diff --git a/packages/ast-spec/src/base/BinaryExpressionBase.ts b/packages/ast-spec/src/base/BinaryExpressionBase.ts
deleted file mode 100644
index 926491d4985..00000000000
--- a/packages/ast-spec/src/base/BinaryExpressionBase.ts
+++ /dev/null
@@ -1,8 +0,0 @@
-import type { Expression } from '../unions/Expression';
-import type { BaseNode } from './BaseNode';
-
-export interface BinaryExpressionBase extends BaseNode {
-  operator: string;
-  left: Expression;
-  right: Expression;
-}
diff --git a/packages/ast-spec/src/expression/AssignmentExpression/spec.ts b/packages/ast-spec/src/expression/AssignmentExpression/spec.ts
index 8d76be21bdc..eb3c8effa99 100644
--- a/packages/ast-spec/src/expression/AssignmentExpression/spec.ts
+++ b/packages/ast-spec/src/expression/AssignmentExpression/spec.ts
@@ -1,7 +1,8 @@
 import type { AST_NODE_TYPES } from '../../ast-node-types';
-import type { BinaryExpressionBase } from '../../base/BinaryExpressionBase';
+import type { BaseNode } from '../../base/BaseNode';
+import type { Expression } from '../../unions/Expression';
 
-export interface AssignmentExpression extends BinaryExpressionBase {
+export interface AssignmentExpression extends BaseNode {
   type: AST_NODE_TYPES.AssignmentExpression;
   operator:
     | '-='
@@ -20,4 +21,6 @@ export interface AssignmentExpression extends BinaryExpressionBase {
     | '>>>='
     | '|='
     | '||=';
+  left: Expression;
+  right: Expression;
 }
diff --git a/packages/ast-spec/src/expression/BinaryExpression/spec.ts b/packages/ast-spec/src/expression/BinaryExpression/spec.ts
index 5df33cc3e16..fa43c88bcf5 100644
--- a/packages/ast-spec/src/expression/BinaryExpression/spec.ts
+++ b/packages/ast-spec/src/expression/BinaryExpression/spec.ts
@@ -1,6 +1,11 @@
 import type { AST_NODE_TYPES } from '../../ast-node-types';
-import type { BinaryExpressionBase } from '../../base/BinaryExpressionBase';
+import type { BaseNode } from '../../base/BaseNode';
+import type { PrivateIdentifier } from '../../special/PrivateIdentifier/spec';
+import type { Expression } from '../../unions/Expression';
 
-export interface BinaryExpression extends BinaryExpressionBase {
+export interface BinaryExpression extends BaseNode {
   type: AST_NODE_TYPES.BinaryExpression;
+  operator: string;
+  left: Expression | PrivateIdentifier;
+  right: Expression;
 }
diff --git a/packages/ast-spec/src/expression/LogicalExpression/spec.ts b/packages/ast-spec/src/expression/LogicalExpression/spec.ts
index 6d2b56b50dd..1eda8e3b615 100644
--- a/packages/ast-spec/src/expression/LogicalExpression/spec.ts
+++ b/packages/ast-spec/src/expression/LogicalExpression/spec.ts
@@ -1,7 +1,10 @@
 import type { AST_NODE_TYPES } from '../../ast-node-types';
-import type { BinaryExpressionBase } from '../../base/BinaryExpressionBase';
+import type { BaseNode } from '../../base/BaseNode';
+import type { Expression } from '../../unions/Expression';
 
-export interface LogicalExpression extends BinaryExpressionBase {
+export interface LogicalExpression extends BaseNode {
   type: AST_NODE_TYPES.LogicalExpression;
   operator: '??' | '&&' | '||';
+  left: Expression;
+  right: Expression;
 }
diff --git a/packages/eslint-plugin/src/rules/no-base-to-string.ts b/packages/eslint-plugin/src/rules/no-base-to-string.ts
index 11962363c64..644bb88152a 100644
--- a/packages/eslint-plugin/src/rules/no-base-to-string.ts
+++ b/packages/eslint-plugin/src/rules/no-base-to-string.ts
@@ -166,7 +166,10 @@ export default util.createRule<Options, MessageIds>({
 
