Allow compile on save with decorator emit

Author: Ben Lichtman

src/server/session.ts

@@ -1599,6 +1599,10 @@ namespace ts.server {
                 return emptyArray;
             }
 
+            function dtsChangeCanAffectEmit(compilationSettings: CompilerOptions) {
+                return getEmitDeclarations(compilationSettings) || !!compilationSettings.emitDecoratorMetadata;
+            }
+
             return combineProjectOutput(
                 info,
                 path => this.projectService.getScriptInfoForPath(path)!,
@@ -1610,8 +1614,8 @@ namespace ts.server {
 
                     const compilationSettings = project.getCompilationSettings();
 
-                    if (!!compilationSettings.noEmit || fileExtensionIs(info.fileName, Extension.Dts) && !getEmitDeclarations(compilationSettings)) {
-                        // avoid triggering emit when a change is made in a .d.ts when declaration emit is disabled
+                    if (!!compilationSettings.noEmit || fileExtensionIs(info.fileName, Extension.Dts) && !dtsChangeCanAffectEmit(compilationSettings)) {
+                        // avoid triggering emit when a change is made in a .d.ts when declaration emit and decorator metadata emit are disabled
                         return undefined;
                     }
 

src/testRunner/unittests/tsserver/compileOnSave.ts

@@ -598,6 +598,15 @@ namespace ts.projectSystem {
                 /*expectDTSEmit*/ true
                 );
             });
+
+            it("should return results if change is made in a global declaration file with decorator emit enabled", () => {
+                testDTS(
+                /*dtsFileContents*/ "declare const x: string;",
+                /*tsFileContents*/ "var y = 1;",
+                /*opts*/ { experimentalDecorators: true, emitDecoratorMetadata: true },
+                /*expectDTSEmit*/ true
+                );
+            });
         });
 
         describe("tsserverProjectSystem emit with outFile or out setting", () => {