fix(isolated_declarations): Support expando functions (#4910)

I mentioned this issue in a
https://github.com/oxc-project/oxc/issues/4607#issuecomment-2264863618
but I later realized this is actually a separate issue from the one that
I originally reported.

It seems that https://github.com/oxc-project/oxc/pull/3872 added support
for reporting expando function errors, but didn't add support for cases
where expando functions are allowed. This PR adds support for not
reporting errors when there is a namespace declaration that declares the
variable being assigned to.

[TypeScript
playground](https://www.typescriptlang.org/play/?noCheck=true&isolatedDeclarations=true&ts=5.5.4#code/KYDwDg9gTgLgBAMwK4DsDGMCWEWIhACgEoAuOANwkwBM4BvAXwCgF8A6AQzDABsBPOAF44xIQD56zJqEiw4aHAGd4AIw5QhI0hSq1BExkzVQ2aDjx6bR+yUzszo8FBwC2wRWA5pgcAHIBleiY4OAc5BRRlOABzfCttShpxWxDYiDYeYBRomAALTQBGAAYmKTCnV3dPbzwIIJDM1XVNABYAJgBuYNDwRzhGuDUAL0Kioq6pVnTjVs6WdmHR8bsAehW4VHLgaiYIqKG44WIyRL0DZgP0mAh-GChMbPjkxiA)
for reference

---------

Co-authored-by: MichaelMitchell-at <=>

crates/oxc_isolated_declarations/src/lib.rs

@@ -26,7 +26,7 @@ use oxc_allocator::Allocator;
 use oxc_ast::{ast::*, AstBuilder, Visit};
 use oxc_diagnostics::OxcDiagnostic;
 use oxc_span::{Atom, SourceType, SPAN};
-use rustc_hash::FxHashSet;
+use rustc_hash::{FxHashMap, FxHashSet};
 
 use crate::scope::ScopeTree;
 
@@ -404,7 +404,76 @@ impl<'a> IsolatedDeclarations<'a> {
         })
     }
 
+    fn get_assignable_properties_for_namespaces(
+        stmts: &'a oxc_allocator::Vec<'a, Statement<'a>>,
+    ) -> FxHashMap<&'a str, FxHashSet<Atom>> {
+        let mut assignable_properties_for_namespace = FxHashMap::<&str, FxHashSet<Atom>>::default();
+        for stmt in stmts {
+            let Statement::ExportNamedDeclaration(decl) = stmt else { continue };
+            let Some(Declaration::TSModuleDeclaration(decl)) = &decl.declaration else { continue };
+            if decl.kind != TSModuleDeclarationKind::Namespace {
+                continue;
+            }
+            let TSModuleDeclarationName::Identifier(ident) = &decl.id else { continue };
+            let Some(TSModuleDeclarationBody::TSModuleBlock(block)) = &decl.body else {
+                continue;
+            };
+            for stmt in &block.body {
+                let Statement::ExportNamedDeclaration(decl) = stmt else { continue };
+                match &decl.declaration {
+                    Some(Declaration::VariableDeclaration(var)) => {
+                        for declarator in &var.declarations {
+                            if let Some(name) = declarator.id.get_identifier() {
+                                assignable_properties_for_namespace
+                                    .entry(&ident.name)
+                                    .or_default()
+                                    .insert(name);
+                            }
+                        }
+                    }
+                    Some(Declaration::FunctionDeclaration(func)) => {
+                        if let Some(id) = func.id.as_ref() {
+                            assignable_properties_for_namespace
+                                .entry(&ident.name)
+                                .or_default()
+                                .insert(id.name.clone());
+                        }
+                    }
+                    Some(Declaration::ClassDeclaration(cls)) => {
+                        if let Some(id) = cls.id.as_ref() {
+                            assignable_properties_for_namespace
+                                .entry(&ident.name)
+                                .or_default()
+                                .insert(id.name.clone());
+                        }
+                    }
+                    Some(Declaration::TSEnumDeclaration(decl)) => {
+                        assignable_properties_for_namespace
+                            .entry(&ident.name)
+                            .or_default()
+                            .insert(decl.id.name.clone());
+                    }
+                    Some(Declaration::UsingDeclaration(decl)) => {
+                        for declarator in &decl.declarations {
+                            if let Some(name) = declarator.id.get_identifier() {
+                                assignable_properties_for_namespace
+                                    .entry(&ident.name)
+                                    .or_default()
+                                    .insert(name);
+                            }
+                        }
+                    }
+                    _ => {}
+                }
+            }
+        }
+        assignable_properties_for_namespace
+    }
+
     pub fn report_error_for_expando_function(&self, stmts: &oxc_allocator::Vec<'a, Statement<'a>>) {
+        let assignable_properties_for_namespace =
+            IsolatedDeclarations::get_assignable_properties_for_namespaces(stmts);
+
         let mut can_expando_function_names = FxHashSet::default();
         for stmt in stmts {
             match stmt {
@@ -468,7 +537,13 @@ impl<'a> IsolatedDeclarations<'a> {
                             &assignment.left
                         {
                             if let Expression::Identifier(ident) = &static_member_expr.object {
-                                if can_expando_function_names.contains(&ident.name) {
+                                if can_expando_function_names.contains(&ident.name)
+                                    && !assignable_properties_for_namespace
+                                        .get(&ident.name.as_str())
+                                        .map_or(false, |properties| {
+                                            properties.contains(&static_member_expr.property.name)
+                                        })
+                                {
                                     self.error(function_with_assigning_properties(
                                         static_member_expr.span,
                                     ));

crates/oxc_isolated_declarations/tests/fixtures/expando-function.ts

@@ -10,6 +10,20 @@ export namespace NS {
   goo.length = 10
 }
 
+export namespace foo {
+  // declaration must be exported
+  let bar = 42;
+  export let baz = 100;
+}
+
+// namespace must be exported
+namespace foo {
+  export let bar = 42;
+}
+
+foo.bar = 42;
+foo.baz = 100;
+
 // unexported
 const zoo = (): void => {}
 zoo.toString = () => {}

crates/oxc_isolated_declarations/tests/snapshots/expando-function.snap

@@ -9,6 +9,9 @@ export declare const bar: () => void;
 export declare namespace NS {
 	export const goo: () => void;
 }
+export declare namespace foo {
+	export let baz: number;
+}
 
 
 ==================== Errors ====================
@@ -42,3 +45,13 @@ export declare namespace NS {
    : ^^^^^^^^
  6 | 
    `----
+
+  x TS9023: Assigning properties to functions without declaring them is not
+  | supported with --isolatedDeclarations. Add an explicit declaration for the
+  | properties assigned to this function.
+    ,-[24:1]
+ 23 | 
+ 24 | foo.bar = 42;
+    : ^^^^^^^
+ 25 | foo.baz = 100;
+    `----