Suppress completion after error tokens (#752)

* Fix #668 (partial)

* Tests

* Revert "Tests"

This reverts commit 7ffc9db.

* Suppress completion after error tokens

Author: Mikhail Arkhipov

src/LanguageServer/Impl/Completion/CompletionSource.cs

@@ -40,7 +40,7 @@ public CompletionResult GetCompletions(IDocumentAnalysis analysis, SourceLocatio
                     // no completions on integer ., the user is typing a float
                     return CompletionResult.Empty;
                 case ConstantExpression ce2 when ce2.Value is string:
-                    // no completions in strings
+                // no completions in strings
                 case null when context.Ast.IsInsideComment(context.Location):
                 case null when context.Ast.IsInsideString(context.Location):
                     return CompletionResult.Empty;
@@ -82,10 +82,11 @@ public CompletionResult GetCompletions(IDocumentAnalysis analysis, SourceLocatio
                     return result;
                 default: {
                         var result = ErrorExpressionCompletion.GetCompletions(scope, statement, expression, context);
-                        return result == CompletionResult.Empty
-                            ? TopLevelCompletion.GetCompletions(statement, scope, context)
-                            : result;
-                    }
+                        if (result == null) {
+                            return CompletionResult.Empty;
+                        }
+                        return result == CompletionResult.Empty ? TopLevelCompletion.GetCompletions(statement, scope, context) : result;
+                }
             }
         }
     }

src/LanguageServer/Impl/Completion/ErrorExpressionCompletion.cs

@@ -83,6 +83,9 @@ public static CompletionResult GetCompletions(ScopeStatement scope, Node stateme
                 case TokenKind.KeywordAs:
                     return lastToken.Key.Start <= context.Position && context.Position <= lastToken.Key.End ? null : CompletionResult.Empty;
 
+                case TokenKind.Error:
+                    return null;
+
                 default:
                     return CompletionResult.Empty;
             }

src/LanguageServer/Test/CompletionTests.cs

@@ -854,6 +854,15 @@ public async Task NoCompletionInString() {
             result.Should().HaveNoCompletion();
         }
 
+        [TestMethod, Priority(0)]
+        public async Task NoCompletionInOpenString() {
+
+            var analysis = await GetAnalysisAsync("'''.");
+            var cs = new CompletionSource(new PlainTextDocumentationSource(), ServerSettings.completion);
+            var result = cs.GetCompletions(analysis, new SourceLocation(1, 5));
+            result.Should().HaveNoCompletion();
+        }
+
         [TestMethod, Priority(0)]
         public async Task NoCompletionInComment() {