diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 9cb08b3e44..a3abe1dc5b 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -10097,9 +10097,9 @@ namespace ts {
             return false;
         }
 
-        function getPropertyTypeForIndexType(originalObjectType: Type, objectType: Type, indexType: Type, fullIndexType: Type, suppressNoImplicitAnyError: boolean, accessNode: ElementAccessExpression | IndexedAccessTypeNode | PropertyName | BindingName | SyntheticExpression | undefined, accessFlags: AccessFlags) {
+        function getPropertyNameFromIndex(indexType: Type, accessNode: StringLiteral | Identifier | ObjectBindingPattern | ArrayBindingPattern | ComputedPropertyName | NumericLiteral | IndexedAccessTypeNode | ElementAccessExpression | SyntheticExpression | undefined) {
             const accessExpression = accessNode && accessNode.kind === SyntaxKind.ElementAccessExpression ? accessNode : undefined;
-            const propName = isTypeUsableAsPropertyName(indexType) ?
+            return isTypeUsableAsPropertyName(indexType) ?
                 getPropertyNameFromType(indexType) :
                 accessExpression && checkThatExpressionIsProperSymbolReference(accessExpression.argumentExpression, indexType, /*reportError*/ false) ?
                     getPropertyNameForKnownSymbolName(idText((<PropertyAccessExpression>accessExpression.argumentExpression).name)) :
@@ -10107,6 +10107,11 @@ namespace ts {
                         // late bound names are handled in the first branch, so here we only need to handle normal names
                         getPropertyNameForPropertyNameNode(accessNode) :
                         undefined;
+        }
+
+        function getPropertyTypeForIndexType(originalObjectType: Type, objectType: Type, indexType: Type, fullIndexType: Type, suppressNoImplicitAnyError: boolean, accessNode: ElementAccessExpression | IndexedAccessTypeNode | PropertyName | BindingName | SyntheticExpression | undefined, accessFlags: AccessFlags) {
+            const accessExpression = accessNode && accessNode.kind === SyntaxKind.ElementAccessExpression ? accessNode : undefined;
+            const propName = getPropertyNameFromIndex(indexType, accessNode);
             if (propName !== undefined) {
                 const prop = getPropertyOfType(objectType, propName);
                 if (prop) {
@@ -25323,7 +25328,7 @@ namespace ts {
             forEach(node.types, checkSourceElement);
         }
 
-        function checkIndexedAccessIndexType(type: Type, accessNode: Node) {
+        function checkIndexedAccessIndexType(type: Type, accessNode: IndexedAccessTypeNode | ElementAccessExpression) {
             if (!(type.flags & TypeFlags.IndexedAccess)) {
                 return type;
             }
@@ -25339,9 +25344,20 @@ namespace ts {
             }
             // Check if we're indexing with a numeric type and if either object or index types
             // is a generic type with a constraint that has a numeric index signature.
-            if (getIndexInfoOfType(getApparentType(objectType), IndexKind.Number) && isTypeAssignableToKind(indexType, TypeFlags.NumberLike)) {
+            const apparentObjectType = getApparentType(objectType);
+            if (getIndexInfoOfType(apparentObjectType, IndexKind.Number) && isTypeAssignableToKind(indexType, TypeFlags.NumberLike)) {
                 return type;
             }
+            if (isGenericObjectType(objectType)) {
+                const propertyName = getPropertyNameFromIndex(indexType, accessNode);
+                if (propertyName) {
+                    const propertySymbol = forEachType(apparentObjectType, t => getPropertyOfType(t, propertyName));
+                    if (propertySymbol && getDeclarationModifierFlagsFromSymbol(propertySymbol) & ModifierFlags.NonPublicAccessibilityModifier) {
+                        error(accessNode, Diagnostics.Private_or_protected_member_0_cannot_be_accessed_on_a_type_parameter, unescapeLeadingUnderscores(propertyName));
+                        return errorType;
+                    }
+                }
+            }
             error(accessNode, Diagnostics.Type_0_cannot_be_used_to_index_type_1, typeToString(indexType), typeToString(objectType));
             return errorType;
         }
diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index 529484956e..2ea72a2449 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/src/compiler/diagnosticMessages.json
@@ -2963,6 +2963,10 @@
         "category": "Error",
         "code": 4104
     },
+    "Private or protected member '{0}' cannot be accessed on a type parameter.": {
+        "category": "Error",
+        "code": 4105
+    },
 
     "The current host does not support the '{0}' option.": {
         "category": "Error",
diff --git a/tests/baselines/reference/indexedAccessPrivateMemberOfGenericConstraint.errors.txt b/tests/baselines/reference/indexedAccessPrivateMemberOfGenericConstraint.errors.txt
new file mode 100644
index 0000000000..56bf43e77a
--- /dev/null
+++ b/tests/baselines/reference/indexedAccessPrivateMemberOfGenericConstraint.errors.txt
@@ -0,0 +1,27 @@
+tests/cases/compiler/indexedAccessPrivateMemberOfGenericConstraint.ts(9,24): error TS4105: Private or protected member 'a' cannot be accessed on a type parameter.
