Allow lambdas to return auto ref

Bugzilla: https://issues.dlang.org/show_bug.cgi?id=21243
DMD PR: dlang/dmd#14454

Author: pbackus

spec/expression.dd

@@ -1894,9 +1894,9 @@ $(H3 $(LNAME2 function_literals, Function Literals))
 
 $(GRAMMAR
 $(GNAME FunctionLiteral):
-    $(D function) $(D ref)$(OPT) $(GLINK2 type, Type)$(OPT) $(GLINK ParameterWithAttributes)$(OPT) $(GLINK FunctionLiteralBody2)
-    $(D delegate) $(D ref)$(OPT) $(GLINK2 type, Type)$(OPT) $(GLINK ParameterWithMemberAttributes)$(OPT) $(GLINK FunctionLiteralBody2)
-    $(D ref)$(OPT) $(GLINK ParameterWithMemberAttributes) $(GLINK FunctionLiteralBody2)
+    $(D function) $(GLINK RefOrAutoRef)$(OPT) $(GLINK2 type, Type)$(OPT) $(GLINK ParameterWithAttributes)$(OPT) $(GLINK FunctionLiteralBody2)
+    $(D delegate) $(GLINK RefOrAutoRef)$(OPT) $(GLINK2 type, Type)$(OPT) $(GLINK ParameterWithMemberAttributes)$(OPT) $(GLINK FunctionLiteralBody2)
+    $(GLINK RefOrAutoRef)$(OPT) $(GLINK ParameterWithMemberAttributes) $(GLINK FunctionLiteralBody2)
     $(GLINK2 statement, BlockStatement)
     $(IDENTIFIER) $(D =>) $(GLINK AssignExpression)
 
@@ -1909,6 +1909,10 @@ $(GNAME ParameterWithMemberAttributes):
 $(GNAME FunctionLiteralBody2):
     $(D =>) $(GLINK AssignExpression)
     $(GLINK2 function, SpecifiedFunctionBody)
+
+$(GNAME RefOrAutoRef):
+    $(D ref)
+    $(D auto ref)
 )
 
     $(P $(I FunctionLiteral)s (also known as $(LNAME2 lambdas, $(I Lambdas))) enable embedding anonymous functions