fix issue with optionality of CompilerHost

src/compiler/program.ts

@@ -959,15 +959,16 @@ namespace ts {
 
         let resolvedTypeReferenceDirectives: Map<ResolvedTypeReferenceDirective> = {};
         let fileProcessingDiagnostics = createDiagnosticCollection();
-        let skipDefaultLib = options.noLib;
-        const programDiagnostics = createDiagnosticCollection();
-        const currentDirectory = host.getCurrentDirectory();
-        const supportedExtensions = getSupportedExtensions(options);
 
         const start = new Date().getTime();
 
         host = host || createCompilerHost(options);
 
+        let skipDefaultLib = options.noLib;
+        const programDiagnostics = createDiagnosticCollection();
+        const currentDirectory = host.getCurrentDirectory();
+        const supportedExtensions = getSupportedExtensions(options);
+
         // Map storing if there is emit blocking diagnostics for given input
         const hasEmitBlockingDiagnostics = createFileMap<boolean>(getCanonicalFileName);
 

tests/cases/unittests/reuseProgramStructure.ts

@@ -368,5 +368,11 @@ module ts {
             assert.isTrue(!program_3.structureIsReused);
             checkResolvedTypeDirectivesCache(program_1, "/a.ts", { "typedefs": { resolvedFileName: "/types/typedefs/index.d.ts", primary: true } });
         });
-    })
+    });
+
+    describe("host is optional", () => {
+        it("should work if host is not provided", () => {
+            createProgram([], {});
+        })
+    });
 }