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    <p>There is some special handling in the source to suppress the "duplicate argument name" error. The error message only appears in <code>shadowing_lvar_gen</code> inside <code>parse.y</code>, <a href="https://github.com/ruby/ruby/blob/ruby_1_9_3/parse.y#L8415" rel="noreferrer">the 1.9.3 version looks like this</a>:</p> <pre><code>static ID shadowing_lvar_gen(struct parser_params *parser, ID name) { if (idUScore == name) return name; /* ... */ </code></pre> <p>and <code>idUScore</code> is <a href="https://github.com/ruby/ruby/blob/ruby_1_9_3/id.c#L51" rel="noreferrer">defined in <code>id.c</code></a> like this:</p> <pre><code>REGISTER_SYMID(idUScore, "_"); </code></pre> <p>You'll see similar special handling in <a href="https://github.com/ruby/ruby/blob/ruby_1_9_3/parse.y#L9113" rel="noreferrer"><code>warn_unused_var</code></a>:</p> <pre><code>static void warn_unused_var(struct parser_params *parser, struct local_vars *local) { /* ... */ for (i = 0; i &lt; cnt; ++i) { if (!v[i] || (u[i] &amp; LVAR_USED)) continue; if (idUScore == v[i]) continue; rb_compile_warn(ruby_sourcefile, (int)u[i], "assigned but unused variable - %s", rb_id2name(v[i])); } } </code></pre> <p>You'll notice that the warning is suppressed on the second line of the <code>for</code> loop.</p> <p>The only special handling of <code>_</code> that I could find in the 1.9.3 source is above: the duplicate name error is suppressed and the unused variable warning is suppressed. Other than those two things, <code>_</code> is just a plain old variable like any other. I don't know of any documentation about the (minor) specialness of <code>_</code>.</p> <p>In Ruby 2.0, the <code>idUScore == v[i]</code> test in <a href="https://github.com/ruby/ruby/blob/ruby_2_0_0/parse.y#L9456" rel="noreferrer"><code>warn_unused_var</code></a> is replaced with a call to <code>is_private_local_id</code>:</p> <pre><code>if (is_private_local_id(v[i])) continue; rb_warn4S(ruby_sourcefile, (int)u[i], "assigned but unused variable - %s", rb_id2name(v[i])); </code></pre> <p>and <a href="https://github.com/ruby/ruby/blob/ruby_2_0_0/parse.y#L8606" rel="noreferrer"><code>is_private_local_id</code></a> suppresses warnings for variables that begin with <code>_</code>:</p> <pre><code>if (name == idUScore) return 1; /* ... */ return RSTRING_PTR(s)[0] == '_'; </code></pre> <p>rather than just <code>_</code> itself. So 2.0 loosens things up a bit.</p>
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    1. POWhere and how is the _ (underscore) variable specified?
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    1. POWhere and how is the _ (underscore) variable specified?
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    1. COI wonder if the difference in behavior of `lambda { |_, _| _ }.call(4, 2)` between 1.8 & 1.9 is just an unintentional side-effect then? As under "normal" circumstances where the variable name can't be duplicated the order in which they're assigned is inconsequential.
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    2. COWow, you beat me to it. +1
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    3. CO@AndrewMarshall: Yes, I think the "4 vs 2" issue is just an artifact of how 1.8 and 1.9 handle the stack. The only time it would be noticeable is `|_,_,...|` because the duplicate-error has been suppressed.
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