fix(es/typescript): Handle ASI hazards in fast type strip (#9707)

**Related issue:**

- Closes: #9681

Author: magic-akari

.changeset/neat-cheetahs-suffer.md

@@ -0,0 +1,6 @@
+---
+swc_fast_ts_strip: patch
+swc_core: patch
+---
+
+fix(es/typescript): Handle ASI hazards in fast type strip

crates/swc_fast_ts_strip/src/lib.rs

@@ -561,6 +561,17 @@ impl Visit for TsStrip {
         }
 
         'type_params: {
+            // ```TypeScript
+            // let f = async <
+            //    T
+            // >(v: T) => v;
+            // ```
+
+            // ```TypeScript
+            // let f = async (
+            //
+            //   v   ) => v;
+            // ```
             if let Some(tp) = &n.type_params {
                 self.add_replacement(tp.span);
 
@@ -673,7 +684,12 @@ impl Visit for TsStrip {
             return;
         }
 
-        self.strip_class_modifier(n.span.lo, n.key.span_lo());
+        // TODO(AST): constructor can not be optional
+        debug_assert!(!n.is_optional);
+
+        if n.accessibility.is_some() {
+            self.strip_class_modifier(n.span.lo, n.key.span_lo());
+        }
 
         n.visit_children_with(self);
     }
@@ -684,20 +700,49 @@ impl Visit for TsStrip {
             return;
         }
 
+        let has_modifier = n.is_override || n.accessibility.is_some();
+
         // @foo public m(): void {}
         let start_pos = n
             .function
             .decorators
             .last()
             .map_or(n.span.lo, |d| d.span.hi);
 
-        self.strip_class_modifier(start_pos, n.key.span_lo());
+        if has_modifier {
+            self.strip_class_modifier(start_pos, n.key.span_lo());
+        }
 
         if n.is_optional {
             let mark_index = self.get_next_token_index(n.key.span_hi());
             self.strip_optional_mark(mark_index);
         }
 
+        // It's dangerous to strip TypeScript modifiers if the key is computed, a
+        // generator, or `in`/`instanceof` keyword. However, it is safe to do so
+        // if the key is preceded by a `static` keyword or decorators.
+        //
+        // `public [foo]()`
+        // `;      [foo]()`
+        //
+        // `public *foo()`
+        // `;      *foo()`
+        //
+        // `public in()`
+        // `;      in()`
+        if has_modifier
+            && !n.is_static
+            && n.function.decorators.is_empty()
+            && (n.key.is_computed()
+                || n.function.is_generator
+                || n.key
+                    .as_ident()
+                    .filter(|k| matches!(k.sym.as_ref(), "in" | "instanceof"))
+                    .is_some())
+        {
+            self.add_overwrite(start_pos, b';');
+        }
+
         n.visit_children_with(self);
     }
 
@@ -707,9 +752,12 @@ impl Visit for TsStrip {
             return;
         }
 
+        let has_modifier = n.readonly || n.is_override || n.accessibility.is_some();
         let start_pos = n.decorators.last().map_or(n.span.lo, |d| d.span.hi);
 
-        self.strip_class_modifier(start_pos, n.key.span_lo());
+        if has_modifier {
+            self.strip_class_modifier(start_pos, n.key.span_lo());
+        }
 
         if n.is_optional {
             let mark_index = self.get_next_token_index(n.key.span_hi());
@@ -720,23 +768,53 @@ impl Visit for TsStrip {
             self.strip_definite_mark(mark_index);
         }
 
-        if n.value.is_none() && n.key.as_ident().filter(|k| k.sym == "static").is_some() {
-            if let Some(type_ann) = &n.type_ann {
-                self.add_overwrite(type_ann.span.lo, b';');
+        // It's dangerous to strip types if the key is `get`, `set`, or `static`.
+        if n.value.is_none() {
+            if let Some(key) = n.key.as_ident() {
+                if matches!(key.sym.as_ref(), "get" | "set" | "static") {
+                    // `get: number`
+                    // `get;       `
+                    if let Some(type_ann) = &n.type_ann {
+                        self.add_overwrite(type_ann.span.lo, b';');
+                    }
+                }
             }
         }
 
