From 37e7b987160ce8688c9a0acf06e0ba96efbe0916 Mon Sep 17 00:00:00 2001
From: Jeremy Evans <code@jeremyevans.net>
Date: Thu, 17 Oct 2019 15:41:06 -0700
Subject: [PATCH] Refine error message for subclassing a non-class

Previously, code such as:

```ruby
class C1; end
class C2 < C1.new; end
```

Resulted in an error message:

```
TypeError (superclass must be a Class (C1 given))
```

This may confuse users as C1 is a class.

This changes the error message to:

```
TypeError (superclass must be an instance of Class (given an instance of C1))
```

This makes it clear that the wrong type of of was given.

Fixes [Bug #14726]
---
 class.c                          | 2 +-
 spec/ruby/core/class/new_spec.rb | 2 +-
 vm_insnhelper.c                  | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/class.c b/class.c
index 010faed3eb..40f21d56cc 100644
--- a/class.c
+++ b/class.c
@@ -220,7 +220,7 @@ void
 rb_check_inheritable(VALUE super)
 {
     if (!RB_TYPE_P(super, T_CLASS)) {
-	rb_raise(rb_eTypeError, "superclass must be a Class (%"PRIsVALUE" given)",
+        rb_raise(rb_eTypeError, "superclass must be an instance of Class (given an instance of %"PRIsVALUE")",
 		 rb_obj_class(super));
     }
     if (RBASIC(super)->flags & FL_SINGLETON) {
diff --git a/spec/ruby/core/class/new_spec.rb b/spec/ruby/core/class/new_spec.rb
index 8191ce6a37..aa9a1881e7 100644
--- a/spec/ruby/core/class/new_spec.rb
+++ b/spec/ruby/core/class/new_spec.rb
@@ -95,7 +95,7 @@ def message2; "hello"; end
   end
 
   it "raises a TypeError when given a non-Class" do
-    error_msg = /superclass must be a Class/
+    error_msg = /superclass must be a.*Class/
     -> { Class.new("")         }.should raise_error(TypeError, error_msg)
     -> { Class.new(1)          }.should raise_error(TypeError, error_msg)
     -> { Class.new(:symbol)    }.should raise_error(TypeError, error_msg)
diff --git a/vm_insnhelper.c b/vm_insnhelper.c
index c7dd213035..437266bce7 100644
--- a/vm_insnhelper.c
+++ b/vm_insnhelper.c
@@ -3596,7 +3596,7 @@ vm_define_class(ID id, rb_num_t flags, VALUE cbase, VALUE super)
 
     if (VM_DEFINECLASS_HAS_SUPERCLASS_P(flags) && !RB_TYPE_P(super, T_CLASS)) {
 	rb_raise(rb_eTypeError,
-		 "superclass must be a Class (%"PRIsVALUE" given)",
+                 "superclass must be an instance of Class (given an instance of %"PRIsVALUE")",
 		 rb_obj_class(super));
     }
 
-- 
2.23.0