         if (util.getTypeName(typeChecker, leftType) === 'string') {
           checkExpression(node.right, rightType);
-        } else if (util.getTypeName(typeChecker, rightType) === 'string') {
+        } else if (
+          util.getTypeName(typeChecker, rightType) === 'string' &&
+          node.left.type !== AST_NODE_TYPES.PrivateIdentifier
+        ) {
           checkExpression(node.left, leftType);
         }
       },
diff --git a/packages/eslint-plugin/src/rules/no-confusing-non-null-assertion.ts b/packages/eslint-plugin/src/rules/no-confusing-non-null-assertion.ts
index f578c5d3a1d..1535a643397 100644
--- a/packages/eslint-plugin/src/rules/no-confusing-non-null-assertion.ts
+++ b/packages/eslint-plugin/src/rules/no-confusing-non-null-assertion.ts
@@ -38,7 +38,7 @@ export default util.createRule({
         node: TSESTree.BinaryExpression | TSESTree.AssignmentExpression,
       ): void {
         function isLeftHandPrimaryExpression(
-          node: TSESTree.Expression,
+          node: TSESTree.Expression | TSESTree.PrivateIdentifier,
         ): boolean {
           return node.type === AST_NODE_TYPES.TSNonNullExpression;
         }
diff --git a/packages/eslint-plugin/src/rules/no-unnecessary-boolean-literal-compare.ts b/packages/eslint-plugin/src/rules/no-unnecessary-boolean-literal-compare.ts
index 8583e657f91..41367d4115b 100644
--- a/packages/eslint-plugin/src/rules/no-unnecessary-boolean-literal-compare.ts
+++ b/packages/eslint-plugin/src/rules/no-unnecessary-boolean-literal-compare.ts
@@ -21,7 +21,7 @@ type Options = [
 ];
 
 interface BooleanComparison {
-  expression: TSESTree.Expression;
+  expression: TSESTree.Expression | TSESTree.PrivateIdentifier;
   literalBooleanInComparison: boolean;
   forTruthy: boolean;
   negated: boolean;
diff --git a/packages/eslint-plugin/src/rules/prefer-for-of.ts b/packages/eslint-plugin/src/rules/prefer-for-of.ts
index f00868d0409..e1c3a058016 100644
--- a/packages/eslint-plugin/src/rules/prefer-for-of.ts
+++ b/packages/eslint-plugin/src/rules/prefer-for-of.ts
@@ -33,7 +33,10 @@ export default util.createRule({
       );
     }
 
-    function isLiteral(node: TSESTree.Expression, value: number): boolean {
+    function isLiteral(
+      node: TSESTree.Expression | TSESTree.PrivateIdentifier,
+      value: number,
+    ): boolean {
       return node.type === AST_NODE_TYPES.Literal && node.value === value;
     }
 
diff --git a/packages/eslint-plugin/src/rules/prefer-string-starts-ends-with.ts b/packages/eslint-plugin/src/rules/prefer-string-starts-ends-with.ts
index c90bb0a1a74..9bf63c08431 100644
--- a/packages/eslint-plugin/src/rules/prefer-string-starts-ends-with.ts
+++ b/packages/eslint-plugin/src/rules/prefer-string-starts-ends-with.ts
@@ -286,7 +286,9 @@ export default createRule({
       return { isEndsWith, isStartsWith, text };
     }
 
-    function getLeftNode(node: TSESTree.Expression): TSESTree.MemberExpression {
+    function getLeftNode(
+      node: TSESTree.Expression | TSESTree.PrivateIdentifier,
+    ): TSESTree.MemberExpression {
       if (node.type === AST_NODE_TYPES.ChainExpression) {
         return getLeftNode(node.expression);
       }
diff --git a/packages/eslint-plugin/src/rules/restrict-plus-operands.ts b/packages/eslint-plugin/src/rules/restrict-plus-operands.ts
index 51b4da9be46..30c155e3dcd 100644
--- a/packages/eslint-plugin/src/rules/restrict-plus-operands.ts
+++ b/packages/eslint-plugin/src/rules/restrict-plus-operands.ts
@@ -93,7 +93,9 @@ export default util.createRule<Options, MessageIds>({
      * Helper function to get base type of node
      * @param node the node to be evaluated.
      */
-    function getNodeType(node: TSESTree.Expression): BaseLiteral {
+    function getNodeType(
+      node: TSESTree.Expression | TSESTree.PrivateIdentifier,
+    ): BaseLiteral {
       const tsNode = service.esTreeNodeToTSNodeMap.get(node);
       const type = util.getConstrainedTypeAtLocation(typeChecker, tsNode);
 
