fix(isolated-declarations): inferring an incorrect return type when there is an arrow function inside a function

crates/oxc_isolated_declarations/src/return_type.rs

@@ -1,7 +1,7 @@
 use oxc_ast::{
     ast::{
-        BindingIdentifier, Expression, Function, FunctionBody, ReturnStatement, TSType,
-        TSTypeAliasDeclaration, TSTypeName, TSTypeQueryExprName,
+        ArrowFunctionExpression, BindingIdentifier, Expression, Function, FunctionBody,
+        ReturnStatement, TSType, TSTypeAliasDeclaration, TSTypeName, TSTypeQueryExprName,
     },
     AstBuilder, Visit,
 };
@@ -138,6 +138,10 @@ impl<'a> Visit<'a> for FunctionReturnType<'a> {
         // We don't care about nested functions
     }
 
+    fn visit_arrow_expression(&mut self, _expr: &ArrowFunctionExpression<'a>) {
+        // We don't care about nested functions
+    }
+
     fn visit_return_statement(&mut self, stmt: &ReturnStatement<'a>) {
         self.return_statement_count += 1;
         if self.return_statement_count > 1 {

crates/oxc_isolated_declarations/tests/fixtures/infer-return-type.ts

@@ -4,17 +4,24 @@ function foo() {
 // inferred type is number
 
 function bar() {
-  if (true) {
+  if (a) {
    return;
   }
   return 1;
 }
 // inferred type is number | undefined
 
 function baz() {
- if (true) {
+ if (a) {
   return null;
  }
  return 1;
 }
-// We can't infer return type if there are multiple return statements with different types
+// We can't infer return type if there are multiple return statements with different types
+
+function qux() {
+  const a = (() => {
+    return 1;
+  })();
+ return `Hello, world!`;
+}

crates/oxc_isolated_declarations/tests/snapshots/infer-return-type.snap

@@ -7,6 +7,7 @@ input_file: crates/oxc_isolated_declarations/tests/fixtures/infer-return-type.ts
 declare function foo(): number;
 declare function bar(): ((number) | (undefined));
 declare function baz();
+declare function qux(): string;
 
 
 ==================== Errors ====================
@@ -17,5 +18,5 @@ declare function baz();
  13 | 
  14 | function baz() {
     :          ^^^
- 15 |  if (true) {
+ 15 |  if (a) {
     `----