Implement static name resolution (#958)

This doesn't actually use the results of name resolution, but it does verify that they are present.
Also ensures that name resolution and type checking are applied to deduced function parameters and the implicit call to `Main()`.

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

executable_semantics/ast/static_scope.h

@@ -28,18 +28,38 @@ class NamedEntity : public virtual AstNode {
   }
 };
 
-// The set of declared names in a scope. This is not aware of child scopes, but
-// does include directions to parent or related scopes for lookup purposes.
+// Maps the names visible in a given scope to the entities they name.
+// A scope may have parent scopes, whose names will also be visible in the
+// child scope.
 class StaticScope {
  public:
+  // Defines `name` to be `entity` in this scope, or reports a compilation error
+  // if `name` is already defined in this scope.
   void Add(std::string name, Nonnull<const NamedEntity*> entity);
 
+  // Make `parent` a parent of this scope.
+  // REQUIRES: `parent` is not already a parent of this scope.
+  void AddParent(Nonnull<StaticScope*> parent) {
+    parent_scopes_.push_back(parent);
+  }
+
+  // Returns the nearest definition of `name` in the ancestor graph of this
+  // scope, or reports a compilation error at `source_loc` there isn't exactly
+  // one such definition.
+  auto Resolve(const std::string& name, SourceLocation source_loc) const
+      -> Nonnull<const NamedEntity*>;
+
  private:
+  // Equivalent to Resolve, but returns `nullopt` instead of raising an error
+  // if no definition can be found. Still raises a compilation error if more
+  // than one definition is found.
+  auto TryResolve(const std::string& name, SourceLocation source_loc) const
+      -> std::optional<Nonnull<const NamedEntity*>>;
+
   // Maps locally declared names to their entities.
   std::unordered_map<std::string, Nonnull<const NamedEntity*>> declared_names_;
 
   // A list of scopes used for name lookup within this scope.
-  // TODO: This is unused, but is intended for name lookup cross-scope.
   std::vector<Nonnull<StaticScope*>> parent_scopes_;
 };