diff --git a/packages/shared-fixtures/fixtures/typescript/basics/private-fields-in-in.src.ts b/packages/shared-fixtures/fixtures/typescript/basics/private-fields-in-in.src.ts
new file mode 100644
index 00000000000..b75db0be222
--- /dev/null
+++ b/packages/shared-fixtures/fixtures/typescript/basics/private-fields-in-in.src.ts
@@ -0,0 +1,6 @@
+class Foo {
+  #prop1;
+  method(arg) {
+    return #prop1 in arg;
+  }
+}
diff --git a/packages/typescript-estree/tests/lib/__snapshots__/semantic-diagnostics-enabled.test.ts.snap b/packages/typescript-estree/tests/lib/__snapshots__/semantic-diagnostics-enabled.test.ts.snap
index ad3342300b6..d3a84fbb12a 100644
--- a/packages/typescript-estree/tests/lib/__snapshots__/semantic-diagnostics-enabled.test.ts.snap
+++ b/packages/typescript-estree/tests/lib/__snapshots__/semantic-diagnostics-enabled.test.ts.snap
@@ -2072,6 +2072,8 @@ exports[`Parse all fixtures with "errorOnTypeScriptSyntacticAndSemanticIssues" e
 
 exports[`Parse all fixtures with "errorOnTypeScriptSyntacticAndSemanticIssues" enabled fixtures/typescript/basics/parenthesized-use-strict.src 1`] = `"TEST OUTPUT: No semantic or syntactic issues found"`;
 
+exports[`Parse all fixtures with "errorOnTypeScriptSyntacticAndSemanticIssues" enabled fixtures/typescript/basics/private-fields-in-in.src 1`] = `"TEST OUTPUT: No semantic or syntactic issues found"`;
+
 exports[`Parse all fixtures with "errorOnTypeScriptSyntacticAndSemanticIssues" enabled fixtures/typescript/basics/readonly-arrays.src 1`] = `"TEST OUTPUT: No semantic or syntactic issues found"`;
 
