checker: check missing return values in functions (#5803)

vlib/v/checker/checker.v

@@ -3060,7 +3060,7 @@ fn (c &Checker) fetch_and_verify_orm_fields(info table.Struct, pos token.Positio
 }
 
 fn (mut c Checker) fn_decl(mut node ast.FnDecl) {
-	c.returns = true
+	c.returns = false
 	if node.is_generic && c.cur_generic_type == 0 { // need the cur_generic_type check to avoid inf. recursion
 		// loop thru each generic type and generate a function
 		for gen_type in c.table.fn_gen_types[node.name] {

vlib/v/checker/tests/function_missing_return_type.out

@@ -0,0 +1,5 @@
+vlib/v/checker/tests/function_missing_return_type.v:1:1: error: missing return at end of function `h`
+    1 | fn h() int {
+      | ~~~~~~~~~~
+    2 | }
+    3 |

vlib/v/checker/tests/function_missing_return_type.vv

@@ -0,0 +1,7 @@
+fn h() int {
+}
+
+fn main() {
+	d := h()
+	println('$d')
+}