retain @__PURE__ call when return value is used (#3874)

lib/compress.js

@@ -3499,6 +3499,7 @@ merge(Compressor.prototype, {
             if (fn instanceof AST_Lambda) {
                 if (fn.evaluating) return this;
                 if (fn.name && fn.name.definition().recursive_refs > 0) return this;
+                if (this.is_expr_pure(compressor)) return this;
                 var stat = fn.first_statement();
                 if (!(stat instanceof AST_Return)) return this;
                 var args = eval_args(this.args);

test/compress/pure_funcs.js

@@ -782,3 +782,28 @@ inline_pure_call_3: {
         "undefined",
     ]
 }
+
+inline_pure_call_4: {
+    options = {
+        evaluate: true,
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a = /*@__PURE__*/ function() {
+            return console.log("PASS"), 42;
+        }();
+        console.log(a);
+    }
+    expect: {
+        var a = function() {
+            return console.log("PASS"), 42;
+        }();
+        console.log(a);
+    }
+    expect_stdout: [
+        "PASS",
+        "42",
+    ]
+}