fix corner case in collapse_vars (#4909)

fixes #4908

lib/compress.js

@@ -1911,6 +1911,7 @@ merge(Compressor.prototype, {
                             right: ref,
                         });
                     }
+                    ref.fixed = false;
                     def.replaced++;
                     return ref;
                 }
@@ -1991,6 +1992,7 @@ merge(Compressor.prototype, {
                             && !compressor.exposed(def);
                         value_def.last_ref = false;
                         value_def.single_use = false;
+                        CHANGED = true;
                     }
                     if (replaced && !remove_candidate(candidate)) statements.splice(stat_index, 1);
                 }

test/compress/collapse_vars.js

@@ -9049,7 +9049,7 @@ issue_4874: {
     }
     expect: {
         var a;
-        null;
+        a = null;
         (function(b) {
             for (var c in a && a[console.log("PASS")])
                 console;
@@ -9107,3 +9107,26 @@ issue_4895: {
     }
     expect_stdout: "42 42"
 }
+
+issue_4908: {
+    options = {
+        collapse_vars: true,
+        join_vars: true,
+        reduce_vars: true,
+        toplevel: true,
+    }
+    input: {
+        var a = 0;
+        var b;
+        console || a++;
+        var c = d = a, d = [ c && c, d += 42 ];
+        console.log(d[1]);
+    }
+    expect: {
+        var a = 0, b;
+        console || a++;
+        var c = d = a, d = [ d && d, d += 42 ];
+        console.log(d[1]);
+    }
+    expect_stdout: "42"
+}

test/compress/functions.js

@@ -6107,7 +6107,7 @@ reduce_cross_reference_2: {
 reduce_cross_reference_2_toplevel: {
     options = {
         collapse_vars: true,
-        passes: 2,
+        passes: 3,
         pure_getters: "strict",
         reduce_vars: true,
         sequences: true,
@@ -6150,7 +6150,7 @@ reduce_cross_reference_3: {
 reduce_cross_reference_3_toplevel: {
     options = {
         collapse_vars: true,
-        passes: 2,
+        passes: 3,
         pure_getters: "strict",
         reduce_vars: true,
         sequences: true,