Use correct concat function for blocks evaluation

Fixes pallets#1701

CHANGES.rst

@@ -5,6 +5,8 @@ Version 3.2.0
 
 Unreleased
 
+-   Use correct concat function for blocks evaluation. :issue:`1701`
+
 
 Version 3.1.2
 -------------

src/jinja2/runtime.py

@@ -362,7 +362,7 @@ def super(self) -> t.Union["BlockReference", "Undefined"]:
 
     @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
         )
 
@@ -376,7 +376,9 @@ def __call__(self) -> str:
         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)