Preserve const enums should keep import refs

for exported const enums exported via export default

Move some functionality around, small cleanup

Remove unneeded const enum check

Author: weswigham

src/compiler/checker.ts

@@ -2253,7 +2253,7 @@ namespace ts {
             return links.target;
         }
 
-        function markExportAsReferenced(node: ImportEqualsDeclaration | ExportAssignment | ExportSpecifier) {
+        function markExportAsReferenced(node: ImportEqualsDeclaration | ExportSpecifier) {
             const symbol = getSymbolOfNode(node);
             const target = resolveAlias(symbol);
             if (target) {
@@ -2275,15 +2275,10 @@ namespace ts {
                 links.referenced = true;
                 const node = getDeclarationOfAliasSymbol(symbol);
                 if (!node) return Debug.fail();
-                if (node.kind === SyntaxKind.ExportAssignment) {
-                    // export default <symbol>
-                    checkExpressionCached((<ExportAssignment>node).expression);
-                }
-                else if (node.kind === SyntaxKind.ExportSpecifier) {
-                    // export { <symbol> } or export { <symbol> as foo }
-                    checkExpressionCached((<ExportSpecifier>node).propertyName || (<ExportSpecifier>node).name);
-                }
-                else if (isInternalModuleImportEqualsDeclaration(node)) {
+                // We defer checking of the reference of an `import =` until the import itself is referenced,
+                // This way a chain of imports can be elided if ultimately the final input is only used in a type
+                // position.
+                if (isInternalModuleImportEqualsDeclaration(node)) {
                     // import foo = <symbol>
                     checkExpressionCached(<Expression>node.moduleReference);
                 }
@@ -17430,7 +17425,7 @@ namespace ts {
         }
 
         function markAliasReferenced(symbol: Symbol, location: Node) {
-            if (isNonLocalAlias(symbol, /*excludes*/ SymbolFlags.Value) && !isInTypeQuery(location) && !isConstEnumOrConstEnumOnlyModule(resolveAlias(symbol))) {
+            if (isNonLocalAlias(symbol, /*excludes*/ SymbolFlags.Value) && !isInTypeQuery(location) && (compilerOptions.preserveConstEnums || !isConstEnumOrConstEnumOnlyModule(resolveAlias(symbol)))) {
                 markAliasSymbolAsReferenced(symbol);
             }
         }
@@ -19941,8 +19936,8 @@ namespace ts {
                 // if jsx emit was not react as there wont be error being emitted
                 reactSym.isReferenced = SymbolFlags.All;
 
-                // If react symbol is alias, mark it as referenced
-                if (reactSym.flags & SymbolFlags.Alias && !isConstEnumOrConstEnumOnlyModule(resolveAlias(reactSym))) {
+                // If react symbol is alias, mark it as refereced
+                if (reactSym.flags & SymbolFlags.Alias) {
                     markAliasSymbolAsReferenced(reactSym);
                 }
             }
@@ -24163,7 +24158,7 @@ namespace ts {
             return result;
         }
 
-        function checkExpressionCached(node: Expression, checkMode?: CheckMode): Type {
+        function checkExpressionCached(node: Expression | QualifiedName, checkMode?: CheckMode): Type {
             const links = getNodeLinks(node);
             if (!links.resolvedType) {
                 if (checkMode && checkMode !== CheckMode.Normal) {
@@ -24471,7 +24466,8 @@ namespace ts {
                 (node.parent.kind === SyntaxKind.PropertyAccessExpression && (<PropertyAccessExpression>node.parent).expression === node) ||
                 (node.parent.kind === SyntaxKind.ElementAccessExpression && (<ElementAccessExpression>node.parent).expression === node) ||
                 ((node.kind === SyntaxKind.Identifier || node.kind === SyntaxKind.QualifiedName) && isInRightSideOfImportOrExportAssignment(<Identifier>node) ||
-                    (node.parent.kind === SyntaxKind.TypeQuery && (<TypeQueryNode>node.parent).exprName === node));
+                    (node.parent.kind === SyntaxKind.TypeQuery && (<TypeQueryNode>node.parent).exprName === node)) ||
+                (node.parent.kind === SyntaxKind.ExportSpecifier); // We allow reexporting const enums
 
             if (!ok) {
                 error(node, Diagnostics.const_enums_can_only_be_used_in_property_or_index_access_expressions_or_the_right_hand_side_of_an_import_declaration_or_export_assignment_or_type_query);
@@ -29049,6 +29045,10 @@ namespace ts {
                 }
                 else {
                     markExportAsReferenced(node);
+                    const target = symbol && (symbol.flags & SymbolFlags.Alias ? resolveAlias(symbol) : symbol);
+                    if (!target || target === unknownSymbol || target.flags & SymbolFlags.Value) {
+                        checkExpressionCached(node.propertyName || node.name);
+                    }
                 }
             }
         }
@@ -29075,7 +29075,17 @@ namespace ts {
                 grammarErrorOnFirstToken(node, Diagnostics.An_export_assignment_cannot_have_modifiers);
             }
             if (node.expression.kind === SyntaxKind.Identifier) {
-                markExportAsReferenced(node);
+                const id = node.expression as Identifier;
+                const sym = resolveEntityName(id, SymbolFlags.All, /*ignoreErrors*/ true, /*dontResolveAlias*/ true, node);
+                if (sym) {
+                    markAliasReferenced(sym, id);
+                    // If not a value, we're interpreting the identifier as a type export, along the lines of (`export { Id as default }`)
+                    const target = sym.flags & SymbolFlags.Alias ? resolveAlias(sym) : sym;
+                    if (target === unknownSymbol || target.flags & SymbolFlags.Value) {
+                        // However if it is a value, we need to check it's being used correctly
+                        checkExpressionCached(node.expression);
+                    }
+                }
 
