Test octal warnings in REPL

RT#119339

- 0-prefixed number must warn about usin 0o prefix
- This warning must not suggest the user uses invalid octal, e.g. 0o9
- 0o prefix on valid octal must work in REPL

S19-command-line/repl.t

@@ -1,9 +1,39 @@
 use v6;
 use Test;
-plan 1;
+plan 4;
 
 # Sanity check that the repl is working at all.
 my $cmd = $*DISTRO.is-win
     ?? "echo exit(42)   | $*EXECUTABLE 1>&2"
     !! "echo 'exit(42)' | $*EXECUTABLE >/dev/null 2>&1";
 is shell($cmd).exitcode, 42, 'exit(42) in executed REPL got run';
+
+{
+    # RT #119339
+    subtest { plan 2;
+        my $proc = run $*EXECUTABLE, :in, :out, :err;
+        $proc.in.print: "say 069\nexit\n";
+        like $proc.out.slurp-rest, /'69'/, 'stdout contains the number';
+        like $proc.err.slurp-rest, /'Potential difficulties:'
+            .* "Leading 0 is not allowed. For octals, use '0o' prefix,"
+            .* 'but note that 69 is not a valid octal number'
+        /, 'stderr contains correct warning';
+    }, 'prefix 0 on invalid octal warns in REPL';
+
+    subtest { plan 2;
+        my $proc = run $*EXECUTABLE, :in, :out, :err;
+        $proc.in.print: "say 067\nexit\n";
+        like $proc.out.slurp-rest, /'67'/, 'stdout contains the number';
+        like $proc.err.slurp-rest, /'Potential difficulties:'
+            .* 'Leading 0 does not indicate octal in Perl 6.'
+            .* 'Please use 0o67 if you mean that.'
+        /, 'stderr contains correct warning';
+    }, 'prefix 0 on valid octal warns in REPL';
+
+    subtest { plan 2;
+        my $proc = run $*EXECUTABLE, :in, :out, :err;
+        $proc.in.print: "say 0o67\nexit\n";
+        like $proc.out.slurp-rest, /'55'/, 'stdout contains the number';
+        is   $proc.err.slurp-rest,     '', 'stderr is empty';
+    }, 'prefix 0o on valid octal works fine in REPL';
+}