interpreter: Correct return value state handling

In cases where a script module function returns
void, while also calling an internal function which
does not return void, the return type of the internal
function ends up as the expected return type of the
outer void-returning function, yielding type failure.

Admittedly this is a very contrived example and only
likely to show up when prototyping new script modules
but it was nonetheless causing me difficulties in
exactly that situation so I have fixed it and
extended the script_module test to cover this case.

src/functions/common.c

@@ -763,6 +763,7 @@ func_obj_call(struct workspace *wk, struct obj_func *f, obj args, obj *res)
 	*res = wk->returned;
 	/* LO("%s returned %o\n", f->name, wk->returned); */
 	wk->returning = false;
+	wk->returned = 0;
 
 	if (ret && !typecheck_custom(wk, wk->return_node, *res, f->return_type,
 		"function returned invalid type, expected %s, got %s")) {

src/script/modules/_test.meson

@@ -100,6 +100,17 @@ func multi_return(k str) -> int
     return d[k]
 endfunc
 
+func returns_string() -> str
+    return '10'
+endfunc
+
+# this should return void and muon should not raise an error stating that
+# it expected a return of 'str' due to the internal call to `returns_string`
+func returns_void()
+    val_str = returns_string()
+    message(val_str)
+endfunc
+
 func project() -> dict[any]
     return meson.project()
 endfunc
@@ -120,5 +131,6 @@ return {
     'glob_arg': glob_arg,
     'typed_list': typed_list,
     'multi_return': multi_return,
+    'returns_void': returns_void,
     'project': project,
 }

tests/project/muon/script_module/meson.build

@@ -84,6 +84,9 @@ assert(m.multi_return('a') == 1)
 assert(m.multi_return('b') == 2)
 assert(m.multi_return('c') == 0)
 
+# test internal calls not changing interpreter state.
+m.returns_void()
+
 # test meson.project()
 assert('c' not in m.project()['compilers'])
 add_languages('c')