checker: fix generic fn with nested generic fn call (fix #18285) (#18314)

vlib/v/checker/fn.v

@@ -1354,13 +1354,23 @@ fn (mut c Checker) fn_call(mut node ast.CallExpr, mut continue_check &bool) ast.
 				}
 			}
 			return node.return_type
-		} else if typ := c.table.resolve_generic_to_concrete(func.return_type, func.generic_names,
-			concrete_types)
-		{
-			if typ.has_flag(.generic) {
-				node.return_type = typ
+		} else {
+			if node.concrete_types.len > 0 && !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)
+				{
+					node.return_type = typ
+					return typ
+				}
+			}
+			if typ := c.table.resolve_generic_to_concrete(func.return_type, func.generic_names,
+				concrete_types)
+			{
+				if typ.has_flag(.generic) {
+					node.return_type = typ
+				}
+				return typ
 			}
-			return typ
 		}
 	}
 	return func.return_type

vlib/v/tests/generic_fn_with_nested_generic_fn_call_test.v

@@ -0,0 +1,14 @@
+struct Test {}
+
+fn unmarshal[T]() ! {
+	get_number[int]()!
+}
+
+fn get_number[T]() !T {
+	return T(42)
+}
+
+fn test_generic_fn_with_nested_generic_fn_call() {
+	unmarshal[Test]()!
+	assert true
+}