Merge pull request #31942 from andrewbranch/bug/30882

Improve error message on indexed access to private members of type parameters

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;
         }

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",