fix corner case in if_return (#5598)

fixes #5597

lib/compress.js

@@ -3452,16 +3452,15 @@ Compressor.prototype.compress = function(node) {
             var parent = compressor.parent();
             var self = compressor.self();
             var declare_only, jump, merge_jump;
-            var in_tail = in_lambda && self instanceof AST_Block && self.body === statements;
+            var in_iife = in_lambda && parent && parent.TYPE == "Call" && parent.expression === self;
-            var in_iife = in_tail && parent && parent.TYPE == "Call" && parent.expression === self;
+            var chain_if_returns = in_lambda && compressor.option("conditionals") && compressor.option("sequences");
-            var chain_if_returns = in_tail && compressor.option("conditionals") && compressor.option("sequences");
             var multiple_if_returns = has_multiple_if_returns(statements);
             for (var i = statements.length; --i >= 0;) {
                 var stat = statements[i];
                 var j = next_index(i);
                 var next = statements[j];
 
-                if (in_tail && declare_only && !next && stat instanceof AST_Return
+                if (in_lambda && declare_only && !next && stat instanceof AST_Return
                     && !(self instanceof AST_SwitchBranch)
                     && !(in_try && in_try.bfinally && in_async_generator(in_lambda))) {
                     var body = stat.value;
@@ -3486,6 +3485,7 @@ Compressor.prototype.compress = function(node) {
 
                 if (stat instanceof AST_If) {
                     var ab = aborts(stat.body);
+                    // if (foo()) { bar(); return; } else baz(); moo(); ---> if (foo()) bar(); else { baz(); moo(); }
                     if (can_merge_flow(ab)) {
                         if (ab.label) remove(ab.label.thedef.references, ab);
                         changed = true;
@@ -3500,7 +3500,7 @@ Compressor.prototype.compress = function(node) {
                         statements[i] = stat.transform(compressor);
                         continue;
                     }
-
+                    // if (foo()) { bar(); return x; } return y; ---> if (!foo()) return y; bar(); return x;
                     if (ab && !stat.alternative && next instanceof AST_Jump) {
                         var cond = stat.condition;
                         var preference = i + 1 == j && stat.body instanceof AST_BlockStatement;
@@ -3514,12 +3514,17 @@ Compressor.prototype.compress = function(node) {
                                 body: extract_functions(true, null, j + 1),
                             });
                             statements.splice(i, 1, stat, body);
-                            statements[i] = stat.transform(compressor);
+                            // proceed further only if `TreeWalker.stack` is in a consistent state
+                            //    https://github.com/mishoo/UglifyJS/issues/5595
+                            //    https://github.com/mishoo/UglifyJS/issues/5597
+                            if (!in_lambda || self instanceof AST_Block && self.body === statements) {
+                                statements[i] = stat.transform(compressor);
+                            }
                             continue;
                         }
                     }
-
                     var alt = aborts(stat.alternative);
+                    // if (foo()) bar(); else { baz(); return; } moo(); ---> if (foo()) { bar(); moo(); } else baz();
                     if (can_merge_flow(alt)) {
                         if (alt.label) remove(alt.label.thedef.references, alt);
                         changed = true;
@@ -3534,7 +3539,6 @@ Compressor.prototype.compress = function(node) {
                         statements[i] = stat.transform(compressor);
                         continue;
                     }
-
                     if (compressor.option("typeofs")) {
                         if (ab && !alt) {
                             var stats = make_node(AST_BlockStatement, self, { body: statements.slice(i + 1) });
@@ -3564,7 +3568,7 @@ Compressor.prototype.compress = function(node) {
                     // if (foo()) return x; [ return ; ] ---> return foo() ? x : undefined;
                     // if (foo()) return bar() ? x : void 0; ---> return foo() && bar() ? x : void 0;
                     // if (foo()) return bar() ? void 0 : x; ---> return !foo() || bar() ? void 0 : x;
-                    if (in_tail && declare_only && !next && !stat.alternative && (in_bool
+                    if (in_lambda && declare_only && !next && !stat.alternative && (in_bool
                         || value && multiple_if_returns
                         || value instanceof AST_Conditional && (is_undefined(value.consequent, compressor)
                             || is_undefined(value.alternative, compressor)))) {
@@ -3645,7 +3649,7 @@ Compressor.prototype.compress = function(node) {
             function can_drop_abort(ab) {
                 if (ab instanceof AST_Exit) {
                     if (merge_jump = match_return(ab)) return true;
-                    if (!in_tail) return false;
+                    if (!in_lambda) return false;
                     if (!(ab instanceof AST_Return)) return false;
                     var value = ab.value;
                     if (value && !is_undefined(value.tail_node())) return false;
@@ -3658,7 +3662,7 @@ Compressor.prototype.compress = function(node) {
                 if (!(ab instanceof AST_LoopControl)) return false;
                 if (jump && self instanceof AST_SwitchBranch) {
                     if (jump instanceof AST_Exit) {
-                        if (!in_tail) return false;
+                        if (!in_lambda) return false;
                         if (jump.value) return false;
                     } else if (compressor.loopcontrol_target(jump) !== parent) {
                         return false;

test/compress/if_return.js

@@ -2279,3 +2279,29 @@ issue_5595: {
     }
     expect_stdout: "PASS"
 }
+
+issue_5597: {
+    options = {
+        conditionals: true,
+        if_return: true,
+        unused: true,
+    }
+    input: {
+        function f(a) {
+            if (a) L: {
+                return;
+                var b;
+            } else
+                return "FAIL";
+        }
+        console.log(f(42) || "PASS");
+    }
+    expect: {
+        function f(a) {
+            if (!a)
+                return "FAIL";
+        }
+        console.log(f(42) || "PASS");
+    }
+    expect_stdout: "PASS"
+}