Add None with type NoneType during builtin PostWalk (microsoft#447)

Fixes microsoft#442.
Replaces microsoft#429, which only masks the underlying True/False bug in microsoft#391.

I also removed the hack in microsoft#336 which checked for `ConstantExpression`.

src/Analysis/Engine/Impl/Analyzer/ExpressionEvaluator.cs

@@ -150,7 +150,7 @@ internal class ExpressionEvaluator {
                         res = refs.Types;
                     } else {
                         // ... warn the user
-                        warn = !(node is ConstantExpression); // Don't warn when None.
+                        warn = true;
                     }
                 }
             }

src/Analysis/Engine/Impl/Interpreter/Ast/AstBuiltinsPythonModule.cs

@@ -90,22 +90,21 @@ public AstBuiltinsPythonModule(PythonLanguageVersion version)
 #endif
 
             var walker = new AstAnalysisWalker(
-                interpreter, ast, this, filePath, null, _members, 
-                includeLocations, 
-                warnAboutUndefinedValues: true, 
+                interpreter, ast, this, filePath, null, _members,
+                includeLocations,
+                warnAboutUndefinedValues: true,
                 suppressBuiltinLookup: true
             );
             walker.CreateBuiltinTypes = true;
             return walker;
         }
 
         protected override void PostWalk(PythonWalker walker) {
-            IPythonType boolType = null;
+            AstPythonBuiltinType boolType = null;
+            AstPythonBuiltinType noneType = null;
 
             foreach (BuiltinTypeId typeId in Enum.GetValues(typeof(BuiltinTypeId))) {
-                IMember m;
-                AstPythonBuiltinType biType;
-                if (_members.TryGetValue("__{0}__".FormatInvariant(typeId), out m) && (biType = m as AstPythonBuiltinType) != null) {
+                if (_members.TryGetValue("__{0}__".FormatInvariant(typeId), out IMember m) && m is AstPythonBuiltinType biType) {
                     if (typeId != BuiltinTypeId.Str &&
                         typeId != BuiltinTypeId.StrIterator) {
                         biType.TrySetTypeId(typeId);
@@ -117,7 +116,11 @@ public AstBuiltinsPythonModule(PythonLanguageVersion version)
                     _hiddenNames.Add("__{0}__".FormatInvariant(typeId));
 
                     if (typeId == BuiltinTypeId.Bool) {
-                        boolType = m as IPythonType;
+                        boolType = boolType ?? biType;
+                    }
+
+                    if (typeId == BuiltinTypeId.NoneType) {
+                        noneType = noneType ?? biType;
                     }
                 }
             }
@@ -127,6 +130,10 @@ public AstBuiltinsPythonModule(PythonLanguageVersion version)
                 _members["True"] = _members["False"] = new AstPythonConstant(boolType);
             }
 
+            if (noneType != null) {
+                _members["None"] = new AstPythonConstant(noneType);
+            }
+
             base.PostWalk(walker);
         }
 

src/Analysis/Engine/Test/LanguageServerTests.cs

@@ -363,18 +363,15 @@ public class LanguageServerTests {
             }
         }
 
-        [TestMethod, Priority(0)]
-        public async Task TypeHintNoneDiagnostic() {
+        [DataRow("def f(b: None) -> None:\n    b: None")]
+        [DataRow("from typing import Generator\ndef fun() -> Generator[int, None, None]:\n    yield from range(10)")]
+        [DataTestMethod, Priority(0)]
+        public async Task TypeHintNoneDiagnostic(string code) {
             if (this is LanguageServerTests_V2) {
                 // No type hints in Python 2.
                 return;
             }
 
-            var code = @"
-def f(b: None) -> None:
-    b: None
-";
-
             var diags = new Dictionary<Uri, PublishDiagnosticsEventArgs>();
             using (var s = await CreateServer((Uri)null, null, diags)) {
                 var u = await s.OpenDefaultDocumentAndGetUriAsync(code);