 exports[`Parse all fixtures with "errorOnTypeScriptSyntacticAndSemanticIssues" enabled fixtures/typescript/basics/readonly-tuples.src 1`] = `"TEST OUTPUT: No semantic or syntactic issues found"`;
diff --git a/packages/typescript-estree/tests/snapshots/typescript/basics/private-fields-in-in.src.ts.shot b/packages/typescript-estree/tests/snapshots/typescript/basics/private-fields-in-in.src.ts.shot
new file mode 100644
index 00000000000..27c37ed05ad
--- /dev/null
+++ b/packages/typescript-estree/tests/snapshots/typescript/basics/private-fields-in-in.src.ts.shot
@@ -0,0 +1,599 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`typescript basics private-fields-in-in.src 1`] = `
+Object {
+  "body": Array [
+    Object {
+      "body": Object {
+        "body": Array [
+          Object {
+            "computed": false,
+            "declare": false,
+            "key": Object {
+              "loc": Object {
+                "end": Object {
+                  "column": 8,
+                  "line": 2,
+                },
+                "start": Object {
+                  "column": 2,
+                  "line": 2,
+                },
+              },
+              "name": "prop1",
+              "range": Array [
+                14,
+                20,
+              ],
+              "type": "PrivateIdentifier",
+            },
+            "loc": Object {
+              "end": Object {
+                "column": 9,
+                "line": 2,
+              },
+              "start": Object {
+                "column": 2,
+                "line": 2,
+              },
+            },
+            "override": false,
+            "range": Array [
+              14,
+              21,
+            ],
+            "readonly": undefined,
+            "static": false,
+            "type": "PropertyDefinition",
+            "value": null,
+          },
+          Object {
+            "computed": false,
+            "key": Object {
+              "loc": Object {
+                "end": Object {
+                  "column": 8,
+                  "line": 3,
+                },
+                "start": Object {
+                  "column": 2,
+                  "line": 3,
+                },
+              },
+              "name": "method",
+              "range": Array [
+                24,
+                30,
+              ],
+              "type": "Identifier",
+            },
+            "kind": "method",
+            "loc": Object {
+              "end": Object {
+                "column": 3,
+                "line": 5,
+              },
+              "start": Object {
+                "column": 2,
+                "line": 3,
+              },
+            },
+            "override": false,
+            "range": Array [
+              24,
+              67,
+            ],
+            "static": false,
+            "type": "MethodDefinition",
+            "value": Object {
+              "async": false,
+              "body": Object {
+                "body": Array [
+                  Object {
+                    "argument": Object {
+                      "left": Object {
+                        "loc": Object {
+                          "end": Object {
+                            "column": 17,
+                            "line": 4,
+                          },
+                          "start": Object {
+                            "column": 11,
+                            "line": 4,
+                          },
+                        },
+                        "name": "prop1",
+                        "range": Array [
+                          49,
+                          55,
+                        ],
+                        "type": "PrivateIdentifier",
+                      },
+                      "loc": Object {
+                        "end": Object {
+                          "column": 24,
+                          "line": 4,
+                        },
+                        "start": Object {
+                          "column": 11,
+                          "line": 4,
+                        },
+                      },
+                      "operator": "in",
+                      "range": Array [
+                        49,
+                        62,
+                      ],
+                      "right": Object {
+                        "loc": Object {
+                          "end": Object {
+                            "column": 24,
+                            "line": 4,
+                          },
+                          "start": Object {
+                            "column": 21,
+                            "line": 4,
+                          },
+                        },
+                        "name": "arg",
+                        "range": Array [
+                          59,
+                          62,
+                        ],
+                        "type": "Identifier",
+                      },
+                      "type": "BinaryExpression",
+                    },
+                    "loc": Object {
+                      "end": Object {
+                        "column": 25,
+                        "line": 4,
+                      },
+                      "start": Object {
+                        "column": 4,
+                        "line": 4,
+                      },
+                    },
+                    "range": Array [
+                      42,
+                      63,
+                    ],
+                    "type": "ReturnStatement",
+                  },
+                ],
+                "loc": Object {
+                  "end": Object {
+                    "column": 3,
+                    "line": 5,
+                  },
+                  "start": Object {
+                    "column": 14,
+                    "line": 3,
+                  },
+                },
+                "range": Array [
+                  36,
+                  67,
+                ],
+                "type": "BlockStatement",
+              },
+              "expression": false,
+              "generator": false,
+              "id": null,
+              "loc": Object {
+                "end": Object {
+                  "column": 3,
+                  "line": 5,
+                },
+                "start": Object {
+                  "column": 8,
+                  "line": 3,
+                },
+              },
+              "params": Array [
+                Object {
+                  "loc": Object {
+                    "end": Object {
+                      "column": 12,
+                      "line": 3,
+                    },
+                    "start": Object {
+                      "column": 9,
+                      "line": 3,
+                    },
+                  },
+                  "name": "arg",
+                  "range": Array [
+                    31,
+                    34,
+                  ],
+                  "type": "Identifier",
+                },
+              ],
+              "range": Array [
+                30,
+                67,
+              ],
