fix corner cases in aliasing of global variables (#4039)

fixes #4038

test/compress/collapse_vars.js

@@ -8317,3 +8317,61 @@ issue_4012: {
     }
     expect_stdout: "PASS"
 }
+
+global_assign: {
+    options = {
+        collapse_vars: true,
+    }
+    input: {
+        this.A = "FAIL";
+        A = "PASS";
+        B = "FAIL";
+        console.log(A);
+    }
+    expect: {
+        this.A = "FAIL";
+        A = "PASS";
+        B = "FAIL";
+        console.log(A);
+    }
+    expect_stdout: "PASS"
+}
+
+global_read: {
+    options = {
+        collapse_vars: true,
+    }
+    input: {
+        var a = 0;
+        a = this.A;
+        A = 1;
+        a ? console.log("FAIL") : console.log("PASS");
+    }
+    expect: {
+        var a = 0;
+        a = this.A;
+        A = 1;
+        a ? console.log("FAIL") : console.log("PASS");
+    }
+    expect_stdout: "PASS"
+}
+
+issue_4038: {
+    options = {
+        collapse_vars: true,
+    }
+    input: {
+        var a = 0;
+        a = this;
+        a = a.A;
+        A = 1;
+        a ? console.log("FAIL") : console.log("PASS");
+    }
+    expect: {
+        var a = 0;
+        a = (a = this).A;
+        A = 1;
+        a ? console.log("FAIL") : console.log("PASS");
+    }
+    expect_stdout: "PASS"
+}

test/compress/reduce_vars.js

@@ -7402,7 +7402,27 @@ issue_4030: {
     }
     expect: {
         A = "PASS";
-        console.log("PASS");
+        console.log(A);
+    }
+    expect_stdout: "PASS"
+}
+
+global_assign: {
+    options = {
+        evaluate: true,
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        A = "FAIL";
+        this.A = "PASS";
+        console.log(A);
+    }
+    expect: {
+        A = "FAIL";
+        this.A = "PASS";
+        console.log(A);
     }
     expect_stdout: "PASS"
 }

test/ufuzz/index.js

@@ -1175,7 +1175,8 @@ function log(options) {
 }
 
 function sort_globals(code) {
-    return "var " + sandbox.run_code("throw Object.keys(this).sort();" + code).join(",") + ";" + code;
+    var globals = sandbox.run_code("throw Object.keys(this).sort();" + code);
+    return globals.length ? "var " + globals.join(",") + ";" + code : code;
 }
 
 function fuzzy_match(original, uglified) {