Start on tests for definite return values

S06-signature/definite-return.t

@@ -0,0 +1,106 @@
+use v6;
+
+use Test;
+
+plan 9;
+
+# L<S06/Signatures>
+
+# Nil is treated as definite value in return specifications
+#?rakudo skip 'definite values as return specifications dont parse yet'
+{
+    my sub return-nil(--> Nil) {
+        1
+    }
+
+    ok return-nil() =:= Nil, 'A function with a definite return value should ignore the result of its last statement';
+}
+
+#?rakudo skip 'definite values as return specifications dont parse yet'
+{
+    my sub return-two(--> 2) {
+        3
+    }
+
+    is return-two(), 2, 'A function with a non-Nil definite return value should ignore the result of its last statement';
+}
+
+#?rakudo skip 'definite values as return specifications dont parse yet'
+{
+    my $code = q:to/PERL/;
+    my sub return-nil(--> Nil) {
+        return 1
+    }
+PERL
+    eval_dies_ok($code, 'A function with a definite return value may not use return with a value')
+}
+
+#?rakudo skip 'definite values as return specifications dont parse yet'
+{
+    my $code = q:to/PERL/;
+    my sub return-failure(--> Nil) {
+        return Failure.new(X::Adhoc.new(4))
+    }
+PERL
+    eval_dies_ok($code, 'A function with a definite return value may not use return with a value, even a Failure')
+}
+
+#?rakudo skip 'definite values as return specifications dont parse yet'
+{
+    my sub fail-five(--> 5) {
+        fail 5
+    }
+
+    my $failure = fail-five();
+    isnt $failure, 5, 'Failures bypass the return value of function with a definite return value in the signature';
+    ok $failure === Failure, 'Failures bypass the return value of function with a definite return value in the signature';
+}
+
+#?rakudo skip 'definite values as return specifications dont parse yet'
+{
+    my sub return-six(--> 6) {
+        return
+    }
+
+    is return-six(), 6, 'Value-less returns are allowed in functions with definite return value in their signatures';
+}
+
+
+#?rakudo skip 'definite values as return specifications dont parse yet'
+{
+    my sub return-seven(--> $result) {
+        $result = 7;
+        8
+    }
+
+    is return-seven(), 7, 'Functions using a mutable value as their return value use that value as their return value';
+}
+
+
+#?rakudo skip 'definite values as return specifications dont parse yet'
+{
+    my $result = 9;
+
+    my sub return-eight(--> $result) {
+        $result = 8;
+        19
+    }
+
+    is eight(), 8, 'Variables used as return specifications *always* shadow existing variables with the same name';
+}
+
+# returns vs -->
+# indefinite vs immutable vs mutable
+# what are some wacky immutable values that we could return? (*)
+# mutable: existing variable (can you put a type on this?)
+# mutable: new variable (can you put a type on this?)
+# mutable: default value (pre-existing vs new)
+# mutable: non variable (think rw function or something)
+# what other things can we do to variables/parameters that we could do to the return variable?
+# anonymous subs, nested subs
+# if !indefinite, final statement should be in sink (and not item/list) context
+# S26 for return params...
+# coerce Bool from Junction in return value?
+# use of CALLER::/OUTER:: as return value (FROGGS' example)
+# Should sub return-two(--> 2) { 3 } emit an error/warning?
+# traits - how to distinguish a trait on the return value vs on the sub?