Fix comment emit for namespaces & enums.

Keep emit flags set on the original node on a namespace or enum node.
This prevents dropping flags like NoComments, which caused duplicated
comment emits.

Additionally, TypeScript would emit synthetic comments twice, once for
the variable declaration, once for the module statement. This explicitly
clears away synthetic comments on namespaces and enums if their
synthetic comments have already been emitted on the corresponding
variable statement.

src/compiler/transformers/ts.ts

@@ -2633,7 +2633,8 @@ namespace ts {
             // If needed, we should emit a variable declaration for the enum. If we emit
             // a leading variable declaration, we should not emit leading comments for the
             // enum body.
-            if (addVarForEnumOrModuleDeclaration(statements, node)) {
+            const varAdded = addVarForEnumOrModuleDeclaration(statements, node);
+            if (varAdded) {
                 // We should still emit the comments if we are emitting a system module.
                 if (moduleKind !== ModuleKind.System || currentLexicalScope !== currentSourceFile) {
                     emitFlags |= EmitFlags.NoLeadingComments;
@@ -2691,8 +2692,13 @@ namespace ts {
             );
 
             setOriginalNode(enumStatement, node);
+            if (varAdded) {
+                // If a variable was added, synthetic comments are mitted on it, not on the moduleStatement.
+                setSyntheticLeadingComments(enumStatement, undefined);
+                setSyntheticTrailingComments(enumStatement, undefined);
+            }
             setTextRange(enumStatement, node);
-            setEmitFlags(enumStatement, emitFlags);
+            addEmitFlags(enumStatement, emitFlags);
             statements.push(enumStatement);
 
             // Add a DeclarationMarker for the enum to preserve trailing comments and mark
@@ -2882,7 +2888,7 @@ namespace ts {
                 //     })(m1 || (m1 = {})); // trailing comment module
                 //
                 setCommentRange(statement, node);
-                setEmitFlags(statement, EmitFlags.NoTrailingComments | EmitFlags.HasEndOfDeclarationMarker);
+                addEmitFlags(statement, EmitFlags.NoTrailingComments | EmitFlags.HasEndOfDeclarationMarker);
                 statements.push(statement);
                 return true;
             }
@@ -2922,7 +2928,8 @@ namespace ts {
             // If needed, we should emit a variable declaration for the module. If we emit
             // a leading variable declaration, we should not emit leading comments for the
             // module body.
-            if (addVarForEnumOrModuleDeclaration(statements, node)) {
+            const varAdded = addVarForEnumOrModuleDeclaration(statements, node);
+            if (varAdded) {
                 // We should still emit the comments if we are emitting a system module.
                 if (moduleKind !== ModuleKind.System || currentLexicalScope !== currentSourceFile) {
                     emitFlags |= EmitFlags.NoLeadingComments;
@@ -2979,8 +2986,13 @@ namespace ts {
             );
 
             setOriginalNode(moduleStatement, node);
+            if (varAdded) {
+                // If a variable was added, synthetic comments are mitted on it, not on the moduleStatement.
+                setSyntheticLeadingComments(moduleStatement, undefined);
+                setSyntheticTrailingComments(moduleStatement, undefined);
+            }
             setTextRange(moduleStatement, node);
-            setEmitFlags(moduleStatement, emitFlags);
+            addEmitFlags(moduleStatement, emitFlags);
             statements.push(moduleStatement);
 
             // Add a DeclarationMarker for the namespace to preserve trailing comments and mark

src/testRunner/unittests/transform.ts

@@ -357,6 +357,27 @@ class Clazz {
                         }
                     }).outputText;
         });
+
+        testBaseline("transformAddCommentToNamespace", () => {
+            return transpileModule(`
+// namespace comment.
+namespace Foo {
+    export const x = 1;
+}
+// another comment.
+namespace Foo {
+    export const y = 1;
+}
+`, {
+                        transformers: {
+                            before: [addSyntheticComment(n => isModuleDeclaration(n))],
+                        },
+                        compilerOptions: {
+                            target: ScriptTarget.ES2015,
+                            newLine: NewLineKind.CarriageReturnLineFeed,
+                        }
+                    }).outputText;
+        });
     });
 }
 

tests/baselines/reference/transformApi/transformsCorrectly.transformAddCommentToNamespace.js

@@ -0,0 +1,9 @@
+/*comment*/
+var Foo;
+(function (Foo) {
+    Foo.x = 1;
+})(Foo || (Foo = {}));
+/*comment*/
+(function (Foo) {
+    Foo.y = 1;
+})(Foo || (Foo = {}));