Support optional else-branch for if-then-elif-end (#2598)

docs/content/manual/manual.yml

@@ -2147,7 +2147,7 @@ sections:
 
           `if A then B end` is the same as `if A then B else .  end`.
           That is, the `else` branch is optional, and if absent is the
-          same as `.`.
+          same as `.`. This also applies to `elif` with absent ending `else` branch.
 
           Checking for false or null is a simpler notion of
           "truthiness" than is found in JavaScript or Python, but it

src/parser.y

@@ -367,9 +367,6 @@ Term "as" Patterns '|' Exp {
 "if" Exp "then" Exp ElseBody {
   $$ = gen_cond($2, $4, $5);
 } |
-"if" Exp "then" Exp "end" {
-  $$ = gen_cond($2, $4, gen_noop());
-} |
 "if" Exp "then" error {
   FAIL(@$, "Possibly unterminated 'if' statement");
   $$ = $2;
@@ -625,6 +622,9 @@ ElseBody:
 } |
 "else" Exp "end" {
   $$ = $2;
+} |
+"end" {
+  $$ = gen_noop();
 }
 
 ExpD:

tests/jq.test

@@ -1116,6 +1116,13 @@ null
 7
 [7]
 
+[if false then 3 elif false then 4 end]
+7
+[7]
+
+[if false then 3 elif false then 4 else . end]
+7
+[7]
 
 [.[] | [.foo[] // .bar]]
 [{"foo":[1,2], "bar": 42}, {"foo":[1], "bar": null}, {"foo":[null,false,3], "bar": 18}, {"foo":[], "bar":42}, {"foo": [null,false,null], "bar": 41}]