Explorer: support index-based fields for FieldPath and Members (#2417)

Features
* Support addressing positional members (i.e. tuple fields) using an index.
* Addresses a couple of `TODO`s relative to positional members

Changes:
* Add a new `IndexedValue` mirroring `NamedValue`
* Adds `FieldPath::index() -> int`
* Adds a `Member` variant for `IndexedValue`

explorer/interpreter/value.cpp

@@ -14,6 +14,7 @@
 #include "explorer/common/arena.h"
 #include "explorer/common/error_builders.h"
 #include "explorer/interpreter/action.h"
+#include "explorer/interpreter/field_path.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/Support/Casting.h"
@@ -48,12 +49,32 @@ static auto FindClassField(Nonnull<const NominalClassValue*> object,
   return std::nullopt;
 }
 
-static auto GetMember(Nonnull<Arena*> arena, Nonnull<const Value*> v,
-                      const FieldPath::Component& field,
-                      SourceLocation source_loc, Nonnull<const Value*> me_value)
+static auto GetPositionalMember(Nonnull<const Value*> v,
+                                const FieldPath::Component& field,
+                                SourceLocation source_loc)
     -> ErrorOr<Nonnull<const Value*>> {
-  std::string_view f = field.name();
+  switch (v->kind()) {
+    case Value::Kind::TupleValue: {
+      const auto& tuple = cast<TupleValue>(*v);
+      const auto index = field.member().index();
+      if (index < 0 || index >= static_cast<int>(tuple.elements().size())) {
+        return ProgramError(source_loc)
+               << "index " << index << " out of range for " << *v;
+      }
+      return tuple.elements()[index];
+    }
+    default:
+      return ProgramError(source_loc)
+             << "Invalid positional argument for value " << *v;
+  }
+}
 
+static auto GetNamedMember(Nonnull<Arena*> arena, Nonnull<const Value*> v,
+                           const FieldPath::Component& field,
+                           SourceLocation source_loc,
+                           Nonnull<const Value*> me_value)
+    -> ErrorOr<Nonnull<const Value*>> {
+  const auto f = field.member().name();
   if (field.witness().has_value()) {
     const auto* witness = cast<Witness>(*field.witness());
 
@@ -152,6 +173,15 @@ static auto GetMember(Nonnull<Arena*> arena, Nonnull<const Value*> v,
   }
 }
 
+static auto GetMember(Nonnull<Arena*> arena, Nonnull<const Value*> v,
+                      const FieldPath::Component& field,
+                      SourceLocation source_loc, Nonnull<const Value*> me_value)
+    -> ErrorOr<Nonnull<const Value*>> {
+  return field.member().HasPosition()
+             ? GetPositionalMember(v, field, source_loc)
+             : GetNamedMember(arena, v, field, source_loc, me_value);
+}
+
 auto Value::GetMember(Nonnull<Arena*> arena, const FieldPath& path,
                       SourceLocation source_loc,
                       Nonnull<const Value*> me_value) const
@@ -178,11 +208,11 @@ static auto SetFieldImpl(
       std::vector<NamedValue> elements = cast<StructValue>(*value).elements();
       auto it =
           llvm::find_if(elements, [path_begin](const NamedValue& element) {
-            return element.name == (*path_begin).name();
+            return (*path_begin).IsNamed(element.name);
           });
       if (it == elements.end()) {
         return ProgramError(source_loc)
-               << "field " << (*path_begin).name() << " not in " << *value;
+               << "field " << *path_begin << " not in " << *value;
       }
       CARBON_ASSIGN_OR_RETURN(
           it->value, SetFieldImpl(arena, it->value, path_begin + 1, path_end,
@@ -207,17 +237,16 @@ static auto SetFieldImpl(
       }
       // Failed to match, show full object content
       return ProgramError(source_loc)
-             << "field " << (*path_begin).name() << " not in " << *value;
+             << "field " << *path_begin << " not in " << *value;
     }
     case Value::Kind::TupleType:
     case Value::Kind::TupleValue: {
       std::vector<Nonnull<const Value*>> elements =
           cast<TupleValueBase>(*value).elements();
-      // TODO(geoffromer): update FieldPath to hold integers as well as strings.
-      int index = std::stoi(std::string((*path_begin).name()));
-      if (index < 0 || static_cast<size_t>(index) >= elements.size()) {
-        return ProgramError(source_loc) << "index " << (*path_begin).name()
-                                        << " out of range in " << *value;
+      const auto index = (*path_begin).member().index();
+      if (index < 0 || index >= static_cast<int>(elements.size())) {
+        return ProgramError(source_loc)
+               << "index " << index << " out of range in " << *value;
       }
       CARBON_ASSIGN_OR_RETURN(
           elements[index], SetFieldImpl(arena, elements[index], path_begin + 1,
@@ -541,7 +570,7 @@ void Value::Print(llvm::raw_ostream& out) const {
       if (member_name.interface().has_value()) {
         out << *member_name.interface().value();
       }
-      out << "." << member_name.name();
+      out << "." << member_name;
       if (member_name.base_type().has_value() &&
           member_name.interface().has_value()) {
         out << ")";
@@ -587,7 +616,7 @@ void Value::Print(llvm::raw_ostream& out) const {
           << cast<TypeOfParameterizedEntityName>(*this).name();
       break;
     case Value::Kind::TypeOfMemberName: {
-      out << "member name " << cast<TypeOfMemberName>(*this).member().name();
+      out << "member name " << cast<TypeOfMemberName>(*this).member();
       break;
     }
     case Value::Kind::StaticArrayType: {