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Programs that must fail #12

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Both compilers refuse this program — correctly

The program is broken on purpose. The right behaviour is a compile-time error instead of output, and that is what both compilers give.

The program

sub f($x, $x) { };

Run executes the program in your browser, with the Raku++ build this site ships. Edit it and try variations.

What each compiler printed

Rakudo 2026.08

(printed nothing)

Raku++ 5.0.0-g312cde8e

(printed nothing)

Error messages

Rakudo 2026.08

===SORRY!=== Error while compiling invalid-00000051.raku
Redeclaration of symbol '$x'.
at invalid-00000051.raku:1
------> sub f($x, $x<HERE>) { };
    expecting any of:
        shape declaration

Raku++ 5.0.0-g312cde8e

===SORRY!=== Parse error at line 1: Redeclaration of symbol '$x'
      1 | sub f($x, $x) { };
  at invalid-00000051.raku:1

The wording of an error message is up to each compiler; what matters is that both refuse the program.

invalid-v1 · seed 51 · recorded 2026-09-29 with Rakudo 2026.08 and Raku++ 5.0.0-g312cde8e · Source on GitHub ↗