cgen: fix generic functions returning none or error values with generic result types (#16613)

vlib/v/gen/c/cgen.v

@@ -4387,7 +4387,7 @@ fn (g &Gen) expr_is_multi_return_call(expr ast.Expr) bool {
 }
 
 fn (mut g Gen) gen_result_error(target_type ast.Type, expr ast.Expr) {
-	styp := g.typ(target_type)
+	styp := g.typ(g.unwrap_generic(target_type))
 	g.write('(${styp}){ .is_error=true, .err=')
 	g.expr(expr)
 	g.write(', .data={EMPTY_STRUCT_INITIALIZATION} }')
@@ -4395,7 +4395,7 @@ fn (mut g Gen) gen_result_error(target_type ast.Type, expr ast.Expr) {
 
 // NB: remove this when optional has no errors anymore
 fn (mut g Gen) gen_optional_error(target_type ast.Type, expr ast.Expr) {
-	styp := g.typ(target_type)
+	styp := g.typ(g.unwrap_generic(target_type))
 	g.write('(${styp}){ .state=2, .err=')
 	g.expr(expr)
 	g.write(', .data={EMPTY_STRUCT_INITIALIZATION} }')

vlib/v/tests/generic_fn_returning_option_and_result_test.v

@@ -0,0 +1,27 @@
+module main
+
+fn get_value[T]() ?T {
+	return none
+}
+
+fn get_value_result[T]() !T {
+	return error('no result')
+}
+
+fn test_generic_function_that_returns_an_option() {
+	value := get_value[&int]() or { &int(0) }
+	assert value == unsafe { nil }
+	sval := get_value[string]() or { 'abc' }
+	assert sval == 'abc'
+	uval := get_value[u64]() or { 123 }
+	assert uval == 123
+}
+
+fn test_generic_function_that_returns_an_error() {
+	sval := get_value_result[string]() or { 'xyz' }
+	assert sval == 'xyz'
+	ival := get_value_result[int]() or { 456 }
+	assert ival == 456
+	pval := get_value_result[&int]() or { &int(789) }
+	assert u64(pval) == u64(&int(789))
+}