Disable Array.Empty params array optimization in expression lambdas

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

@@ -615,7 +615,11 @@ private static ImmutableArray<RefKind> GetRefKindsOrNull(ArrayBuilder<RefKind> r
             var paramArrayType = parameters[paramsParam].Type;
             var arrayArgs = paramArray.ToImmutableAndFree();
 
-            if (arrayArgs.Length == 0) // if this is a zero-length array, rather than using "new T[0]", optimize with "Array.Empty<T>()" if it's available
+            // If this is a zero-length array, rather than using "new T[0]", optimize with "Array.Empty<T>()" 
+            // if it's available.  However, we also disable the optimization if we're in an expression lambda, the 
+            // point of which is just to represent the semantics of an operation, and we don't know that all consumers
+            // of expression lambdas will appropriately understand Array.Empty<T>().
+            if (arrayArgs.Length == 0 && !_inExpressionLambda)
             {
                 ArrayTypeSymbol ats = paramArrayType as ArrayTypeSymbol;
                 if (ats != null) // could be null if there's a semantic error, e.g. the params parameter type isn't an array

src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenExprLambdaTests.cs

@@ -2273,6 +2273,19 @@ static void Main()
             CompileAndVerify(
                 text,
                 new[] { ExpressionAssemblyRef }, expectedOutput: TrimExpectedOutput(expectedOutput));
+
+            // Also verify with the assemblies on which the tests are running, as there's a higher
+            // liklihood that they have Array.Empty, and we want to verify that Array.Empty is not used
+            // in expression lambdas.  This can be changed to use the mscorlib 4.6 metadata once it's
+            // available in the Roslyn tests.
+            CompileAndVerify(CreateCompilation(
+                text,
+                references: new[] {
+                    MetadataReference.CreateFromAssembly(typeof(object).Assembly),
+                    MetadataReference.CreateFromAssembly(typeof(System.Linq.Enumerable).Assembly)
+                },
+                options: TestOptions.ReleaseExe),
+                expectedOutput: TrimExpectedOutput(expectedOutput));
         }
 
         [WorkItem(544270, "DevDiv")]