Modifying frozen objects will raise FrozenError.

`FrozenError` is a subclass of `RuntimeError` which used to be
raised.  [Ruby2.5]

include/mruby.h

@@ -1164,6 +1164,7 @@ MRB_API void mrb_print_error(mrb_state *mrb);
 #define E_SYNTAX_ERROR              (mrb_exc_get(mrb, "SyntaxError"))
 #define E_LOCALJUMP_ERROR           (mrb_exc_get(mrb, "LocalJumpError"))
 #define E_REGEXP_ERROR              (mrb_exc_get(mrb, "RegexpError"))
+#define E_FROZEN_ERROR              (mrb_exc_get(mrb, "FrozenError"))
 
 #define E_NOTIMP_ERROR              (mrb_exc_get(mrb, "NotImplementedError"))
 #ifndef MRB_WITHOUT_FLOAT

mrbgems/mruby-string-ext/mrblib/string.rb

@@ -95,7 +95,7 @@ class String
   #    "hello".lstrip!      #=> nil
   #
   def lstrip!
-    raise RuntimeError, "can't modify frozen String" if frozen?
+    raise FrozenError, "can't modify frozen String" if frozen?
     s = self.lstrip
     (s == self) ? nil : self.replace(s)
   end
@@ -125,7 +125,7 @@ class String
   #  <code>nil</code> if <i>str</i> was not altered.
   #
   def strip!
-    raise RuntimeError, "can't modify frozen String" if frozen?
+    raise FrozenError, "can't modify frozen String" if frozen?
     s = self.strip
     (s == self) ? nil : self.replace(s)
   end
@@ -199,7 +199,7 @@ class String
   #    string                  #=> "thsa sting"
   #
   def slice!(arg1, arg2=nil)
-    raise RuntimeError, "can't modify frozen String" if frozen?
+    raise FrozenError, "can't modify frozen String" if frozen?
     raise "wrong number of arguments (for 1..2)" if arg1.nil? && arg2.nil?
 
     if !arg1.nil? && !arg2.nil?

mrbgems/mruby-struct/src/struct.c

@@ -89,7 +89,7 @@ static void
 mrb_struct_modify(mrb_state *mrb, mrb_value strct)
 {
   if (MRB_FROZEN_P(mrb_basic_ptr(strct))) {
-    mrb_raise(mrb, E_RUNTIME_ERROR, "can't modify frozen struct");
+    mrb_raise(mrb, E_FROZEN_ERROR, "can't modify frozen struct");
   }
 
   mrb_write_barrier(mrb, mrb_basic_ptr(strct));

mrblib/10error.rb

@@ -51,6 +51,9 @@ end
 class NotImplementedError < ScriptError
 end
 
+class FrozenError < RuntimeError
+end
+
 class StopIteration < IndexError
   attr_accessor :result
 end

mrblib/string.rb

@@ -96,7 +96,7 @@ class String
   #
   # ISO 15.2.10.5.19
   def gsub!(*args, &block)
-    raise RuntimeError, "can't modify frozen String" if frozen?
+    raise FrozenError, "can't modify frozen String" if frozen?
     return to_enum(:gsub!, *args) if args.length == 1 && !block
     str = self.gsub(*args, &block)
     return nil if str == self
@@ -159,7 +159,7 @@ class String
   #
   # ISO 15.2.10.5.37
   def sub!(*args, &block)
-    raise RuntimeError, "can't modify frozen String" if frozen?
+    raise FrozenError, "can't modify frozen String" if frozen?
     str = self.sub(*args, &block)
     return nil if str == self
     self.replace(str)

src/array.c

@@ -114,7 +114,7 @@ static void
 ary_modify_check(mrb_state *mrb, struct RArray *a)
 {
   if (MRB_FROZEN_P(a)) {
-    mrb_raise(mrb, E_RUNTIME_ERROR, "can't modify frozen array");
+    mrb_raise(mrb, E_FROZEN_ERROR, "can't modify frozen array");
   }
 }
 

src/class.c

@@ -430,9 +430,9 @@ mrb_define_method_raw(mrb_state *mrb, struct RClass *c, mrb_sym mid, mrb_method_
 
   if (MRB_FROZEN_P(c)) {
     if (c->tt == MRB_TT_MODULE)
-      mrb_raise(mrb, E_RUNTIME_ERROR, "can't modify frozen module");
+      mrb_raise(mrb, E_FROZEN_ERROR, "can't modify frozen module");
     else
-      mrb_raise(mrb, E_RUNTIME_ERROR, "can't modify frozen class");
+      mrb_raise(mrb, E_FROZEN_ERROR, "can't modify frozen class");
   }
   if (!h) h = c->mt = kh_init(mt, mrb);
   k = kh_put(mt, mrb, h, mid);

src/hash.c

@@ -286,7 +286,7 @@ static void
 mrb_hash_modify(mrb_state *mrb, mrb_value hash)
 {
   if (MRB_FROZEN_P(mrb_hash_ptr(hash))) {
-    mrb_raise(mrb, E_RUNTIME_ERROR, "can't modify frozen hash");
+    mrb_raise(mrb, E_FROZEN_ERROR, "can't modify frozen hash");
   }
   mrb_hash_tbl(mrb, hash);
 }

src/string.c

@@ -494,7 +494,7 @@ static void
 check_frozen(mrb_state *mrb, struct RString *s)
 {
   if (MRB_FROZEN_P(s)) {
-    mrb_raise(mrb, E_RUNTIME_ERROR, "can't modify frozen string");
+    mrb_raise(mrb, E_FROZEN_ERROR, "can't modify frozen string");
   }
 }
 

src/variable.c

@@ -261,7 +261,7 @@ mrb_obj_iv_set(mrb_state *mrb, struct RObject *obj, mrb_sym sym, mrb_value v)
   iv_tbl *t = obj->iv;
 
   if (MRB_FROZEN_P(obj)) {
-    mrb_raisef(mrb, E_RUNTIME_ERROR, "can't modify frozen %S", mrb_obj_value(obj));
+    mrb_raisef(mrb, E_FROZEN_ERROR, "can't modify frozen %S", mrb_obj_value(obj));
   }
   if (!t) {
     t = obj->iv = iv_new(mrb);