Merge pull request #4317 from AlekseyTs/Issue4132

Properly adjust result type of a user-defined conversion for lifted case.

src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter_Conversion.cs

@@ -554,11 +554,23 @@ private BoundExpression MakeImplicitConversion(BoundExpression rewrittenOperand,
                         @checked);
                 }
 
+                TypeSymbol userDefinedConversionRewrittenType = conversion.Method.ReturnType;
+
+                if (rewrittenOperand.Type != conversion.Method.ParameterTypes[0])
+                {
+                    Debug.Assert(rewrittenOperand.Type.IsNullableType());
+                    Debug.Assert(rewrittenOperand.Type.GetNullableUnderlyingType() == conversion.Method.ParameterTypes[0]);
+                    Debug.Assert(!userDefinedConversionRewrittenType.IsNullableType());
+
+                    // Lifted conversion, wrap return type in Nullable
+                    userDefinedConversionRewrittenType = ((NamedTypeSymbol)rewrittenOperand.Type.OriginalDefinition).Construct(userDefinedConversionRewrittenType);
+                }
+
                 BoundExpression userDefined = RewriteUserDefinedConversion(
                     syntax,
                     rewrittenOperand,
                     conversion.Method,
-                    conversion.Method.ReturnType,
+                    userDefinedConversionRewrittenType,
                     conversion.Kind);
 
                 if (userDefined.Type != conversion.BestUserDefinedConversionAnalysis.ToType)

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

@@ -8676,5 +8676,57 @@ public static void Main (string[] args)
 System.Int32
 ");
         }
+
+        [Fact, WorkItem(4132, "https://github.com/dotnet/roslyn/issues/4132")]
+        public void Issue4132()
+        {
+            string source = @"
+using System;
+
+namespace NullableMathRepro
+{
+    class Program
+    {
+        static void Main(string[] args)
+        {
+            int? x = 0;
+            x += 5;
+            Console.WriteLine(""'x' is {0}"", x);
+
+            IntHolder? y = 0;
+            y += 5;
+            Console.WriteLine(""'y' is {0}"", y);
+        }
+    }
+
+    struct IntHolder
+    {
+        private int x;
+
+        public static implicit operator int (IntHolder ih)
+        {
+            Console.WriteLine(""operator int (IntHolder ih)"");
+            return ih.x;
+        }
+
+        public static implicit operator IntHolder(int i)
+        {
+            Console.WriteLine(""operator IntHolder(int i)"");
+            return new IntHolder { x = i };
+        }
+
+        public override string ToString()
+        {
+            return x.ToString();
+        }
+    }
+}";
+            CompileAndVerify(source: source, expectedOutput:
+@"'x' is 5
+operator IntHolder(int i)
+operator int (IntHolder ih)
+operator IntHolder(int i)
+'y' is 5");
+        }
     }
 }

src/Compilers/VisualBasic/Test/Semantic/Semantics/CompoundAssignment.vb

@@ -2701,9 +2701,68 @@ Position get for item 'Foo'
 Position set for item 'Foo'
 False
 False
+]]>)
+        End Sub
+
+        <Fact, WorkItem(4132, "https://github.com/dotnet/roslyn/issues/4132")>
+        Public Sub Issue4132()
+
+            Dim compilationDef =
+<compilation>
+    <file name="a.vb">
+Imports System
+
+Namespace NullableMathRepro
+    Module Program
+        Sub Main()
+            Dim x As Integer? = 0
+            x += 5
+            Console.WriteLine("'x' is {0}", x)
+
+            Dim y As IntHolder? = 0
+            y += 5
+            Console.WriteLine("'y' is {0}", y)
+        End Sub
+    End Module
+
+    Structure IntHolder
+        Private x As Integer
+
+        Public Shared Widening Operator CType(ih As IntHolder) As Integer
+            Console.WriteLine("operator int (IntHolder ih)")
+            Return ih.x
+        End Operator
+
+        Public Shared Widening Operator CType(i As Integer) As IntHolder
+            Console.WriteLine("operator IntHolder(int i)")
+            Return New IntHolder With {.x = i}
+        End Operator
+
+        Public Shared Operator +(x As IntHolder, y As Integer) As Integer
+            Return CInt(x) + y
+        End Operator
+        Public Overrides Function ToString() As String
+            Return x.ToString()
+        End Function
+
+    End Structure
+End Namespace
+    </file>
+</compilation>
+
+            Dim compilation = CompilationUtils.CreateCompilationWithMscorlibAndVBRuntime(compilationDef, TestOptions.ReleaseExe)
+
+            CompileAndVerify(compilation,
+            <![CDATA[
+'x' is 5
+operator IntHolder(int i)
+operator int (IntHolder ih)
+operator IntHolder(int i)
+'y' is 5
 ]]>)
         End Sub
 
     End Class
 
 End Namespace
+