ConcatPair.coalesce() idempotent

Change Restore.diff

diff --git a/django/db/models/functions.py b/django/db/models/functions.py
index ac24aa7..8139ec0 100644
--- a/django/db/models/functions.py
+++ b/django/db/models/functions.py
@@ -44,8 +44,12 @@ class ConcatPair(Func):
     def as_sqlite(self, compiler, connection):
         self.arg_joiner = ' || '
         self.template = '%(expressions)s'
-        self.coalesce()
-        return super(ConcatPair, self).as_sql(compiler, connection)
+        original = self.get_source_expressions()
+        coalesced = self.coalesce()
+        self.set_source_expressions(coalesced)
+        sql, params = super(ConcatPair, self).as_sql(compiler, connection)
+        self.set_source_expressions(original)
+        return sql, params

     def as_mysql(self, compiler, connection):
         # Use CONCAT_WS with an empty separator so that NULLs are ignored.
@@ -55,9 +59,9 @@ class ConcatPair(Func):

     def coalesce(self):
         # null on either side results in null for expression, wrap with coalesce
-        expressions = [
-            Coalesce(expression, Value('')) for expression in self.get_source_expressions()]
-        self.set_source_expressions(expressions)
+        return [
+            Coalesce(expression, Value('')) for expression in self.get_source_expressions()
+        ]

Copy.diff

diff --git a/django/db/models/functions.py b/django/db/models/functions.py
index ac24aa7..f560643 100644
--- a/django/db/models/functions.py
+++ b/django/db/models/functions.py
@@ -42,10 +42,10 @@ class ConcatPair(Func):
         super(ConcatPair, self).__init__(left, right, **extra)

     def as_sqlite(self, compiler, connection):
-        self.arg_joiner = ' || '
-        self.template = '%(expressions)s'
-        self.coalesce()
-        return super(ConcatPair, self).as_sql(compiler, connection)
+        coalesced = self.coalesce()
+        coalesced.arg_joiner = ' || '
+        coalesced.template = '%(expressions)s'
+        return super(ConcatPair, coalesced).as_sql(compiler, connection)

     def as_mysql(self, compiler, connection):
         # Use CONCAT_WS with an empty separator so that NULLs are ignored.
@@ -55,9 +55,11 @@ class ConcatPair(Func):

     def coalesce(self):
         # null on either side results in null for expression, wrap with coalesce
+        c = self.copy()
         expressions = [
-            Coalesce(expression, Value('')) for expression in self.get_source_expressions()]
-        self.set_source_expressions(expressions)
+            Coalesce(expression, Value('')) for expression in c.get_source_expressions()]
+        c.set_source_expressions(expressions)
+        return c