Don't treat class name as a completion list blocker if the position c…

…omes after it (microsoft#20762)

src/services/completions.ts

@@ -1760,7 +1760,10 @@ namespace ts.Completions {
                     return true;
             }
 
-            return isDeclarationName(contextToken) && !isJsxAttribute(contextToken.parent);
+            return isDeclarationName(contextToken)
+                && !isJsxAttribute(contextToken.parent)
+                // Don't block completions if we're in `class C /**/`, because we're *past* the end of the identifier and might want to complete `extends`.
+                && !(isClassLike(contextToken.parent) && position > previousToken.end);
         }
 
         function isFunctionLikeButNotConstructor(kind: SyntaxKind) {

tests/cases/fourslash/completionsKeywordsExtends.ts

@@ -0,0 +1,11 @@
+/// <reference path="fourslash.ts" />
+
+////class C/*a*/ /*b*/ { }
+
+// Tests that `isCompletionListBlocker` is true *at* the class name, but false *after* it.
+
+goTo.marker("a");
+verify.completionListIsEmpty();
+
+goTo.marker("b");
+verify.completionListContains("extends");