enhance collapse_vars (#5312)

lib/compress.js

@@ -2058,6 +2058,11 @@ Compressor.prototype.compress = function(node) {
                     assign.right = rvalue;
                     return assign;
                 }
+                // Stop signals related to AST_SymbolRef
+                if (should_stop_ref(node, parent)) {
+                    abort = true;
+                    return node;
+                }
                 // These node types have child nodes that execute sequentially,
                 // but are otherwise not safe to scan into or beyond them.
                 if (is_last_node(node, parent) || may_throw(node)) {
@@ -2184,7 +2189,8 @@ Compressor.prototype.compress = function(node) {
                     var stop_if_hit = null;
                     var lhs = get_lhs(candidate);
                     var side_effects = lhs && lhs.has_side_effects(compressor);
-                    var scan_lhs = lhs && !side_effects && !is_lhs_read_only(lhs, compressor);
+                    var scan_lhs = lhs && (!side_effects || lhs instanceof AST_SymbolRef)
+                            && !is_lhs_read_only(lhs, compressor);
                     var scan_rhs = foldable(candidate);
                     if (!scan_lhs && !scan_rhs) continue;
                     var compound = candidate instanceof AST_Assign && candidate.operator.slice(0, -1);
@@ -2318,7 +2324,13 @@ Compressor.prototype.compress = function(node) {
                 if (node instanceof AST_DestructuredKeyVal) return node.key instanceof AST_Node;
                 if (node instanceof AST_DWLoop) return true;
                 if (node instanceof AST_LoopControl) return true;
-                if (node instanceof AST_SymbolRef) {
+                if (node instanceof AST_Try) return true;
+                if (node instanceof AST_With) return true;
+                return false;
+            }
+
+            function should_stop_ref(node, parent) {
+                if (!(node instanceof AST_SymbolRef)) return false;
                 if (node.is_declared(compressor)) {
                     if (node.fixed_value()) return false;
                     if (can_drop_symbol(node)) {
@@ -2332,10 +2344,6 @@ Compressor.prototype.compress = function(node) {
                 scan_rhs = false;
                 return false;
             }
-                if (node instanceof AST_Try) return true;
-                if (node instanceof AST_With) return true;
-                return false;
-            }
 
             function in_conditional(node, parent) {
                 if (parent instanceof AST_Assign) return parent.left !== node && lazy_op[parent.operator.slice(0, -1)];

test/compress/assignments.js

@@ -673,8 +673,7 @@ issue_4827_1: {
         c &&= b = a, console.log(b);
     }
     expect: {
-        A = "FAIL";
+        var a = A = "FAIL", b = "PASS", c;
-        var a = A, b = "PASS", c;
         c &&= b = a, console.log(b);
     }
     expect_stdout: "PASS"

test/compress/collapse_vars.js

@@ -2579,7 +2579,7 @@ side_effects_property: {
     expect_stdout: true
 }
 
-undeclared: {
+undeclared_1: {
     options = {
         collapse_vars: true,
         unused: true,
@@ -2594,8 +2594,68 @@ undeclared: {
     }
     expect: {
         function f(x, y) {
+            return (b = y) + x;
+        }
+    }
+}
+
+undeclared_2: {
+    options = {
+        collapse_vars: true,
+        unused: true,
+    }
+    input: {
+        function f(x, y) {
+            var a;
+            a = x;
             b = y;
-            return b + x;
+            return a + b;
+        }
+    }
+    expect: {
+        function f(x, y) {
+            return x + (b = y);
+        }
+    }
+}
+
+undeclared_3: {
+    options = {
+        collapse_vars: true,
+        unused: true,
+    }
+    input: {
+        function f(x, y) {
+            var a;
+            a = x;
+            b = y;
+            return b + a();
+        }
+    }
+    expect: {
+        function f(x, y) {
+            return (b = y) + x();
+        }
+    }
+}
+
+undeclared_4: {
+    options = {
+        collapse_vars: true,
+        unused: true,
+    }
+    input: {
+        function f(x, y) {
+            var a;
+            a = x;
+            b = y;
+            return a() + b;
+        }
+    }
+    expect: {
+        function f(x, y) {
+            b = y;
+            return x() + b;
         }
     }
 }
@@ -2987,9 +3047,8 @@ compound_assignment_4: {
         console.log(a);
     }
     expect: {
-        A = "PASS";
         var a = "";
-        (a += (a = "FAIL", A)).p;
+        (a += (a = "FAIL", A = "PASS")).p;
         console.log(a);
     }
     expect_stdout: "PASS"

test/compress/hoist_vars.js

@@ -250,8 +250,7 @@ issue_4517: {
     expect: {
         console.log(function() {
             var a = 2;
-            A = a;
+            return (A = a) + typeof !1;
-            return A + typeof !1;
         }());
     }
     expect_stdout: "2boolean"