Reject modifier conditionals in if/unless predicates

Prism accepted a modifier conditional as the predicate of an `if`/`unless`
(and `elsif`), while parse.y rejects it:

    if a if b then end      # parse.y: SyntaxError, prism: accepted
    unless a unless b then end

The `if`/`unless`/`elsif` predicate was parsed with a floor of
`PM_BINDING_POWER_MODIFIER`, which is inclusive of the modifier
conditionals (`if`/`unless`/`while`/`until`, left binding power
`PM_BINDING_POWER_MODIFIER`), so they were absorbed into the predicate.
`while`/`until` predicates already use `PM_BINDING_POWER_COMPOSITION` and
reject these correctly.

Raise the floor to `PM_BINDING_POWER_MODIFIER + 1` so the predicate still
absorbs `and`/`or` (and tighter operators) but excludes the modifier
conditionals, matching parse.y and aligning `if`/`unless` with
`while`/`until`.

Author: kai-matsudate

src/prism.c

@@ -15486,7 +15486,7 @@ parse_conditional(pm_parser_t *parser, pm_context_t context, size_t opening_newl
     pm_token_t keyword = parser->previous;
     pm_token_t then_keyword = { 0 };
 
-    pm_node_t *predicate = parse_predicate(parser, PM_BINDING_POWER_MODIFIER, context, &then_keyword, (uint16_t) (depth + 1));
+    pm_node_t *predicate = parse_predicate(parser, PM_BINDING_POWER_MODIFIER + 1, context, &then_keyword, (uint16_t) (depth + 1));
     pm_statements_node_t *statements = NULL;
 
     if (!match3(parser, PM_TOKEN_KEYWORD_ELSIF, PM_TOKEN_KEYWORD_ELSE, PM_TOKEN_KEYWORD_END)) {
@@ -15524,7 +15524,7 @@ parse_conditional(pm_parser_t *parser, pm_context_t context, size_t opening_newl
             pm_token_t elsif_keyword = parser->current;
             parser_lex(parser);
 
-            pm_node_t *predicate = parse_predicate(parser, PM_BINDING_POWER_MODIFIER, PM_CONTEXT_ELSIF, &then_keyword, (uint16_t) (depth + 1));
+            pm_node_t *predicate = parse_predicate(parser, PM_BINDING_POWER_MODIFIER + 1, PM_CONTEXT_ELSIF, &then_keyword, (uint16_t) (depth + 1));
             pm_accepts_block_stack_push(parser, true);
 
             pm_statements_node_t *statements = parse_statements(parser, PM_CONTEXT_ELSIF, (uint16_t) (depth + 1));

test/prism/errors/modifier_conditional_in_predicate.txt

@@ -0,0 +1,12 @@
+if a if b then end
+     ^~ expected `then` or `;` or '\n'
+     ^~ unexpected 'if', ignoring it
+          ^~~~ unexpected 'then', expecting end-of-input
+          ^~~~ unexpected 'then', ignoring it
+
+unless a unless b then end
+         ^~~~~~ expected `then` or `;` or '\n'
+         ^~~~~~ unexpected 'unless', ignoring it
+                  ^~~~ unexpected 'then', expecting end-of-input
+                  ^~~~ unexpected 'then', ignoring it
+