this ternary operator is an undefined behaviour

Let me quote ISO/IEC 9899:2018 section 6.5.15:

> Constraints
>
> The first operand shall have scalar type.
> One of the following shall hold for the second and third operands:
> — both operands have arithmetic type;
> — both operands have the same structure or union type;
> — both operands have void type;
(snip)

Here, `*option` is a const struct rb_compile_option_struct. OTOH
`COMPILE_OPTION_DEFAULT` is a struct rb_compile_option_struct, without
const.   These two are _not_ the "same structure or union type".  Hence
the expression renders undefined behaviour.  COMPILE_OPTION_DEFAULT is
not a const because `RubyVM::InstructionSequence.compile_option=`
touches its internals on-the-fly.  There is no way to meet the
constraints quoted above.

Using ternary operator here was a mistake at the first place.  Let's
just replace it with a normal `if` statement.

1 files changed, 6 insertions, 1 deletions

diff --git a/iseq.c b/iseq.c
index a294f08527..b06f23c33d 100644
--- a/iseq.c
+++ b/iseq.c
@@ -825,7 +825,12 @@ rb_iseq_new_with_opt(const rb_ast_body_t *ast, VALUE name, VALUE path, VALUE rea
     rb_iseq_t *iseq = iseq_alloc();
     rb_compile_option_t new_opt;
 
-    new_opt = option ? *option : COMPILE_OPTION_DEFAULT;
+    if (option) {
+        new_opt = *option;
+    }
+    else {
+        new_opt = COMPILE_OPTION_DEFAULT;
+    }
     if (ast && ast->compile_option) rb_iseq_make_compile_option(&new_opt, ast->compile_option);
 
     prepare_iseq_build(iseq, name, path, realpath, first_lineno, node ? &node->nd_loc : NULL, node ? nd_node_id(node) : -1, parent, type, &new_opt);