                 if (getEmitDeclarations(compilerOptions)) {
                     collectLinkedAliases(node.expression as Identifier, /*setVisibility*/ true);

tests/baselines/reference/constEnumPreserveEmitReexport.js

@@ -0,0 +1,32 @@
+//// [tests/cases/compiler/constEnumPreserveEmitReexport.ts] ////
+
+//// [ConstEnum.ts]
+export const enum MyConstEnum {
+    Foo,
+    Bar
+};
+//// [ImportExport.ts]
+import { MyConstEnum } from './ConstEnum';
+export default MyConstEnum;
+//// [ReExport.ts]
+export { MyConstEnum as default } from './ConstEnum';
+
+//// [ConstEnum.js]
+"use strict";
+exports.__esModule = true;
+var MyConstEnum;
+(function (MyConstEnum) {
+    MyConstEnum[MyConstEnum["Foo"] = 0] = "Foo";
+    MyConstEnum[MyConstEnum["Bar"] = 1] = "Bar";
+})(MyConstEnum = exports.MyConstEnum || (exports.MyConstEnum = {}));
+;
+//// [ImportExport.js]
+"use strict";
+exports.__esModule = true;
+var ConstEnum_1 = require("./ConstEnum");
+exports["default"] = ConstEnum_1.MyConstEnum;
+//// [ReExport.js]
+"use strict";
+exports.__esModule = true;
+var ConstEnum_1 = require("./ConstEnum");
+exports["default"] = ConstEnum_1.MyConstEnum;

tests/baselines/reference/constEnumPreserveEmitReexport.symbols

@@ -0,0 +1,23 @@
+=== tests/cases/compiler/ConstEnum.ts ===
+export const enum MyConstEnum {
+>MyConstEnum : Symbol(MyConstEnum, Decl(ConstEnum.ts, 0, 0))
+
+    Foo,
+>Foo : Symbol(MyConstEnum.Foo, Decl(ConstEnum.ts, 0, 31))
+
+    Bar
+>Bar : Symbol(MyConstEnum.Bar, Decl(ConstEnum.ts, 1, 8))
+
+};
+=== tests/cases/compiler/ImportExport.ts ===
+import { MyConstEnum } from './ConstEnum';
+>MyConstEnum : Symbol(MyConstEnum, Decl(ImportExport.ts, 0, 8))
+
+export default MyConstEnum;
+>MyConstEnum : Symbol(MyConstEnum, Decl(ImportExport.ts, 0, 8))
+
+=== tests/cases/compiler/ReExport.ts ===
+export { MyConstEnum as default } from './ConstEnum';
+>MyConstEnum : Symbol(MyConstEnum, Decl(ConstEnum.ts, 0, 0))
+>default : Symbol(default, Decl(ReExport.ts, 0, 8))
+

tests/baselines/reference/constEnumPreserveEmitReexport.types

@@ -0,0 +1,23 @@
+=== tests/cases/compiler/ConstEnum.ts ===
+export const enum MyConstEnum {
+>MyConstEnum : MyConstEnum
+
+    Foo,
+>Foo : MyConstEnum.Foo
+
+    Bar
+>Bar : MyConstEnum.Bar
+
+};
+=== tests/cases/compiler/ImportExport.ts ===
+import { MyConstEnum } from './ConstEnum';
+>MyConstEnum : typeof MyConstEnum
+
+export default MyConstEnum;
+>MyConstEnum : MyConstEnum
+
+=== tests/cases/compiler/ReExport.ts ===
+export { MyConstEnum as default } from './ConstEnum';
+>MyConstEnum : typeof import("tests/cases/compiler/ConstEnum").MyConstEnum
+>default : typeof import("tests/cases/compiler/ConstEnum").MyConstEnum
+

tests/baselines/reference/exportAsNamespaceConflict.errors.txt

@@ -1,12 +1,9 @@
-/a.d.ts(2,10): error TS2708: Cannot use namespace 'N' as a value.
 /a.d.ts(3,1): error TS2303: Circular definition of import alias 'N'.
 
 
-==== /a.d.ts (2 errors) ====
+==== /a.d.ts (1 errors) ====
     declare global { namespace N {} }
     export = N;
-             ~
-!!! error TS2708: Cannot use namespace 'N' as a value.
     export as namespace N;
     ~~~~~~~~~~~~~~~~~~~~~~
 !!! error TS2303: Circular definition of import alias 'N'.

tests/cases/compiler/constEnumPreserveEmitReexport.ts

@@ -0,0 +1,11 @@
+// @preserveConstEnums: true
+// @filename: ConstEnum.ts
+export const enum MyConstEnum {
+    Foo,
+    Bar
+};
+// @filename: ImportExport.ts
+import { MyConstEnum } from './ConstEnum';
+export default MyConstEnum;
+// @filename: ReExport.ts
+export { MyConstEnum as default } from './ConstEnum';