Add concat expression support to grammar and parser

This is probably going to be reclassified in the grammar and parser
as something different from the conditional expressions once more
support for math operators is added in.

grammar.ebnf

@@ -222,6 +222,7 @@ conditional_expression
     | conditional_expression_logical
     | conditional_expression_comparator
     | conditional_expression_test
+    | concatenate_expression
     | variable_identifier
     ;
 
@@ -275,6 +276,12 @@ variable_tests_logical_operator
     | "or"
     ;
 
+concatenate_expression
+    =
+    concatenate+:variable_identifier
+    { {SP}* "~" {SP}* concatenate+:variable_identifier }+
+    ;
+
 variable_filter
     =
     "|" {SP}* @:filter

new_parser.py

@@ -280,6 +280,16 @@ def parse_block_with(ast):
 def parse_comment(ast):
     return
 
+def parse_concatenate_expression(ast):
+    vars = [
+        parse_variable(var) for var in ast['concatenate']
+    ]
+
+    return nodes.Concat(
+        vars,
+        lineno=lineno_from_parseinfo(ast['parseinfo'])
+    )
+
 def parse_conditional_expression(ast):
     if 'variable' in ast:
         return parse_variable(ast)
@@ -287,6 +297,9 @@ def parse_conditional_expression(ast):
     if 'comparator' in ast:
         return parse_conditional_expression_comparator(ast)
 
+    if 'concatenate' in ast:
+        return parse_concatenate_expression(ast)
+
     if 'test_expression' in ast:
         return parse_conditional_expression_if(ast)
 

test_template.jinja

@@ -65,4 +65,5 @@ across lines #}
 ...
 </ul>
 {{ "[{}]".format(page.title) if page.title }}
-{% extends layout_template if layout_template is defined else 'master.html' %}
+{% extends layout_template if layout_template is defined else 'master.html' %}
+{{ "Hello " ~ name ~ "!" }}