[Sema] Implement tailored diagnostics for MissingMemberFailure with subscripts on tuple base types

Author: LucianoPAlmeida

include/swift/AST/DiagnosticsSema.def

@@ -88,6 +88,14 @@ ERROR(could_not_find_subscript_member_did_you_mean,none,
       "did you mean to use the subscript operator?",
       (Type))
 
+ERROR(could_not_find_subscript_member_tuple, none,
+      "tuple type %0 element cannot be accessed using subscript",
+      (Type))
+ERROR(could_not_find_subscript_member_tuple_did_you_mean_use_dot, none,
+      "tuple type %0 element cannot be accessed using subscript; "
+      "did you mean to use '.' to access element?",
+      (Type))
+
 ERROR(could_not_find_enum_case,none,
       "enum type %0 has no case %1; did you mean %2?",
       (Type, DeclNameRef, DeclName))

lib/Sema/CSDiagnostics.cpp

@@ -3212,6 +3212,9 @@ bool MissingMemberFailure::diagnoseAsError() {
   if (diagnoseInLiteralCollectionContext())
     return true;
 
+  if (diagnoseForSubscriptMemberWithTupleBase())
+    return true;
+
   auto baseType = resolveType(getBaseType())->getWithoutSpecifierType();
 
   DeclNameLoc nameLoc(::getLoc(anchor));
@@ -3428,6 +3431,42 @@ bool MissingMemberFailure::diagnoseInLiteralCollectionContext() const {
   return false;
 }
 
+bool MissingMemberFailure::diagnoseForSubscriptMemberWithTupleBase() const {
+  auto locator = getLocator();
+  auto baseType = resolveType(getBaseType())->getWithoutSpecifierType();
+
+  if (!locator->isLastElement<LocatorPathElt::SubscriptMember>())
+    return false;
+
+  auto tupleType = baseType->getAs<TupleType>();
+  // For non-tuple type or empty tuples, let's fallback to the general
+  // diagnostic logic.
+  if (!tupleType || tupleType->getNumElements() == 0)
+    return false;
+
+  auto *SE = castToExpr<SubscriptExpr>(locator->getAnchor());
+  auto *index = SE->getIndex();
+
+  if (SE->getNumArguments() == 1) {
+    auto *literal =
+        dyn_cast<IntegerLiteralExpr>(index->getSemanticsProvidingExpr());
+    if (literal && !literal->isNegative()) {
+      llvm::SmallString<4> dotAccess;
+      llvm::raw_svector_ostream OS(dotAccess);
+      OS << "." << literal->getDigitsText();
+
+      emitDiagnostic(
+          diag::could_not_find_subscript_member_tuple_did_you_mean_use_dot,
+          baseType)
+          .fixItReplace(index->getSourceRange(), OS.str());
+      return true;
+    }
+  }
+
+  emitDiagnostic(diag::could_not_find_subscript_member_tuple, baseType);
+  return true;
+}
+
 bool UnintendedExtraGenericParamMemberFailure::diagnoseAsError() {
   MissingMemberFailure::diagnoseAsError();
 

lib/Sema/CSDiagnostics.h

@@ -1075,6 +1075,14 @@ class MissingMemberFailure : public InvalidMemberRefFailure {
   /// that defaults the element type. e.g. _ = [.e]
   bool diagnoseInLiteralCollectionContext() const;
 
+  /// Tailored diagnostics for missing subscript member on a tuple base type.
+  /// e.g
+  /// ```swift
+  ///   let tuple: (Int, Int) = (0, 0)
+  ///   _ = tuple[0]
+  ///  ```
+  bool diagnoseForSubscriptMemberWithTupleBase() const;
+
   static DeclName findCorrectEnumCaseName(Type Ty,
                                           TypoCorrectionResults &corrections,
                                           DeclNameRef memberName);