enhance `collapse_vars` (#4637)

diff --git a/lib/compress.js b/lib/compress.js
index 94719af..8b5b020 100644 (file)
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -2170,6 +2170,8 @@ merge(Compressor.prototype, {
                         }
                     }
                     if (expr.value) extract_candidates(expr.value);
+                } else if (expr instanceof AST_Yield) {
+                    if (expr.expression) extract_candidates(expr.expression);
                 }
                 hit_stack.pop();
             }
@@ -2197,6 +2199,7 @@ merge(Compressor.prototype, {
                 if (parent instanceof AST_Switch) return node;
                 if (parent instanceof AST_Unary) return node;
                 if (parent instanceof AST_VarDef) return node;
+                if (parent instanceof AST_Yield) return node;
                 return null;
             }
 
@@ -2307,6 +2310,7 @@ merge(Compressor.prototype, {
                 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 check_assignment(parent.name);
+                if (parent instanceof AST_Yield) return node;
                 return null;
 
                 function check_assignment(lhs) {