Handle goto definition of unknown types better (microsoft#1831)

* Remove stale reference

* Don't suppress LHS diagnostics on augmented assign

* Revert "Don't suppress LHS diagnostics on augmented assign"

This reverts commit 6109ac7.

* Handle gotodef on unknown types

* PR feedback

Author: Mikhail Arkhipov

src/LanguageServer/Impl/Sources/DefinitionSource.cs

@@ -123,6 +123,8 @@ private Reference HandleFromImport(IDocumentAnalysis analysis, SourceLocation lo
                 case ImplicitPackageImport packageImport:
                     module = mres.GetImportedModule(packageImport.FullName);
                     break;
+                case ImportNotFound _:
+                    return null;
             }
 
             // Are we in the module name (i.e. A in 'from A import B')?
@@ -262,7 +264,8 @@ private Reference TryFromVariable(string name, IDocumentAnalysis analysis, Sourc
             definingMember = null;
 
             var m = analysis.ExpressionEvaluator.LookupNameInScopes(name, out var scope, LookupOptions.All);
-            if (m == null || scope.Module.ModuleType == ModuleType.Builtins || !(scope.Variables[name] is IVariable v)) {
+            var v = scope?.Variables[name];
+            if (m == null || scope == null || scope.Module.ModuleType == ModuleType.Builtins || v.IsUnknown()) {
                 return null;
             }
 

src/LanguageServer/Test/GoToDefinitionTests.cs

@@ -688,5 +688,27 @@ def c(): ...
             reference.uri.AbsolutePath.Should().Contain("other.py");
             reference.range.Should().Be(3, 4, 3, 5);
         }
+
+        [TestMethod, Priority(0)]
+        public async Task UnknownType() {
+            const string code = @"
+A
+";
+            var analysis = await GetAnalysisAsync(code);
+            var ds = new DefinitionSource(Services);
+            var reference = ds.FindDefinition(analysis, new SourceLocation(2, 1), out _);
+            reference.Should().BeNull();
+        }
+
+        [TestMethod, Priority(0)]
+        public async Task UnknownImportedType() {
+            const string code = @"
+from nonexistent import some
+";
+            var analysis = await GetAnalysisAsync(code);
+            var ds = new DefinitionSource(Services);
+            var reference = ds.FindDefinition(analysis, new SourceLocation(2, 26), out _);
+            reference.Should().BeNull();
+        }
     }
 }