fix corner case in collapse_vars (#5780)

fixes #5779

lib/compress.js

@@ -1761,6 +1761,11 @@ Compressor.prototype.compress = function(node) {
 
     var identifier_atom = makePredicate("Infinity NaN undefined");
     function is_lhs_read_only(lhs, compressor) {
+        if (lhs instanceof AST_Assign) {
+            if (lhs.operator != "=") return true;
+            if (lhs.right.tail_node().is_constant()) return true;
+            return is_lhs_read_only(lhs.left, compressor);
+        }
         if (lhs instanceof AST_Atom) return true;
         if (lhs instanceof AST_ObjectIdentity) return true;
         if (lhs instanceof AST_PropAccess) {

test/compress/collapse_vars.js

@@ -9864,7 +9864,8 @@ issue_5276: {
     }
     expect: {
         var a = A = "PASS";
-        a.p = a.p + null - 42;
+        a.p += null;
+        a.p -= 42;
         console.log(a);
     }
     expect_stdout: "PASS"
@@ -10148,3 +10149,26 @@ issue_5719: {
     }
     expect_stdout: "PASS"
 }
+
+issue_5779: {
+    options = {
+        collapse_vars: true,
+        evaluate: true,
+        pure_getters: "strict",
+        reduce_vars: true,
+        toplevel: true,
+    }
+    input: {
+        var a = A = "foo";
+        a.p = 42;
+        if (a && !a.p)
+            console.log("PASS");
+    }
+    expect: {
+        var a = A = "foo";
+        a.p = 42;
+        if (a, !a.p)
+            console.log("PASS");
+    }
+    expect_stdout: "PASS"
+}