MDEV-25415 CASE function handles NULL inconsistently

SQL Standard specifies that a <simple case> should return the <result>
from the left-most <simple when clause> where the comparison evaluates
to TRUE.

That is, it should not stop comparing on NULLs

Author: vuvova

mysql-test/main/case.result

@@ -1,4 +1,3 @@
-drop table if exists t1, t2;
 select CASE "b" when "a" then 1 when "b" then 2 END as exp;
 exp
 2
@@ -572,6 +571,22 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	filtered	Extra
 Warnings:
 Note	1003	select `test`.`t1`.`a` AS `a`,`test`.`t1`.`b` AS `b` from `test`.`t1` where case `test`.`t1`.`a` when `test`.`t1`.`b` then 1 end = 1 and case when `test`.`t1`.`a` then `test`.`t1`.`b` else 1 end = 3
 DROP TABLE t1;
-#
 # End of 10.3 test
 #
+# MDEV-25415 CASE function handles NULL inconsistently
+#
+select case 'X' when null then 1 when 'X' then 2 else 3 end;
+case 'X' when null then 1 when 'X' then 2 else 3 end
+2
+select case 'X' when 1/1 then 1 when 'X' then 2 else 3 end;
+case 'X' when 1/1 then 1 when 'X' then 2 else 3 end
+2
+Warnings:
+Warning	1292	Truncated incorrect DOUBLE value: 'X'
+select case 'X' when 1/0 then 1 when 'X' then 2 else 3 end;
+case 'X' when 1/0 then 1 when 'X' then 2 else 3 end
+2
+Warnings:
+Warning	1292	Truncated incorrect DOUBLE value: 'X'
+Warning	1365	Division by 0
+# End of 10.11 test

mysql-test/main/case.test

@@ -2,11 +2,6 @@
 # Testing of CASE
 #
 
-
---disable_warnings
-drop table if exists t1, t2;
---enable_warnings
-
 select CASE "b" when "a" then 1 when "b" then 2 END as exp;
 select CASE "c" when "a" then 1 when "b" then 2 END as exp;
 select CASE "c" when "a" then 1 when "b" then 2 ELSE 3 END as exp;
@@ -412,7 +407,13 @@ SELECT * FROM t1 WHERE
 
 DROP TABLE t1;
 
+--echo # End of 10.3 test
 
 --echo #
---echo # End of 10.3 test
+--echo # MDEV-25415 CASE function handles NULL inconsistently
 --echo #
+select case 'X' when null then 1 when 'X' then 2 else 3 end;
+select case 'X' when 1/1 then 1 when 'X' then 2 else 3 end;
+select case 'X' when 1/0 then 1 when 'X' then 2 else 3 end;
+
+--echo # End of 10.11 test

sql/item_cmpfunc.cc

@@ -3096,7 +3096,8 @@ Item *Item_func_case_simple::find_item()
 {
   /* Compare every WHEN argument with it and return the first match */
   uint idx;
-  if (!Predicant_to_list_comparator::cmp(this, &idx, NULL))
+  bool found_unknown_values;
+  if (!Predicant_to_list_comparator::cmp(this, &idx, &found_unknown_values))
     return args[idx + when_count()];
   Item **pos= Item_func_case_simple::else_expr_addr();
   return pos ? pos[0] : 0;

sql/item_cmpfunc.h

@@ -2284,15 +2284,8 @@ class Predicant_to_list_comparator
     @param [OUT] idx  - In case if a value that is equal to the predicant
                         was found, the index of the matching value is returned
                         here. Otherwise, *idx is not changed.
-    @param [IN/OUT] found_unknown_values - how to handle UNKNOWN results.
-                        If found_unknown_values is NULL (e.g. Item_func_case),
-                        cmp() returns immediately when the first UNKNOWN
-                        result is found.
-                        If found_unknown_values is non-NULL (Item_func_in),
-                        cmp() does not return when an UNKNOWN result is found,
-                        sets *found_unknown_values to true, and continues
-                        to compare the remaining pairs to find FALSE
-                        (i.e. the value that is equal to the predicant).
+    @param [OUT] found_unknown_values - set to true if the result of at least
+                        one comparison was UNKNOWN
 
     @retval     false - Found a value that is equal to the predicant
     @retval     true  - Didn't find an equal value
@@ -2309,11 +2302,7 @@ class Predicant_to_list_comparator
         return false; // Found a matching value
       }
       if (rc == UNKNOWN)
-      {
-        if (!found_unknown_values)
-          return true;
         *found_unknown_values= true;
-      }
     }
     return true; // Not found
   }