+        // `private [foo]`
+        // `;       [foo]`
+        //
+        // `private in`
+        // `;       in`
+        if !n.is_static
+            && has_modifier
+            && n.decorators.is_empty()
+            && (n.key.is_computed()
+                || n.key
+                    .as_ident()
+                    .filter(|k| matches!(k.sym.as_ref(), "in" | "instanceof"))
+                    .is_some())
+        {
+            self.add_overwrite(start_pos, b';');
+        }
+
         n.visit_children_with(self);
     }
 
     fn visit_private_method(&mut self, n: &PrivateMethod) {
-        let start_pos = n
-            .function
-            .decorators
-            .last()
-            .map_or(n.span.lo, |d| d.span.hi);
-
-        self.strip_class_modifier(start_pos, n.key.span.lo);
+        debug_assert!(!n.is_override);
+        debug_assert!(!n.is_abstract);
+
+        // Is `private #foo()` valid?
+        if n.accessibility.is_some() {
+            let start_pos = n
+                .function
+                .decorators
+                .last()
+                .map_or(n.span.lo, |d| d.span.hi);
+
+            self.strip_class_modifier(start_pos, n.key.span.lo);
+        }
 
         if n.is_optional {
             let mark_index = self.get_next_token_index(n.key.span.hi);
@@ -747,9 +825,12 @@ impl Visit for TsStrip {
     }
 
     fn visit_private_prop(&mut self, n: &PrivateProp) {
-        let start_pos = n.decorators.last().map_or(n.span.lo, |d| d.span.hi);
+        debug_assert!(!n.is_override);
 
-        self.strip_class_modifier(start_pos, n.key.span.lo);
+        if n.readonly || n.accessibility.is_some() {
+            let start_pos = n.decorators.last().map_or(n.span.lo, |d| d.span.hi);
+            self.strip_class_modifier(start_pos, n.key.span.lo);
+        }
 
         if n.is_optional {
             let mark_index = self.get_next_token_index(n.key.span.hi);

crates/swc_fast_ts_strip/tests/fixture/issue-9681.js

@@ -0,0 +1,40 @@
+class A {
+    foo = 1
+    ;       ["bar"] = 2;
+
+    x = 1
+    ;      ["y"](){}
+
+    z = 1
+           static ["w"](){}
+}
+
+class Test {
+    x = 1
+    ;      *f() {}
+}
+
+class Test2 {
+    out = 'out'
+    ;      in
+           name
+}
+
+class Test3 {
+    out = 'out'
+    ;      in     
+    name;
+
+    x = 'x'
+    ;      instanceof     
+    String     
+}
+
+class Test4 {
+    set;    
+    a(x) {}
+    get;    
+    b() {}
+    static;    
+    c(x) {}
+}

crates/swc_fast_ts_strip/tests/fixture/issue-9681.transform.js

@@ -0,0 +1,33 @@
+class A {
+    foo = 1;
+    ["bar"] = 2;
+    x = 1;
+    ["y"]() {}
+    z = 1;
+    static ["w"]() {}
+}
+class Test {
+    x = 1;
+    *f() {}
+}
+class Test2 {
+    out = 'out';
+    in;
+    name;
+}
+class Test3 {
+    out = 'out';
+    in;
+    name;
+    x = 'x';
+    instanceof;
+    String;
+}
+class Test4 {
+    set;
+    a(x) {}
+    get;
+    b() {}
+    static;
+    c(x) {}
+}

crates/swc_fast_ts_strip/tests/fixture/issue-9681.ts

@@ -0,0 +1,40 @@
+class A {
+    foo = 1
+    private ["bar"] = 2;
+
+    x = 1
+    public ["y"](){}
+
+    z = 1
+    public static ["w"](){}
+}
+
+class Test {
+    x = 1
+    public *f() {}
+}
+
+class Test2 {
+    out = 'out'
+    public in
+    public name
+}
+
+class Test3 {
+    out = 'out'
+    public in: any
+    name;
+
+    x = 'x'
+    public instanceof: any
+    String: any
+}
+
+class Test4 {
+    set: any
+    a(x) {}
+    get: any
+    b() {}
+    static: any
+    c(x) {}
+}