parse.y: allow "command" syntax in endless method definition.

This change allows `def hello = puts "Hello"` without parentheses.
This syntax has been introduced since Ruby3.1.

mrbgems/mruby-compiler/core/parse.y

@@ -1754,6 +1754,44 @@ command_asgn    : lhs '=' command_rhs
                     {
                       $$ = new_op_asgn(p, new_call(p, $1, $3, 0, tCOLON2), $4, $5);
                     }
+                | defn_head f_opt_arglist_paren '=' command
+                    {
+                      $$ = $1;
+                      endless_method_name(p, $1);
+                      void_expr_error(p, $4);
+                      defn_setup(p, $$, $2, $4);
+                      nvars_unnest(p);
+                      p->in_def--;
+                    }
+                | defn_head f_opt_arglist_paren '=' command modifier_rescue arg
+                    {
+                      $$ = $1;
+                      endless_method_name(p, $1);
+                      void_expr_error(p, $4);
+                      void_expr_error(p, $6);
+                      defn_setup(p, $$, $2, new_mod_rescue(p, $4, $6));
+                      nvars_unnest(p);
+                      p->in_def--;
+                    }
+                | defs_head f_opt_arglist_paren '=' command
+                    {
+                      $$ = $1;
+                      void_expr_error(p, $4);
+                      defs_setup(p, $$, $2, $4);
+                      nvars_unnest(p);
+                      p->in_def--;
+                      p->in_single--;
+                    }
+                | defs_head f_opt_arglist_paren '=' command modifier_rescue arg
+                    {
+                      $$ = $1;
+                      void_expr_error(p, $4);
+                      void_expr_error(p, $6);
+                      defs_setup(p, $$, $2, new_mod_rescue(p, $4, $6));
+                      nvars_unnest(p);
+                      p->in_def--;
+                      p->in_single--;
+                    }
                 | backref tOP_ASGN command_rhs
                     {
                       backref_error(p, $1);

test/t/syntax.rb

@@ -706,3 +706,20 @@ assert('argument forwarding') do
   o.a(1,2,3){}
   o.b(1,2,3){}
 end
+
+assert('endless def') do
+  Class.new do
+    def m1 = 42
+    def m2() = 42
+    def m3(x) = x+1
+    def self.s1 = 42
+    def self.s2() = 42
+    def self.s3(x) = x + 1
+    def c1 = 42
+    def cm2() = p 42
+    def cm3(x) = p x+1
+    def self.cs1 = p 42
+    def self.cs2() = p 42
+    def self.cs3(x) = p x + 1
+  end
+end