Use correct concat function for blocks evaluation (#1702)

CHANGES.rst

@@ -14,6 +14,8 @@ Unreleased
     ``Template.generate_async``. :pr:`1960`
 -   Avoid leaving async generators unclosed in blocks, includes and extends.
     :pr:`1960`
+-   The runtime uses the correct ``concat`` function for the current environment
+    when calling block references. :issue:`1701`
 
 
 Version 3.1.4

src/jinja2/runtime.py

@@ -367,7 +367,7 @@ class BlockReference:
 
     @internalcode
     async def _async_call(self) -> str:
-        rv = concat(
+        rv = self._context.environment.concat(  # type: ignore
             [x async for x in self._stack[self._depth](self._context)]  # type: ignore
         )
 
@@ -381,7 +381,9 @@ class BlockReference:
         if self._context.environment.is_async:
             return self._async_call()  # type: ignore
 
-        rv = concat(self._stack[self._depth](self._context))
+        rv = self._context.environment.concat(  # type: ignore
+            self._stack[self._depth](self._context)
+        )
 
         if self._context.eval_ctx.autoescape:
             return Markup(rv)

tests/test_nativetypes.py

@@ -160,3 +160,13 @@ def test_macro(env):
     result = t.render()
     assert result == 2
     assert isinstance(result, int)
+
+
+def test_block(env):
+    t = env.from_string(
+        "{% block b %}{% for i in range(1) %}{{ loop.index }}{% endfor %}"
+        "{% endblock %}{{ self.b() }}"
+    )
+    result = t.render()
+    assert result == 11
+    assert isinstance(result, int)