fix corner case in collapse_vars (#4360)

fixes #4359

lib/compress.js

@@ -1614,6 +1614,14 @@ merge(Compressor.prototype, {
                     var scan_lhs = lhs && !side_effects && !is_lhs_read_only(lhs, compressor);
                     var scan_rhs = foldable(candidate);
                     if (!scan_lhs && !scan_rhs) continue;
+                    var funarg = candidate.name instanceof AST_SymbolFunarg;
+                    var may_throw = return_false;
+                    if (candidate.may_throw(compressor)) {
+                        if (funarg && scope instanceof AST_AsyncFunction) continue;
+                        may_throw = in_try ? function(node) {
+                            return node.has_side_effects(compressor);
+                        } : side_effects_external;
+                    }
                     var read_toplevel = false;
                     var modify_toplevel = false;
                     // Locate symbols which may execute code outside of scanning range
@@ -1622,10 +1630,6 @@ merge(Compressor.prototype, {
                     var rvalue = get_rvalue(candidate);
                     if (!side_effects) side_effects = value_has_side_effects();
                     var replace_all = replace_all_symbols(candidate);
-                    var may_throw = candidate.may_throw(compressor) ? in_try ? function(node) {
-                        return node.has_side_effects(compressor);
-                    } : side_effects_external : return_false;
-                    var funarg = candidate.name instanceof AST_SymbolFunarg;
                     var hit = funarg;
                     var abort = false;
                     var replaced = 0;

test/compress/async.js

@@ -444,3 +444,30 @@ issue_4349_3: {
     expect_stdout: "function"
     node_version: ">=8"
 }
+
+issue_4359: {
+    options = {
+        collapse_vars: true,
+        unused: true,
+    }
+    input: {
+        try {
+            (async function(a) {
+                return a;
+            })(A);
+        } catch (e) {
+            console.log("PASS");
+        }
+    }
+    expect: {
+        try {
+            (async function(a) {
+                return a;
+            })(A);
+        } catch (e) {
+            console.log("PASS");
+        }
+    }
+    expect_stdout: "PASS"
+    node_version: ">=8"
+}