Add example of compile-time error

Fixes dotnet#938

Add an example of a type pattern to demonstrate when a pattern generates a compile-time error.

Author: BillWagner

standard/patterns.md

@@ -22,6 +22,28 @@ A *declaration_pattern* and a *var_pattern* can result in the declaration of a l
 
 Each pattern form defines the set of types for input values that the pattern may be applied to. A pattern `P` is *applicable to* a type `T` if `T` is among the types whose values the pattern may match. It is a compile-time error if a pattern `P` appears in a program to match a pattern input value ([§11.1](patterns.md#111-general)) of type `T` if `P` is not applicable to `T`.
 
+> *Example*: The following example generates a compile-time error because the compile-time type of `v` is `Stream`. A variable of type `Stream` can never be an expression of type `string`:
+>
+> ```csharp
+> Stream v = OpenDataFile(); // compile-time type of 'v' is 'Stream'
+> if (v is string) // compile-time error
+> {
+>     /* code assuming v is a string*/ 
+> }
+> ```
+>
+> However, the following doesn't generate a compile-time error because the compile-time type of `v` is `object`. A variable of type `object` could be an expression of type `string`:
+>
+> ```csharp
+> object v = OpenDataFile();
+> if (v is string s)
+> {
+>     /* code assuming v is a string*/ 
+> }
+> ```
+>
+> *end example*
+
 Each pattern form defines the set of values for which the pattern *matches* the value at runtime.
 
 ### 11.2.2 Declaration pattern