checker: check unused last expression in if (fix #16084) (#16088)

vlib/os/filelock/lib_windows.c.v

@@ -55,9 +55,6 @@ fn open(f string) voidptr {
 	// locking it
 	fd := C.CreateFileW(f_wide, C.GENERIC_READ | C.GENERIC_WRITE, 0, 0, C.OPEN_ALWAYS,
 		C.FILE_ATTRIBUTE_NORMAL, 0)
-	if fd == C.INVALID_HANDLE_VALUE {
-		fd == -1
-	}
 	return fd
 }
 

vlib/v/checker/if.v

@@ -140,6 +140,7 @@ pub fn (mut c Checker) if_expr(mut node ast.IfExpr) ast.Type {
 					c.stmts_ending_with_expression(branch.stmts)
 				} else {
 					c.stmts(branch.stmts)
+					c.check_non_expr_branch_last_stmt(branch.stmts)
 				}
 			} else if c.pref.output_cross_c {
 				mut is_freestanding_block := false
@@ -156,6 +157,7 @@ pub fn (mut c Checker) if_expr(mut node ast.IfExpr) ast.Type {
 					c.stmts_ending_with_expression(branch.stmts)
 				} else {
 					c.stmts(branch.stmts)
+					c.check_non_expr_branch_last_stmt(branch.stmts)
 				}
 			} else if !is_comptime_type_is_expr {
 				node.branches[i].stmts = []
@@ -174,6 +176,7 @@ pub fn (mut c Checker) if_expr(mut node ast.IfExpr) ast.Type {
 				c.stmts_ending_with_expression(branch.stmts)
 			} else {
 				c.stmts(branch.stmts)
+				c.check_non_expr_branch_last_stmt(branch.stmts)
 			}
 			c.smartcast_mut_pos = token.Pos{}
 			c.smartcast_cond_pos = token.Pos{}
@@ -361,3 +364,16 @@ fn (mut c Checker) smartcast_if_conds(node ast.Expr, mut scope ast.Scope) {
 		c.smartcast_if_conds(node.expr, mut scope)
 	}
 }
+
+fn (mut c Checker) check_non_expr_branch_last_stmt(stmts []ast.Stmt) {
+	if stmts.len > 0 {
+		last_stmt := stmts.last()
+		if last_stmt is ast.ExprStmt {
+			if last_stmt.expr is ast.InfixExpr {
+				if last_stmt.expr.op !in [.left_shift, .right_shift, .unsigned_right_shift, .arrow] {
+					c.error('expression evaluated but not used', last_stmt.pos)
+				}
+			}
+		}
+	}
+}

vlib/v/checker/tests/unused_last_expr_stmt_in_if.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/unused_last_expr_stmt_in_if.vv:16:5: error: expression evaluated but not used
+   14 |     if a.mode == .zz {
+   15 |         println('hello')
+   16 |         a.mode == .yy
+      |           ~~~~~~~~~~~
+   17 |     }
+   18 | }

vlib/v/checker/tests/unused_last_expr_stmt_in_if.vv

@@ -0,0 +1,18 @@
+enum MyEnum {
+	xx
+	yy
+	zz
+}
+
+struct Abc {
+mut:
+	mode MyEnum
+}
+
+fn main() {
+	a := Abc{}
+	if a.mode == .zz {
+		println('hello')
+		a.mode == .yy
+	}
+}