*** a/src/pl/plpgsql/src/pl_comp.c
--- b/src/pl/plpgsql/src/pl_comp.c
***************
*** 352,357 **** do_compile(FunctionCallInfo fcinfo,
--- 352,358 ----
  	function->out_param_varno = -1;		/* set up for no OUT param */
  	function->resolve_option = plpgsql_variable_conflict;
  	function->print_strict_params = plpgsql_print_strict_params;
+ 	function->warn_shadow = plpgsql_warn_shadow;
  
  	if (is_dml_trigger)
  		function->fn_is_trigger = PLPGSQL_DML_TRIGGER;
*** a/src/pl/plpgsql/src/pl_gram.y
--- b/src/pl/plpgsql/src/pl_gram.y
***************
*** 335,340 **** static	List			*read_raise_options(void);
--- 335,341 ----
  %token <keyword>	K_USE_VARIABLE
  %token <keyword>	K_USING
  %token <keyword>	K_VARIABLE_CONFLICT
+ %token <keyword>	K_WARN_SHADOW
  %token <keyword>	K_WARNING
  %token <keyword>	K_WHEN
  %token <keyword>	K_WHILE
***************
*** 364,369 **** comp_option		: '#' K_OPTION K_DUMP
--- 365,379 ----
  						else
  							elog(ERROR, "unrecognized print_strict_params option %s", $3);
  					}
+ 				| '#' K_WARN_SHADOW option_value
+ 					{
+ 						if (strcmp($3, "on") == 0)
+ 							plpgsql_curr_compile->warn_shadow = true;
+ 						else if (strcmp($3, "off") == 0)
+ 							plpgsql_curr_compile->warn_shadow = false;
+ 						else
+ 							elog(ERROR, "unrecognized warn_shadow option %s", $3);
+ 					}
  				| '#' K_VARIABLE_CONFLICT K_ERROR
  					{
  						plpgsql_curr_compile->resolve_option = PLPGSQL_RESOLVE_ERROR;
***************
*** 727,732 **** decl_varname	: T_WORD
--- 737,754 ----
  											  $1.ident, NULL, NULL,
  											  NULL) != NULL)
  							yyerror("duplicate declaration");
+ 						if (plpgsql_curr_compile->warn_shadow)
+ 						{
+ 							PLpgSQL_nsitem *nsi;
+ 							nsi = plpgsql_ns_lookup(plpgsql_ns_top(), false,
+ 													$1.ident, NULL, NULL, NULL);
+ 							if (nsi != NULL)
+ 								ereport(WARNING,
+ 										(errcode(ERRCODE_DUPLICATE_ALIAS),
+ 										 errmsg("variable \"%s\" shadows a previously defined variable",
+ 												$1.ident),
+ 										 parser_errposition(@1)));
+ 						}
  					}
  				| unreserved_keyword
  					{
***************
*** 740,745 **** decl_varname	: T_WORD
--- 762,779 ----
  											  $1, NULL, NULL,
  											  NULL) != NULL)
  							yyerror("duplicate declaration");
+ 						if (plpgsql_curr_compile->warn_shadow)
+ 						{
+ 							PLpgSQL_nsitem *nsi;
+ 							nsi = plpgsql_ns_lookup(plpgsql_ns_top(), false,
+ 													$1, NULL, NULL, NULL);
+ 							if (nsi != NULL)
+ 								ereport(WARNING,
+ 										(errcode(ERRCODE_DUPLICATE_ALIAS),
+ 										 errmsg("variable \"%s\" shadows a previously defined variable",
+ 												$1),
+ 										 parser_errposition(@1)));
+ 						}
  					}
  				;
  
*** a/src/pl/plpgsql/src/pl_handler.c
--- b/src/pl/plpgsql/src/pl_handler.c
***************
*** 38,43 **** static const struct config_enum_entry variable_conflict_options[] = {
--- 38,44 ----
  int			plpgsql_variable_conflict = PLPGSQL_RESOLVE_ERROR;
  
  bool		plpgsql_print_strict_params = false;
+ bool		plpgsql_warn_shadow = false;
  
  /* Hook for plugins */
  PLpgSQL_plugin **plugin_ptr = NULL;
***************
*** 76,81 **** _PG_init(void)
--- 77,90 ----
  							 PGC_USERSET, 0,
  							 NULL, NULL, NULL);
  
+ 	DefineCustomBoolVariable("plpgsql.warn_shadow",
+ 							 gettext_noop("Display a warning when a variable declaration shadows another variable."),
+ 							 NULL,
+ 							 &plpgsql_warn_shadow,
+ 							 false,
+ 							 PGC_USERSET, 0,
+ 							 NULL, NULL, NULL);
+ 
  	EmitWarningsOnPlaceholders("plpgsql");
  
  	plpgsql_HashTableInit();
*** a/src/pl/plpgsql/src/pl_scanner.c
--- b/src/pl/plpgsql/src/pl_scanner.c
***************
*** 161,166 **** static const ScanKeyword unreserved_keywords[] = {
--- 161,167 ----
  	PG_KEYWORD("use_column", K_USE_COLUMN, UNRESERVED_KEYWORD)
  	PG_KEYWORD("use_variable", K_USE_VARIABLE, UNRESERVED_KEYWORD)
  	PG_KEYWORD("variable_conflict", K_VARIABLE_CONFLICT, UNRESERVED_KEYWORD)
+ 	PG_KEYWORD("warn_shadow", K_WARN_SHADOW, UNRESERVED_KEYWORD)
  	PG_KEYWORD("warning", K_WARNING, UNRESERVED_KEYWORD)
  };
  
*** a/src/pl/plpgsql/src/plpgsql.h
--- b/src/pl/plpgsql/src/plpgsql.h
***************
*** 738,743 **** typedef struct PLpgSQL_function
--- 738,744 ----
  	PLpgSQL_resolve_option resolve_option;
  
  	bool		print_strict_params;
+ 	bool		warn_shadow;
  
  	int			ndatums;
  	PLpgSQL_datum **datums;
***************
*** 880,885 **** extern IdentifierLookup plpgsql_IdentifierLookup;
--- 881,887 ----
  extern int	plpgsql_variable_conflict;
  
  extern bool plpgsql_print_strict_params;
+ extern bool plpgsql_warn_shadow;
  
  extern bool plpgsql_check_syntax;
  extern bool plpgsql_DumpExecTree;
*** a/src/test/regress/sql/plpgsql.sql
--- b/src/test/regress/sql/plpgsql.sql
***************
*** 2689,2694 **** end$$ language plpgsql;
--- 2689,2715 ----
  
  select footest();
  
+ -- test warnings when shadowing a variable
+ 
+ set plpgsql.warn_shadow to true;
+ 
+ create function test(in1 int, in2 int)
+ 	returns table(out1 int, out2 int) as $$
+ declare
+ IN1 text;
+ IN2 text;
+ out1 text;
+ begin
+ 
+ declare
+ out2 text;
+ in1 text;
+ begin
+ end;
+ end$$ language plpgsql;
+ 
+ reset plpgsql.warn_shadow;
+ 
  -- test scrollable cursor support
  
  create function sc_test() returns setof integer as $$