fix TypeScript empty type parameter skipping issue

CHANGELOG.md

@@ -1,5 +1,18 @@
 # Changelog
 
+## Unreleased
+
+* Allow empty type parameter lists in certain cases ([#3512](https://github.com/evanw/esbuild/issues/3512))
+
+    TypeScript allows interface declarations and type aliases to have empty type parameter lists. Previously esbuild didn't handle this edge case but with this release, esbuild will now parse this syntax:
+
+    ```ts
+    interface Foo<> {}
+    type Bar<> = {}
+    ```
+
+    This fix was contributed by [@magic-akari](https://github.com/magic-akari).
+
 ## 0.19.8
 
 * Add a treemap chart to esbuild's bundle analyzer ([#2848](https://github.com/evanw/esbuild/issues/2848))

internal/js_parser/js_parser.go

@@ -3795,9 +3795,13 @@ func (p *parser) parsePrefix(level js_ast.L, errors *deferredErrors, flags exprF
 		//     <const A>(x) => {}
 		//     <const A extends B>(x) => {}
 		//
+		//   A syntax error:
+		//     <>() => {}
+		//
 		// TSX:
 		//
 		//   A JSX element:
+		//     <>() => {}</>
 		//     <A>(x) => {}</A>
 		//     <A extends/>
 		//     <A extends>(x) => {}</A>
@@ -3816,6 +3820,7 @@ func (p *parser) parsePrefix(level js_ast.L, errors *deferredErrors, flags exprF
 		//   A syntax error:
 		//     <[]>(x)
 		//     <A[]>(x)
+		//     <>() => {}
 		//     <A>(x) => {}
 
 		if p.options.ts.Parse && p.options.jsx.Parse && p.isTSArrowFnJSX() {
@@ -4933,6 +4938,13 @@ func (p *parser) parseJSXNamespacedName() (logger.Range, js_lexer.MaybeSubstring
 	return nameRange, name
 }
 
+func tagOrFragmentHelpText(tag string) string {
+	if tag == "" {
+		return "fragment tag"
+	}
+	return fmt.Sprintf("%q tag", tag)
+}
+
 func (p *parser) parseJSXTag() (logger.Range, string, js_ast.Expr) {
 	loc := p.lexer.Loc()
 
@@ -5226,10 +5238,12 @@ func (p *parser) parseJSXElement(loc logger.Loc) js_ast.Expr {
 			p.lexer.NextInsideJSXElement()
 			endRange, endText, _ := p.parseJSXTag()
 			if startText != endText {
+				startTag := tagOrFragmentHelpText(startText)
+				endTag := tagOrFragmentHelpText(endText)
 				msg := logger.Msg{
 					Kind:  logger.Error,
-					Data:  p.tracker.MsgData(endRange, fmt.Sprintf("Expected closing %q tag to match opening %q tag", endText, startText)),
-					Notes: []logger.MsgData{p.tracker.MsgData(startRange, fmt.Sprintf("The opening %q tag is here:", startText))},
+					Data:  p.tracker.MsgData(endRange, fmt.Sprintf("Unexpected closing %s does not match opening %s", endTag, startTag)),
+					Notes: []logger.MsgData{p.tracker.MsgData(startRange, fmt.Sprintf("The opening %s is here:", startTag))},
 				}
 				msg.Data.Location.Suggestion = startText
 				p.log.AddMsg(msg)
@@ -5247,10 +5261,11 @@ func (p *parser) parseJSXElement(loc logger.Loc) js_ast.Expr {
 			}}
 
 		case js_lexer.TEndOfFile:
+			startTag := tagOrFragmentHelpText(startText)
 			msg := logger.Msg{
 				Kind:  logger.Error,
-				Data:  p.tracker.MsgData(p.lexer.Range(), fmt.Sprintf("Unexpected end of file before a closing %q tag", startText)),
-				Notes: []logger.MsgData{p.tracker.MsgData(startRange, fmt.Sprintf("The opening %q tag is here:", startText))},
+				Data:  p.tracker.MsgData(p.lexer.Range(), fmt.Sprintf("Unexpected end of file before a closing %s", startTag)),
+				Notes: []logger.MsgData{p.tracker.MsgData(startRange, fmt.Sprintf("The opening %s is here:", startTag))},
 			}
 			msg.Data.Location.Suggestion = fmt.Sprintf("</%s>", startText)
 			p.log.AddMsg(msg)

