fix corner case in merge_vars (#5320)

fixes #5319

lib/compress.js

@@ -5927,6 +5927,16 @@ Compressor.prototype.compress = function(node) {
         var references = Object.create(null);
         var prev = Object.create(null);
         var tw = new TreeWalker(function(node, descend) {
+            if (node.variables) {
+                if (node instanceof AST_BlockStatement) {
+                    var save_scope = segment.scope;
+                    segment.scope = node;
+                    descend();
+                    segment.scope = save_scope;
+                    return true;
+                }
+                segment.scope = node;
+            }
             if (node instanceof AST_Assign) {
                 var lhs = node.left;
                 var rhs = node.right;
@@ -6298,6 +6308,7 @@ Compressor.prototype.compress = function(node) {
                 var refs = references[def.id];
                 if (!refs) return;
                 if (refs.start.block !== seg.block) return references[def.id] = false;
+                sym.scope = seg.scope;
                 refs.push(sym);
                 refs.end = seg;
                 if (def.id in prev) {
@@ -6312,6 +6323,7 @@ Compressor.prototype.compress = function(node) {
                 return references[def.id] = false;
             } else {
                 var refs = declarations.get(def.id) || [];
+                sym.scope = seg.scope;
                 refs.push(sym);
                 references[def.id] = refs;
                 if (!read) {

lib/output.js

@@ -103,6 +103,7 @@ function OutputStream(options) {
 
     function make_indent(value) {
         if (typeof value == "number") return new Array(value + 1).join(" ");
+        if (!value) return "";
         if (!/^\s*$/.test(value)) throw new Error("unsupported indentation: " + JSON.stringify("" + value));
         return value;
     }

test/compress/const.js

@@ -1813,3 +1813,33 @@ issue_5260: {
     ]
     node_version: ">=4"
 }
+
+issue_5319: {
+    options = {
+        collapse_vars: true,
+        merge_vars: true,
+    }
+    input: {
+        (function(a, c) {
+            var b = a, c = b;
+            {
+                const a = c;
+                console.log(c());
+            }
+        })(function() {
+            return "PASS";
+        });
+    }
+    expect: {
+        (function(a, c) {
+            var b = a, c;
+            {
+                const a = c = b;
+                console.log(c());
+            }
+        })(function() {
+            return "PASS";
+        });
+    }
+    expect_stdout: true
+}

test/compress/let.js

@@ -1973,3 +1973,36 @@ issue_5260: {
     ]
     node_version: ">=4"
 }
+
+issue_5319: {
+    options = {
+        collapse_vars: true,
+        merge_vars: true,
+    }
+    input: {
+        "use strict";
+        (function(a, c) {
+            var b = a, c = b;
+            {
+                let a = c;
+                console.log(c());
+            }
+        })(function() {
+            return "PASS";
+        });
+    }
+    expect: {
+        "use strict";
+        (function(a, c) {
+            var b = a, c;
+            {
+                let a = c = b;
+                console.log(c());
+            }
+        })(function() {
+            return "PASS";
+        });
+    }
+    expect_stdout: "PASS"
+    node_version: ">=4"
+}