Fix oversight in commit 0dca5d68d.

As coded, fmgr_sql() would get an assertion failure for a SQL function
that has an empty body and is declared to return some type other than
VOID.  Typically you'd never get that far because fmgr_sql_validator()
would reject such a definition (I suspect that's how come I managed to
miss the bug).  But if check_function_bodies is off or the function is
polymorphic, the validation check wouldn't get made.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/0fde377a-3870-4d18-946a-ce008ee5bb88@gmail.com

src/backend/executor/functions.c

@@ -1116,6 +1116,19 @@ sql_compile_callback(FunctionCallInfo fcinfo,
 	 */
 	func->num_queries = list_length(source_list);
 
+	/*
+	 * Edge case: empty function body is OK only if it returns VOID.  Normally
+	 * we validate that the last statement returns the right thing in
+	 * check_sql_stmt_retval, but we'll never reach that if there's no last
+	 * statement.
+	 */
+	if (func->num_queries == 0 && rettype != VOIDOID)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_FUNCTION_DEFINITION),
+				 errmsg("return type mismatch in function declared to return %s",
+						format_type_be(rettype)),
+				 errdetail("Function's final statement must be SELECT or INSERT/UPDATE/DELETE/MERGE RETURNING.")));
+
 	/* Save the source trees in pcontext for now. */
 	MemoryContextSwitchTo(pcontext);
 	func->source_list = copyObject(source_list);
@@ -2103,7 +2116,7 @@ check_sql_stmt_retval(List *queryTreeList,
 	}
 	else
 	{
-		/* Empty function body, or last statement is a utility command */
+		/* Last statement is a utility command, or it rewrote to nothing */
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_FUNCTION_DEFINITION),
 				 errmsg("return type mismatch in function declared to return %s",

src/test/regress/expected/create_function_sql.out

@@ -757,9 +757,23 @@ LINE 2:     AS 'SELECT $2;';
 CREATE FUNCTION test1 (int) RETURNS int LANGUAGE SQL
     AS 'a', 'b';
 ERROR:  only one AS item needed for language "sql"
+CREATE FUNCTION test1 (int) RETURNS int LANGUAGE SQL
+    AS '';
+ERROR:  return type mismatch in function declared to return integer
+DETAIL:  Function's final statement must be SELECT or INSERT/UPDATE/DELETE/MERGE RETURNING.
+CONTEXT:  SQL function "test1"
+-- make sure empty-body case is handled at execution time, too
+SET check_function_bodies = off;
+CREATE FUNCTION test1 (anyelement) RETURNS anyarray LANGUAGE SQL
+    AS '';
+SELECT test1(0);
+ERROR:  return type mismatch in function declared to return integer[]
+DETAIL:  Function's final statement must be SELECT or INSERT/UPDATE/DELETE/MERGE RETURNING.
+CONTEXT:  SQL function "test1" during startup
+RESET check_function_bodies;
 -- Cleanup
 DROP SCHEMA temp_func_test CASCADE;
-NOTICE:  drop cascades to 34 other objects
+NOTICE:  drop cascades to 35 other objects
 DETAIL:  drop cascades to function functest_a_1(text,date)
 drop cascades to function functest_a_2(text[])
 drop cascades to function functest_a_3()
@@ -794,5 +808,6 @@ drop cascades to function create_and_insert()
 drop cascades to table ddl_test
 drop cascades to function alter_and_insert()
 drop cascades to function double_append(anyarray,anyelement)
+drop cascades to function test1(anyelement)
 DROP USER regress_unpriv_user;
 RESET search_path;

src/test/regress/sql/create_function_sql.sql

@@ -449,6 +449,16 @@ CREATE FUNCTION test1 (int) RETURNS int LANGUAGE SQL
 CREATE FUNCTION test1 (int) RETURNS int LANGUAGE SQL
     AS 'a', 'b';
 
+CREATE FUNCTION test1 (int) RETURNS int LANGUAGE SQL
+    AS '';
+
+-- make sure empty-body case is handled at execution time, too
+SET check_function_bodies = off;
+CREATE FUNCTION test1 (anyelement) RETURNS anyarray LANGUAGE SQL
+    AS '';
+SELECT test1(0);
+RESET check_function_bodies;
+
 -- Cleanup
 DROP SCHEMA temp_func_test CASCADE;
 DROP USER regress_unpriv_user;