C#: Add implicit to string test.

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

@@ -0,0 +1,44 @@
+using System;
+
+public class TestImplicitToString
+{
+    public class Container
+    {
+        public override string ToString()
+        {
+            return "Container";
+        }
+    }
+
+    public class FormattableContainer : IFormattable
+    {
+        public string ToString(string format, IFormatProvider formatProvider)
+        {
+            return "Formatted container";
+        }
+
+
+        public override string ToString()
+        {
+            return "Container";
+        }
+    }
+
+    public void M()
+    {
+        var container = new Container();
+
+        var y = "Hello" + container; // Implicit ToString call
+        y = "Hello" + container.ToString();
+        y = $"Hello {container}"; // Implicit ToString() call
+        y = $"Hello {container.ToString()}";
+        y = $"Hello {container:D}"; // Implicit ToString() call.
+
+        var z = "Hello" + y; // No implicit ToString call as `y` is already a string.
+
+        var formattableContainer = new FormattableContainer();
+        y = "Hello" + formattableContainer; // Implicit call to ToString().
+        y = $"Hello {formattableContainer}"; // Implicit call to ToString(string, IFormatProvider). We don't handle this.
+        y = $"Hello {formattableContainer:D}"; // Implicit call to ToString(string, IFormatProvider). We don't handle this.
+    }
+}

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

@@ -0,0 +1,4 @@
+| implicitToString.cs:31:27:31:35 | call to method ToString |
+| implicitToString.cs:33:22:33:30 | call to method ToString |
+| implicitToString.cs:35:22:35:30 | call to method ToString |
+| implicitToString.cs:40:23:40:42 | 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