Don't emit duplicate triple-slash directives when using API to print a .d.ts (#40968)

Author: rbuckton

src/compiler/emitter.ts

@@ -5136,7 +5136,12 @@ namespace ts {
             hasWrittenComment = false;
 
             if (isEmittedNode) {
-                forEachLeadingCommentToEmit(pos, emitLeadingComment);
+                if (pos === 0 && currentSourceFile?.isDeclarationFile) {
+                    forEachLeadingCommentToEmit(pos, emitNonTripleSlashLeadingComment);
+                }
+                else {
+                    forEachLeadingCommentToEmit(pos, emitLeadingComment);
+                }
             }
             else if (pos === 0) {
                 // If the node will not be emitted in JS, remove all the comments(normal, pinned and ///) associated with the node,
@@ -5157,6 +5162,12 @@ namespace ts {
             }
         }
 
+        function emitNonTripleSlashLeadingComment(commentPos: number, commentEnd: number, kind: SyntaxKind, hasTrailingNewLine: boolean, rangePos: number) {
+            if (!isTripleSlashComment(commentPos, commentEnd)) {
+                emitLeadingComment(commentPos, commentEnd, kind, hasTrailingNewLine, rangePos);
+            }
+        }
+
         function shouldWriteComment(text: string, pos: number) {
             if (printerOptions.onlyPrintJsDocStyle) {
                 return (isJSDocLikeText(text, pos) || isPinnedComment(text, pos));

src/testRunner/unittests/printer.ts

@@ -93,6 +93,33 @@ namespace ts {
             });
         });
 
+        describe("No duplicate ref directives when emiting .d.ts->.d.ts", () => {
+            it("without statements", () => {
+                const host = new fakes.CompilerHost(new vfs.FileSystem(true, {
+                    files: {
+                        "/test.d.ts": `/// <reference types="node" />\n/// <reference path="./src/test.d.ts />\n`
+                    }
+                }));
+                const program = createProgram(["/test.d.ts"], { }, host);
+                const file = program.getSourceFile("/test.d.ts")!;
+                const printer = createPrinter({ newLine: NewLineKind.CarriageReturnLineFeed });
+                const output = printer.printFile(file);
+                assert.equal(output.split(/\r?\n/g).length, 3);
+            });
+            it("with statements", () => {
+                const host = new fakes.CompilerHost(new vfs.FileSystem(true, {
+                    files: {
+                        "/test.d.ts": `/// <reference types="node" />\n/// <reference path="./src/test.d.ts />\nvar a: number;\n`
+                    }
+                }));
+                const program = createProgram(["/test.d.ts"], { }, host);
+                const file = program.getSourceFile("/test.d.ts")!;
+                const printer = createPrinter({ newLine: NewLineKind.CarriageReturnLineFeed });
+                const output = printer.printFile(file);
+                assert.equal(output.split(/\r?\n/g).length, 4);
+            });
+        });
+
         describe("printBundle", () => {
             const printsCorrectly = makePrintsCorrectly("printsBundleCorrectly");
             let bundle: Bundle;