
+
+        ref struct S1
+        {
+            private S2 x;
+
+            public S1(S2 arg) => x = arg;
+
+            public S2 M1()
+            {
+                // ok
+                return x;
+            }
+
+            public S2 M2()
+            {
+                var toReturn = x;
+
+                // ok
+                return toReturn;
+            }
+
+            public ref S2 M3()
+            {
+                // not ok
+                return ref x;
+            }
+        }
+
+        ref struct S2{}
+
+    }
+";
+            CreateCompilationWithMscorlibAndSpan(text).VerifyDiagnostics(
+                // (31,28): error CS8170: Struct members cannot return 'this' or other instance members by reference
+                //                 return ref x;
+                Diagnostic(ErrorCode.ERR_RefReturnStructThis, "x").WithArguments("this").WithLocation(31, 28)
+                );
+        }
     }
 }