checker: disallow void return value lambdas for map

vlib/v/checker/fn.v

@@ -2739,6 +2739,14 @@ fn (mut c Checker) check_map_and_filter(is_map bool, elem_typ ast.Type, node ast
 				c.error('array append cannot be used in an expression', arg_expr.pos)
 			}
 		}
+		ast.LambdaExpr {
+			if arg_expr.expr is ast.CallExpr {
+				if is_map && arg_expr.expr.return_type in [ast.void_type, 0] {
+					c.error('type mismatch, `${arg_expr.expr.name}` does not return anything',
+						arg_expr.expr.pos)
+				}
+			}
+		}
 		else {}
 	}
 }

vlib/v/checker/tests/map_lambda_void_return_err.out

@@ -0,0 +1,4 @@
+vlib/v/checker/tests/map_lambda_void_return_err.vv:2:13: error: type mismatch, `println` does not return anything
+    1 | arr := ['a', 'b', 'c']
+    2 | arr.map(|x| println(x))
+      |             ~~~~~~~~~~

vlib/v/checker/tests/map_lambda_void_return_err.vv

@@ -0,0 +1,2 @@
+arr := ['a', 'b', 'c']
+arr.map(|x| println(x))