C#: Add implicit to string test.

Author: michaelnebel

csharp/ql/test/library-tests/implicittostring/implicitToString.cs

@@ -0,0 +1,31 @@
+using System;
+
+public class TestImplicitToString
+{
+    public class Container : IFormattable
+    {
+        public string ToString(string format, IFormatProvider formatProvider)
+        {
+            return "Formatted container";
+        }
+
+
+        public override string ToString()
+        {
+            return "Container";
+        }
+    }
+
+    public void M()
+    {
+        var x = new Container();
+
+        var y = "Hello" + x; // Implicit ToString call
+        y = "Hello" + x.ToString();
+        y = $"Hello {x}"; // Implicit ToString() call
+        y = $"Hello {x.ToString()}";
+        y = $"Hello {x:D}"; // Implicit ToString(string, IFormatProvider) call (we currently don't handle this)
+
+        var z = "Hello" + y; // No implicit ToString call as `y` is already a string.
+    }
+}

csharp/ql/test/library-tests/implicittostring/implicitToString.expected

@@ -0,0 +1,3 @@
+| implicitToString.cs:23:27:23:27 | call to method ToString |
+| implicitToString.cs:25:22:25:22 | call to method ToString |
+| implicitToString.cs:27:22:27:22 | call to method ToString |

csharp/ql/test/library-tests/implicittostring/implicitToString.ql

@@ -0,0 +1,5 @@
+import csharp
+
+from MethodCall c
+where c.isImplicit()
+select c