Allow warnings to be ignored (set-wise)

Towards dotnet#646

standard/basic-concepts.md

@@ -103,6 +103,7 @@ The textual order in which names are declared is generally of no significance. I
 <!-- markdownlint-enable MD028 -->
 > *Note*: As specified above, the declaration space of a block includes any nested blocks. Thus, in the following example, the `F` and `G` methods result in a compile-time error because the name `i` is declared in the outer block and cannot be redeclared in the inner block. However, the `H` and `I` methods are valid since the two `i`’s are declared in separate non-nested blocks.
 >
+> <!-- Example: {template:"standalone-lib", name:"Declarations2", expectedErrors:["CS0136","CS0136"], ignoredWarnings:["CS0219"] } -->
 > ```csharp
 > class A
 > {

tools/ExampleExtractor/ExampleMetadata.cs

@@ -22,6 +22,7 @@ public class ExampleMetadata
     public bool ReplaceEllipsis { get; set; }
     public List<string> ExpectedErrors { get; set; }
     public List<string> ExpectedWarnings { get; set; }
+    public List<string> IgnoredWarnings { get; set; }
     public List<string> ExpectedOutput { get; set; }
     public string ExpectedException { get; set; }
 

tools/ExampleTester/GeneratedExample.cs

@@ -50,15 +50,20 @@ internal async Task<bool> Test()
 
         bool ret = true;
         ret &= ValidateDiagnostics("errors", DiagnosticSeverity.Error, Metadata.ExpectedErrors);
-        ret &= ValidateDiagnostics("warnings", DiagnosticSeverity.Warning, Metadata.ExpectedWarnings);
+        ret &= ValidateDiagnostics("warnings", DiagnosticSeverity.Warning, Metadata.ExpectedWarnings, Metadata.IgnoredWarnings);
         ret &= ValidateOutput();
 
         return ret;
 
-        bool ValidateDiagnostics(string type, DiagnosticSeverity severity, List<string> expected)
+        bool ValidateDiagnostics(string type, DiagnosticSeverity severity, List<string> expected, List<string>? ignored = null)
         {
             expected ??= new List<string>();
-            var actual = compilation.GetDiagnostics().Where(d => d.Severity == severity).Select(d => d.Id).ToList();
+            ignored ??= new List<string>();
+            var actual = compilation.GetDiagnostics()
+                .Where(d => d.Severity == severity)
+                .Select(d => d.Id)
+                .Where(id => !ignored.Contains(id))
+                .ToList();
             return ValidateExpectedAgainstActual(type, expected, actual);
         }