+tests/cases/compiler/indexedAccessPrivateMemberOfGenericConstraint.ts(9,32): error TS4105: Private or protected member 'a' cannot be accessed on a type parameter.
+tests/cases/compiler/indexedAccessPrivateMemberOfGenericConstraint.ts(10,27): error TS4105: Private or protected member 'a' cannot be accessed on a type parameter.
+tests/cases/compiler/indexedAccessPrivateMemberOfGenericConstraint.ts(11,27): error TS4105: Private or protected member 'a' cannot be accessed on a type parameter.
+
+
+==== tests/cases/compiler/indexedAccessPrivateMemberOfGenericConstraint.ts (4 errors) ====
+    class A {
+      private a: number;
+    }
+    
+    class B {
+      private a: string;
+    }
+    
+    type X<T extends A> = [T["a"], (T | B)["a"]];
+                           ~~~~~~
+!!! error TS4105: Private or protected member 'a' cannot be accessed on a type parameter.
+                                   ~~~~~~~~~~~~
+!!! error TS4105: Private or protected member 'a' cannot be accessed on a type parameter.
+    type Y<T extends A | B> = T["a"];
+                              ~~~~~~
+!!! error TS4105: Private or protected member 'a' cannot be accessed on a type parameter.
+    type Z<T extends A & B> = T["a"];
+                              ~~~~~~
+!!! error TS4105: Private or protected member 'a' cannot be accessed on a type parameter.
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/indexedAccessPrivateMemberOfGenericConstraint.js b/tests/baselines/reference/indexedAccessPrivateMemberOfGenericConstraint.js
new file mode 100644
index 0000000000..5d2bfc5891
--- /dev/null
+++ b/tests/baselines/reference/indexedAccessPrivateMemberOfGenericConstraint.js
@@ -0,0 +1,25 @@
+//// [indexedAccessPrivateMemberOfGenericConstraint.ts]
+class A {
+  private a: number;
+}
+
+class B {
+  private a: string;
+}
+
+type X<T extends A> = [T["a"], (T | B)["a"]];
+type Y<T extends A | B> = T["a"];
+type Z<T extends A & B> = T["a"];
+
+
+//// [indexedAccessPrivateMemberOfGenericConstraint.js]
+var A = /** @class */ (function () {
+    function A() {
+    }
+    return A;
+}());
+var B = /** @class */ (function () {
+    function B() {
+    }
+    return B;
+}());
diff --git a/tests/baselines/reference/indexedAccessPrivateMemberOfGenericConstraint.symbols b/tests/baselines/reference/indexedAccessPrivateMemberOfGenericConstraint.symbols
new file mode 100644
index 0000000000..69c433b16d
--- /dev/null
+++ b/tests/baselines/reference/indexedAccessPrivateMemberOfGenericConstraint.symbols
@@ -0,0 +1,37 @@
+=== tests/cases/compiler/indexedAccessPrivateMemberOfGenericConstraint.ts ===
+class A {
+>A : Symbol(A, Decl(indexedAccessPrivateMemberOfGenericConstraint.ts, 0, 0))
+
+  private a: number;
+>a : Symbol(A.a, Decl(indexedAccessPrivateMemberOfGenericConstraint.ts, 0, 9))
+}
