Refs #25517 -- Fixed backport inconsistencies.

django/db/models/expressions.py

@@ -339,6 +339,17 @@ class BaseExpression(object):
     def reverse_ordering(self):
         return self
 
+    def flatten(self):
+        """
+        Recursively yield this expression and all subexpressions, in
+        depth-first order.
+        """
+        yield self
+        for expr in self.get_source_expressions():
+            if expr:
+                for inner_expr in expr.flatten():
+                    yield inner_expr
+
 
 class Expression(BaseExpression, Combinable):
     """

django/db/models/functions.py

@@ -47,8 +47,8 @@ class ConcatPair(Func):
         return super(ConcatPair, coalesced).as_sql(compiler, connection)
 
     def as_mysql(self, compiler, connection):
-        self.coalesce()
-        return super(ConcatPair, self).as_sql(compiler, connection)
+        coalesced = self.coalesce()
+        return super(ConcatPair, coalesced).as_sql(compiler, connection)
 
     def coalesce(self):
         # null on either side results in null for expression, wrap with coalesce