[Parser] Validate a collection element if its a subscript

Author: theblixguy

include/swift/AST/DiagnosticsParse.def

@@ -762,6 +762,8 @@ ERROR(extra_rbracket,PointsToFirstBadToken,
       "unexpected ']' in type; did you mean to write an array type?", ())
 ERROR(extra_colon,PointsToFirstBadToken,
       "unexpected ':' in type; did you mean to write a dictionary type?", ())
+WARNING(subscript_collection_element_invalid_index, none,
+       "index '%0' in subscript expression is out of bounds", (StringRef))
 
 // Tuple Types
 ERROR(expected_rparen_tuple_type_list,none,

include/swift/Parse/Parser.h

@@ -1435,6 +1435,8 @@ class Parser {
 
   UnresolvedDeclRefExpr *parseExprOperator();
 
+  void validateCollectionElement(ParserResult<Expr> element);
+
   //===--------------------------------------------------------------------===//
   // Statement Parsing
 

lib/Parse/ParseExpr.cpp

@@ -3447,8 +3447,10 @@ Parser::parseExprCollectionElement(Optional<bool> &isDictionary) {
   if (!isDictionary.hasValue())
     isDictionary = Tok.is(tok::colon);
 
-  if (!*isDictionary)
+  if (!*isDictionary) {
+    validateCollectionElement(Element);
     return Element;
+  }
 
   if (Element.isNull())
     return Element;
@@ -3471,6 +3473,46 @@ Parser::parseExprCollectionElement(Optional<bool> &isDictionary) {
       TupleExpr::createImplicit(Context, {Element.get(), Value.get()}, {}));
 }
 
+/// validateCollectionElement - Check if a given collection element is valid.
+///
+/// At the moment, this checks whether a given collection element is a subscript
+/// expression and whether we're subscripting into an array. If we are, then it
+/// checks whether the subscript index is valid or not.
+///
+/// For example: `let array [ [0, 1] [42] ]`
+void Parser::validateCollectionElement(ParserResult<Expr> element) {
+  if (element.isNull())
+    return;
+
+  auto elementExpr = element.get();
+  if (!isa<SubscriptExpr>(elementExpr))
+    return;
+
+  auto subscriptExpr = cast<SubscriptExpr>(elementExpr);
+  if (!isa<ArrayExpr>(subscriptExpr->getBase()))
+    return;
+
+  auto baseExpr = cast<ArrayExpr>(subscriptExpr->getBase());
+  auto index = subscriptExpr->getIndex()->getValueProvidingExpr();
+
+  if (!isa<IntegerLiteralExpr>(index))
+    return;
+  auto indexExpr = cast<IntegerLiteralExpr>(index);
+
+  int elementsCount = baseExpr->getNumElements();
+  int indexValue = atoi(indexExpr->getDigitsText().str().c_str());
+
+  auto withinBounds = indexValue >= 0 && indexValue < elementsCount;
+
+  if (!withinBounds) {
+    auto diag = diagnose(subscriptExpr->getLoc(),
+                         diag::subscript_collection_element_invalid_index,
+                         indexExpr->getDigitsText());
+    diag.highlight(subscriptExpr->getSourceRange());
+    element.setIsParseError();
+  }
+}
+
 void Parser::addPatternVariablesToScope(ArrayRef<Pattern *> Patterns) {
   for (Pattern *Pat : Patterns) {
     Pat->forEachVariable([&](VarDecl *VD) {

test/type/array.swift

@@ -114,3 +114,9 @@ func passAssocType<T : HasAssocType>(_ t: T) {
   takesAssocType(t, [T.A](), [T.A?]())
 }
 
+// SR-11134
+
+let sr_11134_1 = [["a"][0]] // Ok
+let sr_11134_2 = [["a"][1]] // expected-warning {{index '1' in subscript expression is out of bounds}}
+
+let sr_11134_3 = [[1, 1, 1], [], [4, 5, 6, 7], [0], [], [] [42]] // expected-warning {{index '42' in subscript expression is out of bounds}}