Add test for #21317

Author: Andy Hanson

src/harness/fourslash.ts

@@ -880,6 +880,14 @@ namespace FourSlash {
             });
         }
 
+        public verifyTypeAtLocation(markerName: string, type: string) {
+            const marker = this.getMarkerByName(markerName);
+            const file = this.getProgram().getSourceFile(marker.fileName)!;
+            const node = ts.findPrecedingToken(marker.position, file);
+            const checker = this.getProgram().getTypeChecker();
+            assert.equal(checker.typeToString(checker.getTypeAtLocation(node)), type);
+        }
+
         public verifyCompletionListContains(entryId: ts.Completions.CompletionEntryIdentifier, text?: string, documentation?: string, kind?: string | { kind?: string, kindModifiers?: string }, spanIndex?: number, hasAction?: boolean, options?: FourSlashInterface.VerifyCompletionListContainsOptions) {
             const completions = this.getCompletionListAtCaret(options);
             if (completions) {
@@ -3989,6 +3997,10 @@ namespace FourSlashInterface {
             super(state);
         }
 
+        public typeAtLocation(markerName: string, type: string) {
+            this.state.verifyTypeAtLocation(markerName, type);
+        }
+
         public completionsAt(markerName: string, completions: ReadonlyArray<ExpectedCompletionEntry>, options?: CompletionsAtOptions) {
             this.state.verifyCompletionsAt(markerName, completions, options);
         }

tests/cases/fourslash/typeAtLocation.ts

@@ -0,0 +1,9 @@
+/// <reference path="fourslash.ts" />
+
+// @strictNullChecks: true
+
+////function test<T extends string | undefined>(x: T) {
+////    x/**/!;
+////}
+
+verify.typeAtLocation("", "string | undefined"); // TODO: GH#21317: should be "T".