+              "type": "FunctionExpression",
+            },
+          },
+        ],
+        "loc": Object {
+          "end": Object {
+            "column": 1,
+            "line": 6,
+          },
+          "start": Object {
+            "column": 10,
+            "line": 1,
+          },
+        },
+        "range": Array [
+          10,
+          69,
+        ],
+        "type": "ClassBody",
+      },
+      "id": Object {
+        "loc": Object {
+          "end": Object {
+            "column": 9,
+            "line": 1,
+          },
+          "start": Object {
+            "column": 6,
+            "line": 1,
+          },
+        },
+        "name": "Foo",
+        "range": Array [
+          6,
+          9,
+        ],
+        "type": "Identifier",
+      },
+      "loc": Object {
+        "end": Object {
+          "column": 1,
+          "line": 6,
+        },
+        "start": Object {
+          "column": 0,
+          "line": 1,
+        },
+      },
+      "range": Array [
+        0,
+        69,
+      ],
+      "superClass": null,
+      "type": "ClassDeclaration",
+    },
+  ],
+  "comments": Array [],
+  "loc": Object {
+    "end": Object {
+      "column": 0,
+      "line": 7,
+    },
+    "start": Object {
+      "column": 0,
+      "line": 1,
+    },
+  },
+  "range": Array [
+    0,
+    70,
+  ],
+  "sourceType": "script",
+  "tokens": Array [
+    Object {
+      "loc": Object {
+        "end": Object {
+          "column": 5,
+          "line": 1,
+        },
+        "start": Object {
+          "column": 0,
+          "line": 1,
+        },
+      },
+      "range": Array [
+        0,
+        5,
+      ],
+      "type": "Keyword",
+      "value": "class",
+    },
+    Object {
+      "loc": Object {
+        "end": Object {
+          "column": 9,
+          "line": 1,
+        },
+        "start": Object {
+          "column": 6,
+          "line": 1,
+        },
+      },
+      "range": Array [
+        6,
+        9,
+      ],
+      "type": "Identifier",
+      "value": "Foo",
+    },
+    Object {
+      "loc": Object {
+        "end": Object {
+          "column": 11,
+          "line": 1,
+        },
+        "start": Object {
+          "column": 10,
+          "line": 1,
+        },
+      },
+      "range": Array [
+        10,
+        11,
+      ],
+      "type": "Punctuator",
+      "value": "{",
+    },
+    Object {
+      "loc": Object {
+        "end": Object {
+          "column": 8,
+          "line": 2,
+        },
+        "start": Object {
+          "column": 2,
+          "line": 2,
+        },
+      },
+      "range": Array [
+        14,
+        20,
+      ],
+      "type": "Identifier",
+      "value": "#prop1",
+    },
+    Object {
+      "loc": Object {
+        "end": Object {
+          "column": 9,
+          "line": 2,
+        },
+        "start": Object {
+          "column": 8,
+          "line": 2,
+        },
+      },
+      "range": Array [
+        20,
+        21,
+      ],
+      "type": "Punctuator",
+      "value": ";",
+    },
+    Object {
+      "loc": Object {
+        "end": Object {
+          "column": 8,
+          "line": 3,
+        },
+        "start": Object {
+          "column": 2,
+          "line": 3,
+        },
+      },
+      "range": Array [
+        24,
+        30,
+      ],
+      "type": "Identifier",
+      "value": "method",
+    },
+    Object {
+      "loc": Object {
+        "end": Object {
+          "column": 9,
+          "line": 3,
+        },
+        "start": Object {
+          "column": 8,
+          "line": 3,
+        },
+      },
+      "range": Array [
+        30,
+        31,
+      ],
+      "type": "Punctuator",
+      "value": "(",
+    },
+    Object {
+      "loc": Object {
+        "end": Object {
+          "column": 12,
+          "line": 3,
+        },
+        "start": Object {
+          "column": 9,
+          "line": 3,
+        },
+      },
+      "range": Array [
+        31,
+        34,
+      ],
+      "type": "Identifier",
+      "value": "arg",
+    },
+    Object {
+      "loc": Object {
+        "end": Object {
+          "column": 13,
+          "line": 3,
+        },
+        "start": Object {
+          "column": 12,
+          "line": 3,
+        },
+      },
+      "range": Array [
+        34,
+        35,
+      ],
+      "type": "Punctuator",
+      "value": ")",
+    },
+    Object {
+      "loc": Object {
+        "end": Object {
+          "column": 15,
+          "line": 3,
+        },
+        "start": Object {
+          "column": 14,
+          "line": 3,
+        },
+      },
+      "range": Array [
+        36,
+        37,
+      ],
+      "type": "Punctuator",
+      "value": "{",
+    },
+    Object {
+      "loc": Object {
+        "end": Object {
+          "column": 10,
+          "line": 4,
+        },
+        "start": Object {
+          "column": 4,
+          "line": 4,
+        },
+      },
+      "range": Array [
+        42,
+        48,
+      ],
+      "type": "Keyword",
+      "value": "return",
+    },
+    Object {
+      "loc": Object {
+        "end": Object {
+          "column": 17,
+          "line": 4,
+        },
+        "start": Object {
+          "column": 11,
+          "line": 4,
+        },
+      },
+      "range": Array [
+        49,
+        55,
+      ],
+      "type": "Identifier",
+      "value": "#prop1",
+    },
+    Object {
+      "loc": Object {
+        "end": Object {
+          "column": 20,
+          "line": 4,
+        },
+        "start": Object {
+          "column": 18,
+          "line": 4,
+        },
+      },
+      "range": Array [
+        56,
+        58,
+      ],
+      "type": "Keyword",
+      "value": "in",
+    },
+    Object {
+      "loc": Object {
+        "end": Object {
+          "column": 24,
+          "line": 4,
+        },
+        "start": Object {
+          "column": 21,
+          "line": 4,
+        },
+      },
+      "range": Array [
+        59,
+        62,
+      ],
+      "type": "Identifier",
+      "value": "arg",
+    },
+    Object {
+      "loc": Object {
+        "end": Object {
+          "column": 25,
+          "line": 4,
+        },
+        "start": Object {
+          "column": 24,
+          "line": 4,
+        },
+      },
+      "range": Array [
+        62,
+        63,
+      ],
+      "type": "Punctuator",
+      "value": ";",
+    },
+    Object {
+      "loc": Object {
+        "end": Object {
+          "column": 3,
+          "line": 5,
+        },
+        "start": Object {
+          "column": 2,
+          "line": 5,
+        },
+      },
+      "range": Array [
+        66,
+        67,
+      ],
+      "type": "Punctuator",
+      "value": "}",
+    },
+    Object {
+      "loc": Object {
+        "end": Object {
+          "column": 1,
+          "line": 6,
+        },
+        "start": Object {
+          "column": 0,
+          "line": 6,
+        },
+      },
+      "range": Array [
+        68,
+        69,
+      ],
+      "type": "Punctuator",
+      "value": "}",
+    },
+  ],
+  "type": "Program",
+}
+`;