fix corner case in `collapse_vars` (#3885)

fixes #3884

2 files changed, 26 insertions, 0 deletions

diff --git a/lib/compress.js b/lib/compress.js
index cb5d9c7b..415f9bdb 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -1243,6 +1243,7 @@ merge(Compressor.prototype, {
                         col: node.start.col
                     });
                     if (candidate instanceof AST_UnaryPostfix) {
+                        delete candidate.expression.fixed;
                         return make_node(AST_UnaryPrefix, candidate, candidate);
                     }
                     if (candidate instanceof AST_VarDef) {
diff --git a/test/compress/collapse_vars.js b/test/compress/collapse_vars.js
index c93a1465..ff0837a2 100644
--- a/test/compress/collapse_vars.js
+++ b/test/compress/collapse_vars.js
@@ -7954,3 +7954,28 @@ mangleable_var: {
     }
     expect_stdout: "PASS"
 }
+
+issue_3884: {
+    options = {
+        collapse_vars: true,
+        evaluate: true,
+        reduce_vars: true,
+        side_effects: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a = 100, b = 1;
+        {
+            a++ + a || a;
+            b <<= a;
+        }
+        console.log(a, b);
+    }
+    expect: {
+        var a = 100;
+        ++a;
+        console.log(a, 32);
+    }
+    expect_stdout: "101 32"
+}