fix corner case in `reduce_vars` (#3667)

fixes #3666

2 files changed, 50 insertions, 15 deletions

diff --git a/lib/compress.js b/lib/compress.js
index f3fe8a8e..fcfe95f7 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -6797,24 +6797,28 @@ merge(Compressor.prototype, {
             var def = self.definition();
             var fixed = self.fixed_value();
             var single_use = def.single_use && !(parent instanceof AST_Call && parent.is_expr_pure(compressor));
-            if (single_use && fixed instanceof AST_Lambda) {
-                if (def.scope !== self.scope
-                    && (!compressor.option("reduce_funcs") || def.escaped.depth == 1 || fixed.inlined)) {
-                    single_use = false;
-                } else if (recursive_ref(compressor, def)) {
-                    single_use = false;
-                } else if (def.scope !== self.scope || def.orig[0] instanceof AST_SymbolFunarg) {
-                    single_use = fixed.is_constant_expression(self.scope);
-                    if (single_use == "f") {
-                        var scope = self.scope;
-                        do if (scope instanceof AST_Defun || scope instanceof AST_Function) {
-                            scope.inlined = true;
-                        } while (scope = scope.parent_scope);
+            if (single_use) {
+                if (fixed instanceof AST_Lambda) {
+                    if (def.scope !== self.scope
+                        && (!compressor.option("reduce_funcs") || def.escaped.depth == 1 || fixed.inlined)) {
+                        single_use = false;
+                    } else if (recursive_ref(compressor, def)) {
+                        single_use = false;
+                    } else if (def.scope !== self.scope || def.orig[0] instanceof AST_SymbolFunarg) {
+                        single_use = fixed.is_constant_expression(self.scope);
+                        if (single_use == "f") {
+                            var scope = self.scope;
+                            do if (scope instanceof AST_Defun || scope instanceof AST_Function) {
+                                scope.inlined = true;
+                            } while (scope = scope.parent_scope);
+                        }
                     }
+                    if (single_use) fixed.parent_scope = self.scope;
+                } else if (!fixed || !fixed.is_constant_expression()) {
+                    single_use = false;
                 }
-                if (single_use) fixed.parent_scope = self.scope;
             }
-            if (single_use && fixed) {
+            if (single_use) {
                 def.single_use = false;
                 fixed._squeezed = true;
                 fixed.single_use = true;
diff --git a/test/compress/reduce_vars.js b/test/compress/reduce_vars.js
index 5984042f..3331732a 100644
--- a/test/compress/reduce_vars.js
+++ b/test/compress/reduce_vars.js
@@ -6847,3 +6847,34 @@ issue_3631_2: {
     }
     expect_stdout: "undefined"
 }
+
+issue_3666: {
+    options = {
+        collapse_vars: true,
+        passes: 2,
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        try {
+            var a = "FAIL";
+        } finally {
+            for (;!a;)
+                var c = a++;
+            var a = "PASS", b = c = "PASS";
+        }
+        console.log(a, b);
+    }
+    expect: {
+        try {
+            var a = "FAIL";
+        } finally {
+            for (;!a;)
+                a++;
+            var b = a = "PASS";
+        }
+        console.log(a, b);
+    }
+    expect_stdout: "PASS PASS"
+}