Be more explicit about AST types (#921)

This was born out of wanting FunctionDeclaration to explicitly have a Block for a body, and became a bit more of specifying types around. Note this forces exec_program to generate a Block for print()'s body, which is probably more correct as now we can expect a standard FunctionDeclaration AST structure, even for the built-ins.

There is a syntactic change here: a continuation's body is now a Block, not just a Statement. I've added an example disallowed test case. I think this is more reasonable syntax.

Other than that, note that optional_else now generates a valid Block. This has me thinking about whether we can eliminate Sequence, but that seemed well out of scope for this.

executable_semantics/syntax/parser.ypp

@@ -101,13 +101,13 @@
 %type <Nonnull<FunctionDeclaration*>> function_declaration
 %type <std::vector<Nonnull<Declaration*>>> declaration_list
 %type <Nonnull<Statement*>> statement
-%type <Nonnull<Statement*>> if_statement
-%type <std::optional<Nonnull<Statement*>>> optional_else
+%type <Nonnull<If*>> if_statement
+%type <std::optional<Nonnull<Block*>>> optional_else
 %type <std::pair<Nonnull<Expression*>, bool>> return_expression
-%type <Nonnull<Statement*>> nonempty_block
-%type <Nonnull<Statement*>> block
-%type <Nonnull<Statement*>> nonempty_statement_list
-%type <std::optional<Nonnull<Statement*>>> statement_list
+%type <Nonnull<Block*>> nonempty_block
+%type <Nonnull<Block*>> block
+%type <Nonnull<Sequence*>> nonempty_statement_list
+%type <std::optional<Nonnull<Sequence*>>> statement_list
 %type <Nonnull<Expression*>> expression
 %type <GenericBinding> generic_binding
 %type <std::vector<GenericBinding>> deduced_params
@@ -116,11 +116,11 @@
 %type <Nonnull<Pattern*>> non_expression_pattern
 %type <std::pair<Nonnull<Expression*>, bool>> return_type
 %type <Nonnull<Expression*>> paren_expression
-%type <Nonnull<Expression*>> struct_literal
+%type <Nonnull<StructLiteral*>> struct_literal
 %type <std::vector<FieldInitializer>> struct_literal_contents
-%type <Nonnull<Expression*>> struct_type_literal
+%type <Nonnull<StructTypeLiteral*>> struct_type_literal
 %type <std::vector<FieldInitializer>> struct_type_literal_contents
-%type <Nonnull<Expression*>> tuple
+%type <Nonnull<TupleLiteral*>> tuple
 %type <std::optional<std::string>> binding_lhs
 %type <Nonnull<BindingPattern*>> variable_declaration
 %type <Nonnull<Member*>> member
@@ -577,7 +577,7 @@ statement:
 | MATCH LEFT_PARENTHESIS expression RIGHT_PARENTHESIS LEFT_CURLY_BRACE
   clause_list RIGHT_CURLY_BRACE
     { $$ = arena->New<Match>(context.source_loc(), $3, $6); }
-| CONTINUATION identifier statement
+| CONTINUATION identifier block
     { $$ = arena->New<Continuation>(context.source_loc(), $2, $3); }
 | RUN expression SEMICOLON
     { $$ = arena->New<Run>(context.source_loc(), $2); }
@@ -592,7 +592,11 @@ optional_else:
   // Empty
     { $$ = std::nullopt; }
 | ELSE if_statement
-    { $$ = $2; }
+    {
+      $$ = arena->New<Block>(
+          context.source_loc(),
+          arena->New<Sequence>(context.source_loc(), $2, std::nullopt));
+    }
 | ELSE block
     { $$ = $2; }
 ;