checker: fix returning expression with void type (fix #15543) (#15554)

vlib/builtin/cfns.c.v

@@ -321,7 +321,7 @@ fn C.WriteConsole() voidptr
 
 fn C.WriteFile() voidptr
 
-fn C._wchdir(dirname &u16)
+fn C._wchdir(dirname &u16) int
 
 fn C._wgetcwd(buffer &u16, maxlen int) int
 

vlib/v/checker/if.v

@@ -194,6 +194,14 @@ pub fn (mut c Checker) if_expr(mut node ast.IfExpr) ast.Type {
 						node.typ = c.expected_type
 					}
 				}
+				if last_expr.typ == ast.void_type && !is_noreturn_callexpr(last_expr.expr)
+					&& !c.skip_flags {
+					// cannot return void type and use it as expr in any circumstances
+					// (e.g. argument expression, variable declaration / assignment)
+					c.error('the final expression in `if` or `match`, must have a value of a non-void type',
+						last_expr.pos)
+					continue
+				}
 				if !c.check_types(last_expr.typ, node.typ) {
 					if node.typ == ast.void_type {
 						// first branch of if expression

vlib/v/checker/tests/if_match_result.out

@@ -25,6 +25,13 @@ vlib/v/checker/tests/if_match_result.vv:11:3: error: assignment mismatch: 1 vari
       |   ^
    12 |     1 { println('') }
    13 |     else { exit(0) }
+vlib/v/checker/tests/if_match_result.vv:16:2: error: the final expression in `if` or `match`, must have a value of a non-void type
+   14 | }
+   15 | _ = if true {
+   16 |     println('')
+      |     ~~~~~~~~~~~
+   17 | } else {
+   18 |     exit(0)
 vlib/v/checker/tests/if_match_result.vv:15:3: error: assignment mismatch: 1 variable(s) 0 value(s)
    13 |     else { exit(0) }
    14 | }