fix corner cases in collapse_vars & dead_code (#4052)

fixes #4051

lib/compress.js

@@ -1834,7 +1834,9 @@ merge(Compressor.prototype, {
                 if (parent instanceof AST_SimpleStatement) return find_stop_unused(parent, level + 1);
                 if (parent instanceof AST_Switch) return find_stop_unused(parent, level + 1);
                 if (parent instanceof AST_Unary) return find_stop_unused(parent, level + 1);
-                if (parent instanceof AST_VarDef) return find_stop_unused(parent, level + 1);
+                if (parent instanceof AST_VarDef) {
+                    return may_throw(parent) ? node : find_stop_unused(parent, level + 1);
+                }
                 return null;
             }
 
@@ -7920,12 +7922,17 @@ merge(Compressor.prototype, {
                     }
                 }
             } else if (self.left instanceof AST_SymbolRef) {
-                if (self.operator == "=" && self.left.equivalent_to(self.right)) return self.right;
+                var parent;
+                if (self.operator == "=" && self.left.equivalent_to(self.right)
+                    && !((parent = compressor.parent()) instanceof AST_UnaryPrefix && parent.operator == "delete")) {
+                    return self.right;
+                }
                 if (self.left.is_immutable()) return strip_assignment();
                 var def = self.left.definition();
                 var scope = def.scope.resolve();
                 var local = scope === compressor.find_parent(AST_Lambda);
-                var level = 0, node, parent = self;
+                var level = 0, node;
+                parent = self;
                 do {
                     node = parent;
                     parent = compressor.parent(level++);

test/compress/collapse_vars.js

@@ -8447,3 +8447,22 @@ issue_4047_2: {
         "NaN",
     ]
 }
+
+issue_4051: {
+    options = {
+        collapse_vars: true,
+    }
+    input: {
+        try {
+            var a = (b = b.p, "FAIL"), b = b;
+        } catch (e) {}
+        console.log(a);
+    }
+    expect: {
+        try {
+            var a = (b = b.p, "FAIL"), b = b;
+        } catch (e) {}
+        console.log(a);
+    }
+    expect_stdout: "undefined"
+}

test/compress/dead-code.js

@@ -1341,3 +1341,24 @@ issue_3967: {
     }
     expect_stdout: "PASS"
 }
+
+issue_4051: {
+    options = {
+        dead_code: true,
+    }
+    input: {
+        try {
+            delete (A = A);
+        } catch (e) {
+            console.log("PASS");
+        }
+    }
+    expect: {
+        try {
+            delete (A = A);
+        } catch (e) {
+            console.log("PASS");
+        }
+    }
+    expect_stdout: "PASS"
+}