Fix not without parentheses binding power

src/prism.c

@@ -19195,10 +19195,12 @@ parse_expression_prefix(pm_parser_t *parser, pm_binding_power_t binding_power, b
             pm_arguments_t arguments = { 0 };
             pm_node_t *receiver = NULL;
 
-            // If we do not accept a command call, then we also do not accept a
-            // not without parentheses. In this case we need to reject this
-            // syntax.
-            if (!accepts_command_call && !match1(parser, PM_TOKEN_PARENTHESIS_LEFT)) {
+            // The `not` keyword without parentheses is only valid in contexts
+            // where it would be parsed as an expression (i.e., at or below
+            // the `not` binding power level). In other contexts (e.g., method
+            // arguments, array elements, assignment right-hand sides),
+            // parentheses are required: `not(x)`.
+            if (binding_power > PM_BINDING_POWER_NOT && !match1(parser, PM_TOKEN_PARENTHESIS_LEFT)) {
                 if (match1(parser, PM_TOKEN_PARENTHESIS_LEFT_PARENTHESES)) {
                     pm_parser_err(parser, PM_TOKEN_END(parser, &parser->previous), 1, PM_ERR_EXPECT_LPAREN_AFTER_NOT_LPAREN);
                 } else {

test/prism/errors/not_without_parens_assignment.txt

@@ -0,0 +1,4 @@
+x = not y
+        ^ expected a `(` after `not`
+        ^ unexpected local variable or method, expecting end-of-input
+

test/prism/errors/not_without_parens_call.txt

@@ -0,0 +1,7 @@
+foo(not y)
+        ^ expected a `(` after `not`
+        ^ unexpected local variable or method; expected a `)` to close the arguments
+        ^ unexpected local variable or method, expecting end-of-input
+         ^ unexpected ')', expecting end-of-input
+         ^ unexpected ')', ignoring it
+

test/prism/errors/not_without_parens_command.txt

@@ -0,0 +1,4 @@
+foo not y
+        ^ expected a `(` after `not`
+        ^ unexpected local variable or method, expecting end-of-input
+

test/prism/errors/not_without_parens_command_call.txt

@@ -0,0 +1,4 @@
+a.b not y
+        ^ expected a `(` after `not`
+        ^ unexpected local variable or method, expecting end-of-input
+

test/prism/errors/not_without_parens_return.txt

@@ -0,0 +1,4 @@
+return not y
+           ^ expected a `(` after `not`
+           ^ unexpected local variable or method, expecting end-of-input
+