internal/js_parser/js_parser_test.go

@@ -5504,11 +5504,14 @@ func TestJSX(t *testing.T) {
 
 	expectParseErrorJSX(t, "<a b=true/>", "<stdin>: ERROR: Expected \"{\" but found \"true\"\n")
 	expectParseErrorJSX(t, "</a>", "<stdin>: ERROR: Expected identifier but found \"/\"\n")
-	expectParseErrorJSX(t, "<></b>", "<stdin>: ERROR: Expected closing \"b\" tag to match opening \"\" tag\n<stdin>: NOTE: The opening \"\" tag is here:\n")
-	expectParseErrorJSX(t, "<a></>", "<stdin>: ERROR: Expected closing \"\" tag to match opening \"a\" tag\n<stdin>: NOTE: The opening \"a\" tag is here:\n")
-	expectParseErrorJSX(t, "<a></b>", "<stdin>: ERROR: Expected closing \"b\" tag to match opening \"a\" tag\n<stdin>: NOTE: The opening \"a\" tag is here:\n")
+	expectParseErrorJSX(t, "<></b>",
+		"<stdin>: ERROR: Unexpected closing \"b\" tag does not match opening fragment tag\n<stdin>: NOTE: The opening fragment tag is here:\n")
+	expectParseErrorJSX(t, "<a></>",
+		"<stdin>: ERROR: Unexpected closing fragment tag does not match opening \"a\" tag\n<stdin>: NOTE: The opening \"a\" tag is here:\n")
+	expectParseErrorJSX(t, "<a></b>",
+		"<stdin>: ERROR: Unexpected closing \"b\" tag does not match opening \"a\" tag\n<stdin>: NOTE: The opening \"a\" tag is here:\n")
 	expectParseErrorJSX(t, "<\na\n.\nb\n>\n<\n/\nc\n.\nd\n>",
-		"<stdin>: ERROR: Expected closing \"c.d\" tag to match opening \"a.b\" tag\n<stdin>: NOTE: The opening \"a.b\" tag is here:\n")
+		"<stdin>: ERROR: Unexpected closing \"c.d\" tag does not match opening \"a.b\" tag\n<stdin>: NOTE: The opening \"a.b\" tag is here:\n")
 	expectParseErrorJSX(t, "<a-b.c>", "<stdin>: ERROR: Expected \">\" but found \".\"\n")
 	expectParseErrorJSX(t, "<a.b-c>", "<stdin>: ERROR: Unexpected \"-\"\n")
 

internal/js_parser/ts_parser.go

@@ -651,6 +651,9 @@ const (
 
 	// TypeScript 5.0
 	allowConstModifier
+
+	// Allow "<>" without any type parameters
+	allowEmptyTypeParameters
 )
 
 type skipTypeScriptTypeParametersResult uint8
@@ -671,6 +674,11 @@ func (p *parser) skipTypeScriptTypeParameters(flags typeParameterFlags) skipType
 	p.lexer.Next()
 	result := couldBeTypeCast
 
+	if (flags&allowEmptyTypeParameters) != 0 && p.lexer.Token == js_lexer.TGreaterThan {
+		p.lexer.Next()
+		return definitelyTypeParameters
+	}
+
 	for {
 		hasIn := false
 		hasOut := false
@@ -1181,7 +1189,7 @@ func (p *parser) skipTypeScriptInterfaceStmt(opts parseStmtOpts) {
 		p.localTypeNames[name] = true
 	}
 
-	p.skipTypeScriptTypeParameters(allowInOutVarianceAnnotations)
+	p.skipTypeScriptTypeParameters(allowInOutVarianceAnnotations | allowEmptyTypeParameters)
 
 	if p.lexer.Token == js_lexer.TExtends {
 		p.lexer.Next()
@@ -1256,7 +1264,7 @@ func (p *parser) skipTypeScriptTypeStmt(opts parseStmtOpts) {
 		p.localTypeNames[name] = true
 	}
 
-	p.skipTypeScriptTypeParameters(allowInOutVarianceAnnotations)
+	p.skipTypeScriptTypeParameters(allowInOutVarianceAnnotations | allowEmptyTypeParameters)
 	p.lexer.Expect(js_lexer.TEquals)
 	p.skipTypeScriptType(js_ast.LLowest)
 	p.lexer.ExpectOrInsertSemicolon()

internal/js_parser/ts_parser_test.go

@@ -323,6 +323,23 @@ func TestTSTypes(t *testing.T) {
 	expectPrintedTS(t, "type Foo = Array<<T>(x: T) => T>\n x", "x;\n")
 	expectPrintedTSX(t, "<Foo<<T>(x: T) => T>/>", "/* @__PURE__ */ React.createElement(Foo, null);\n")
 
+	expectPrintedTS(t, "interface Foo<> {}", "")
+	expectPrintedTSX(t, "interface Foo<> {}", "")
+	expectPrintedTS(t, "type Foo<> = {}", "")
+	expectPrintedTSX(t, "type Foo<> = {}", "")
+	expectParseErrorTS(t, "class Foo<> {}", "<stdin>: ERROR: Expected identifier but found \">\"\n")
+	expectParseErrorTSX(t, "class Foo<> {}", "<stdin>: ERROR: Expected identifier but found \">\"\n")
+	expectParseErrorTS(t, "class Foo { foo<>() {} }", "<stdin>: ERROR: Expected identifier but found \">\"\n")
+	expectParseErrorTSX(t, "class Foo { foo<>() {} }", "<stdin>: ERROR: Expected identifier but found \">\"\n")
+	expectParseErrorTS(t, "type Foo = { foo<>(): void }", "<stdin>: ERROR: Expected identifier but found \">\"\n")
+	expectParseErrorTSX(t, "type Foo = { foo<>(): void }", "<stdin>: ERROR: Expected identifier but found \">\"\n")
+	expectParseErrorTS(t, "type Foo = <>() => {}", "<stdin>: ERROR: Expected identifier but found \">\"\n")
+	expectParseErrorTSX(t, "type Foo = <>() => {}", "<stdin>: ERROR: Expected identifier but found \">\"\n")
+	expectParseErrorTS(t, "let Foo = <>() => {}", "<stdin>: ERROR: Unexpected \">\"\n")
+	expectParseErrorTSX(t, "let Foo = <>() => {}",
+		"<stdin>: ERROR: The character \">\" is not valid inside a JSX element\nNOTE: Did you mean to escape it as \"{'>'}\" instead?\n"+
+			"<stdin>: ERROR: Unexpected end of file before a closing fragment tag\n<stdin>: NOTE: The opening fragment tag is here:\n")
+
 	// Certain built-in types do not accept type parameters
 	expectPrintedTS(t, "x as 1 < 1", "x < 1;\n")
 	expectPrintedTS(t, "x as 1n < 1", "x < 1;\n")