Make invalid & operator type error message consistent with */**

If #to_proc is defined, this uses the following error message format,
matching the error message format used for * when to_a returns non-Array
and for ** when to_hash returns non-Hash:

```
can't convert ClassName to Proc (ClassName#to_proc gives OtherClassName)
```

If #to_proc is not defined, this uses the following error message format,
matching the error message format used when ** is called on a non-Hash
not implementing to_hash.

```
no implicit conversion of ClassName into Proc
```

There isn't a similar error for * when called on a non-Array not
implementing to_a, as Ruby does not raise for that case.

Fixes [Bug #21563]

test/ruby/test_exception.rb

@@ -1525,4 +1525,31 @@ $stderr = $stdout; raise "\x82\xa0"') do |outs, errs, status|
       assert_in_out_err(%W[-r#{lib} #{main}], "", [], [:*, "\n""path=#{main}\n", :*])
     end
   end
+
+  class Ex; end
+
+  def test_exception_message_for_unexpected_implicit_conversion_type
+    a = Ex.new
+    def self.x(a) = nil
+
+    assert_raise_with_message(TypeError, "no implicit conversion of TestException::Ex into Hash") do
+      x(**a)
+    end
+    assert_raise_with_message(TypeError, "no implicit conversion of TestException::Ex into Proc") do
+      x(&a)
+    end
+
+    def a.to_a = 1
+    def a.to_hash = 1
+    def a.to_proc = 1
+    assert_raise_with_message(TypeError, "can't convert TestException::Ex to Array (TestException::Ex#to_a gives Integer)") do
+      x(*a)
+    end
+    assert_raise_with_message(TypeError, "can't convert TestException::Ex to Hash (TestException::Ex#to_hash gives Integer)") do
+      x(**a)
+    end
+    assert_raise_with_message(TypeError, "can't convert TestException::Ex to Proc (TestException::Ex#to_proc gives Integer)") do
+      x(&a)
+    end
+  end
 end

vm_args.c

@@ -1045,9 +1045,17 @@ vm_to_proc(VALUE proc)
         }
 
         if (NIL_P(b) || !rb_obj_is_proc(b)) {
-            rb_raise(rb_eTypeError,
-                     "wrong argument type %s (expected Proc)",
-                     rb_obj_classname(proc));
+            if (me) {
+                VALUE cname = rb_obj_class(proc);
+                rb_raise(rb_eTypeError,
+                         "can't convert %"PRIsVALUE" to Proc (%"PRIsVALUE"#to_proc gives %"PRIsVALUE")",
+                         cname, cname, rb_obj_class(b));
+            }
+            else {
+                rb_raise(rb_eTypeError,
+                         "no implicit conversion of %s into Proc",
+                         rb_obj_classname(proc));
+            }
         }
         return b;
     }