+
+class B {
+>B : Symbol(B, Decl(indexedAccessPrivateMemberOfGenericConstraint.ts, 2, 1))
+
+  private a: string;
+>a : Symbol(B.a, Decl(indexedAccessPrivateMemberOfGenericConstraint.ts, 4, 9))
+}
+
+type X<T extends A> = [T["a"], (T | B)["a"]];
+>X : Symbol(X, Decl(indexedAccessPrivateMemberOfGenericConstraint.ts, 6, 1))
+>T : Symbol(T, Decl(indexedAccessPrivateMemberOfGenericConstraint.ts, 8, 7))
+>A : Symbol(A, Decl(indexedAccessPrivateMemberOfGenericConstraint.ts, 0, 0))
+>T : Symbol(T, Decl(indexedAccessPrivateMemberOfGenericConstraint.ts, 8, 7))
+>T : Symbol(T, Decl(indexedAccessPrivateMemberOfGenericConstraint.ts, 8, 7))
+>B : Symbol(B, Decl(indexedAccessPrivateMemberOfGenericConstraint.ts, 2, 1))
+
+type Y<T extends A | B> = T["a"];
+>Y : Symbol(Y, Decl(indexedAccessPrivateMemberOfGenericConstraint.ts, 8, 45))
+>T : Symbol(T, Decl(indexedAccessPrivateMemberOfGenericConstraint.ts, 9, 7))
+>A : Symbol(A, Decl(indexedAccessPrivateMemberOfGenericConstraint.ts, 0, 0))
+>B : Symbol(B, Decl(indexedAccessPrivateMemberOfGenericConstraint.ts, 2, 1))
+>T : Symbol(T, Decl(indexedAccessPrivateMemberOfGenericConstraint.ts, 9, 7))
+
+type Z<T extends A & B> = T["a"];
+>Z : Symbol(Z, Decl(indexedAccessPrivateMemberOfGenericConstraint.ts, 9, 33))
+>T : Symbol(T, Decl(indexedAccessPrivateMemberOfGenericConstraint.ts, 10, 7))
+>A : Symbol(A, Decl(indexedAccessPrivateMemberOfGenericConstraint.ts, 0, 0))
+>B : Symbol(B, Decl(indexedAccessPrivateMemberOfGenericConstraint.ts, 2, 1))
+>T : Symbol(T, Decl(indexedAccessPrivateMemberOfGenericConstraint.ts, 10, 7))
+
diff --git a/tests/baselines/reference/indexedAccessPrivateMemberOfGenericConstraint.types b/tests/baselines/reference/indexedAccessPrivateMemberOfGenericConstraint.types
new file mode 100644
index 0000000000..abdbaa8486
--- /dev/null
+++ b/tests/baselines/reference/indexedAccessPrivateMemberOfGenericConstraint.types
@@ -0,0 +1,24 @@
+=== tests/cases/compiler/indexedAccessPrivateMemberOfGenericConstraint.ts ===
+class A {
+>A : A
+
+  private a: number;
+>a : number
+}
+
+class B {
+>B : B
+
+  private a: string;
+>a : string
+}
+
+type X<T extends A> = [T["a"], (T | B)["a"]];
+>X : [T["a"], (B | T)["a"]]
+
+type Y<T extends A | B> = T["a"];
+>Y : T["a"]
+
+type Z<T extends A & B> = T["a"];
+>Z : T["a"]
+
diff --git a/tests/cases/compiler/indexedAccessPrivateMemberOfGenericConstraint.ts b/tests/cases/compiler/indexedAccessPrivateMemberOfGenericConstraint.ts
new file mode 100644
index 0000000000..a2fa8f4a46
--- /dev/null
+++ b/tests/cases/compiler/indexedAccessPrivateMemberOfGenericConstraint.ts
@@ -0,0 +1,11 @@
+class A {
+  private a: number;
+}
+
+class B {
+  private a: string;
+}
+
+type X<T extends A> = [T["a"], (T | B)["a"]];
+type Y<T extends A | B> = T["a"];
+type Z<T extends A & B> = T["a"];