fix #3574: ts type parser bug with infer + extends

CHANGELOG.md

@@ -1,5 +1,15 @@
 # Changelog
 
+## Unreleased
+
+* Fix a bug with TypeScript type parsing ([#3574](https://github.com/evanw/esbuild/issues/3574))
+
+    This release fixes a bug with esbuild's TypeScript parser where a conditional type containing a union type that ends with an infer type that ends with a constraint could fail to parse. This was caused by the "don't parse a conditional type" flag not getting passed through the union type parser. Here's an example of valid TypeScript code that previously failed to parse correctly:
+
+    ```ts
+    type InferUnion<T> = T extends { a: infer U extends number } | infer U extends number ? U : never
+    ```
+
 ## 0.19.11
 
 * Fix TypeScript-specific class transform edge case ([#3559](https://github.com/evanw/esbuild/issues/3559))

internal/js_parser/ts_parser.go

@@ -482,14 +482,14 @@ loop:
 				return
 			}
 			p.lexer.Next()
-			p.skipTypeScriptType(js_ast.LBitwiseOr)
+			p.skipTypeScriptTypeWithFlags(js_ast.LBitwiseOr, flags)
 
 		case js_lexer.TAmpersand:
 			if level >= js_ast.LBitwiseAnd {
 				return
 			}
 			p.lexer.Next()
-			p.skipTypeScriptType(js_ast.LBitwiseAnd)
+			p.skipTypeScriptTypeWithFlags(js_ast.LBitwiseAnd, flags)
 
 		case js_lexer.TExclamation:
 			// A postfix "!" is allowed in JSDoc types in TypeScript, which are only

internal/js_parser/ts_parser_test.go

@@ -279,6 +279,10 @@ func TestTSTypes(t *testing.T) {
 	expectPrintedTS(t, "type Foo = Bar extends [infer T extends string] ? T : null", "")
 	expectPrintedTS(t, "type Foo = {} extends infer T extends {} ? A<T> : never", "")
 	expectPrintedTS(t, "type Foo = {} extends (infer T extends {}) ? A<T> : never", "")
+	expectPrintedTS(t, "type Foo<T> = T extends { a: infer U extends number } | { b: infer U extends number } ? U : never", "")
+	expectPrintedTS(t, "type Foo<T> = T extends { a: infer U extends number } & { b: infer U extends number } ? U : never", "")
+	expectPrintedTS(t, "type Foo<T> = T extends { a: infer U extends number } | infer U extends number ? U : never", "")
+	expectPrintedTS(t, "type Foo<T> = T extends { a: infer U extends number } & infer U extends number ? U : never", "")
 	expectPrintedTS(t, "let x: A extends B<infer C extends D> ? D : never", "let x;\n")
 	expectPrintedTS(t, "let x: A extends B<infer C extends D ? infer C : never> ? D : never", "let x;\n")
 	expectPrintedTS(t, "let x: ([e1, e2, ...es]: any) => any", "let x;\n")