fix corner case in reduce_vars (#3667)

fixes #3666

lib/compress.js

@@ -6797,7 +6797,8 @@ 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 (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;
@@ -6813,8 +6814,11 @@ merge(Compressor.prototype, {
                         }
                     }
                     if (single_use) fixed.parent_scope = self.scope;
+                } else if (!fixed || !fixed.is_constant_expression()) {
+                    single_use = false;
                 }
-            if (single_use && fixed) {
+            }
+            if (single_use) {
                 def.single_use = false;
                 fixed._squeezed = true;
                 fixed.single_use = true;

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"
+}