fix corner case in side_effects (#5902)

fixes #5900

lib/compress.js

@@ -9345,14 +9345,21 @@ Compressor.prototype.compress = function(node) {
             return expressions === this.expressions ? this : make_sequence(this, expressions);
         });
         def(AST_Sub, function(compressor, first_in_statement) {
-            var expr = this.expression;
-            if (expr.may_throw_on_access(compressor)) return this;
-            var prop = this.property;
-            expr = expr.drop_side_effect_free(compressor, first_in_statement);
-            if (!expr) return prop.drop_side_effect_free(compressor, first_in_statement);
+            var self = this;
+            var expr = self.expression;
+            if (expr.may_throw_on_access(compressor)) return self;
+            var prop = self.property;
+            var optional = self.optional;
+            if (!optional) {
+                expr = expr.drop_side_effect_free(compressor, first_in_statement);
+                if (!expr) return prop.drop_side_effect_free(compressor, first_in_statement);
+            }
             prop = prop.drop_side_effect_free(compressor);
             if (!prop) return expr;
-            return make_sequence(this, [ expr, prop ]);
+            if (!optional) return make_sequence(self, [ expr, prop ]);
+            self = self.clone();
+            self.property = prop;
+            return self;
         });
         def(AST_SymbolRef, function(compressor) {
             return this.is_declared(compressor) && can_drop_symbol(this, compressor) ? null : this;

test/compress/optional-chains.js

@@ -586,7 +586,7 @@ issue_5292_sub_pure_getters: {
                 console.log("foo");
             }
         };
-        console.log("bar");
+        o?.[console.log("bar")];
     }
 }
 
@@ -647,3 +647,25 @@ issue_5856: {
     expect_stdout: "PASS"
     node_version: ">=14"
 }
+
+issue_5900: {
+    options = {
+        pure_getters: "strict",
+        reduce_vars: true,
+        side_effects: true,
+    }
+    input: {
+        var a = 42, b = "PASS";
+        a[a = null];
+        a?.[b = "FAIL"];
+        console.log(b);
+    }
+    expect: {
+        var a = 42, b = "PASS";
+        a[a = null];
+        a?.[b = "FAIL"];
+        console.log(b);
+    }
+    expect_stdout: "PASS"
+    node_version: ">=14"
+}