checker: fix lambda generic param and return validation

vlib/v/ast/table.v

@@ -37,7 +37,8 @@ pub mut:
 	panic_handler      FnPanicHandler = default_table_panic_handler
 	panic_userdata     voidptr        = unsafe { nil } // can be used to pass arbitrary data to panic_handler;
 	panic_npanics      int
-	cur_fn             &FnDecl = unsafe { nil } // previously stored in Checker.cur_fn and Gen.cur_fn
+	cur_fn             &FnDecl     = unsafe { nil } // previously stored in Checker.cur_fn and Gen.cur_fn
+	cur_lambda         &LambdaExpr = unsafe { nil } // current lambda node
 	cur_concrete_types []Type // current concrete types, e.g. <int, string>
 	gostmts            int    // how many `go` statements there were in the parsed files.
 	// When table.gostmts > 0, __VTHREADS__ is defined, which can be checked with `$if threads {`

vlib/v/checker/checker.v

@@ -2735,6 +2735,13 @@ fn (mut c Checker) unwrap_generic(typ ast.Type) ast.Type {
 			{
 				return t_typ
 			}
+			if c.inside_lambda && c.table.cur_lambda.call_ctx != unsafe { nil } {
+				if t_typ := c.table.resolve_generic_to_concrete(typ, c.table.cur_lambda.func.decl.generic_names,
+					c.table.cur_lambda.call_ctx.concrete_types)
+				{
+					return t_typ
+				}
+			}
 		}
 	}
 	return typ
@@ -2971,8 +2978,10 @@ pub fn (mut c Checker) expr(mut node ast.Expr) ast.Type {
 		}
 		ast.LambdaExpr {
 			c.inside_lambda = true
+			c.table.cur_lambda = unsafe { &node }
 			defer {
 				c.inside_lambda = false
+				c.table.cur_lambda = unsafe { nil }
 			}
 			return c.lambda_expr(mut node, c.expected_type)
 		}

vlib/v/checker/fn.v

@@ -1234,6 +1234,10 @@ fn (mut c Checker) fn_call(mut node ast.CallExpr, mut continue_check &bool) ast.
 		for i, mut call_arg in node.args {
 			if call_arg.expr is ast.CallExpr {
 				node.args[i].typ = c.expr(mut call_arg.expr)
+			} else if mut call_arg.expr is ast.LambdaExpr {
+				if node.concrete_types.len > 0 {
+					call_arg.expr.call_ctx = unsafe { node }
+				}
 			}
 		}
 		c.check_expected_arg_count(mut node, func) or { return func.return_type }
@@ -1608,7 +1612,7 @@ fn (mut c Checker) fn_call(mut node ast.CallExpr, mut continue_check &bool) ast.
 						}
 						if mut call_arg.expr is ast.LambdaExpr {
 							// Calling fn is generic and lambda arg also is generic
-							if node.concrete_types.len > 0
+							if node.concrete_types.len > 0 && call_arg.expr.func != unsafe { nil }
 								&& call_arg.expr.func.decl.generic_names.len > 0 {
 								call_arg.expr.call_ctx = unsafe { node }
 								if c.table.register_fn_concrete_types(call_arg.expr.func.decl.fkey(),

vlib/v/checker/return.v

@@ -261,9 +261,8 @@ fn (mut c Checker) return_stmt(mut node ast.Return) {
 				} else {
 					got_type_sym.name
 				}
-				// ignore generic casting expr on lambda in this phase
-				if c.inside_lambda && exp_type.has_flag(.generic)
-					&& node.exprs[expr_idxs[i]] is ast.CastExpr {
+				// ignore generic lambda return in this phase
+				if c.inside_lambda && exp_type.has_flag(.generic) {
 					continue
 				}
 				c.error('cannot use `${got_type_name}` as ${c.error_type_name(exp_type)} in return argument',

vlib/v/tests/generics/lamda_param_and_ret_test.v

@@ -0,0 +1,7 @@
+import arrays
+
+fn test_main() {
+	items := ['item1', 'item2', 'item3']
+	list := arrays.map_indexed[string, string](items, |i, item| '${i}. ${item}')
+	assert list == ['0. item1', '1. item2', '2. item3']
+}