checker: fix generic fn with generic fn call returning generic map (fix #20106) (#20150)

Author: yuyi98

examples/graphs/bellman-ford.v

@@ -53,7 +53,7 @@ fn print_sol(dist []int) {
 // to all other vertices using Bellman-Ford algorithm.  The
 // function also detects negative weight cycle
 fn bellman_ford[T](graph [][]T, src int) {
-	mut edges := build_map_edges_from_graph(graph)
+	mut edges := build_map_edges_from_graph[int](graph)
 	// this function was done to adapt a graph representation
 	// by a adjacency matrix, to list of adjacency (using a MAP)
 	n_edges := edges.len // number of EDGES

vlib/v/checker/fn.v

@@ -1471,7 +1471,7 @@ fn (mut c Checker) fn_call(mut node ast.CallExpr, mut continue_check &bool) ast.
 	}
 
 	if func.generic_names.len > 0 {
-		if has_generic {
+		if has_generic || node.concrete_types.any(it.has_flag(.generic)) {
 			if typ := c.table.resolve_generic_to_concrete(func.return_type, func.generic_names,
 				node.concrete_types)
 			{

vlib/v/tests/generics_with_generics_fn_return_generics_map_type_test.v

@@ -0,0 +1,28 @@
+fn generic1[K, V](a map[K]V) string {
+	t := expect_map[K, V](a)
+	dump(t)
+	return '${t}'
+}
+
+fn generic2[K, V](a map[K]V) string {
+	t := expect_map(a)
+	dump(t)
+	return '${t}'
+}
+
+fn expect_map[K, V](a map[K]V) map[K]V {
+	return a
+}
+
+fn test_generics_with_generics_fn_return_map_type() {
+	a := {
+		'a': 1
+	}
+	b := {
+		1: 'a'
+	}
+	assert generic1(a) == "{'a': 1}"
+	assert generic1(b) == "{1: 'a'}"
+	assert generic2(a) == "{'a': 1}"
+	assert generic2(b) == "{1: 'a'}"
+}

vlib/v/tests/multiple_generic_resolve_test.v

@@ -9,16 +9,17 @@ fn pre_start[T, R](app T, config R) string {
 	return start(app, config.val)
 }
 
-fn start[T, R](app T, otherthing R) R {
+fn start[T, R](app T, otherthing R) string {
 	println(otherthing)
-	return otherthing
+	return '${otherthing}'
 }
 
-fn test_main() {
+fn test_multiple_generic_resolve() {
 	app := App{}
 	testval := 'hello'
 	config := Config[string]{
 		val: testval
 	}
+
 	assert pre_start(app, config) == 'hello'
 }