parser: fix if-guard redefinition (#9425)

vlib/v/parser/if_match.v

@@ -87,6 +87,9 @@ fn (mut p Parser) if_expr(is_comptime bool) ast.IfExpr {
 			is_guard = true
 			var_pos := p.tok.position()
 			var_name := p.check_name()
+			if p.scope.known_var(var_name) {
+				p.error_with_pos('redefinition of `$var_name`', var_pos)
+			}
 			comments << p.eat_comments({})
 			p.check(.decl_assign)
 			comments << p.eat_comments({})

vlib/v/parser/tests/if_guard_redefinition.out

@@ -0,0 +1,7 @@
+vlib/v/parser/tests/if_guard_redefinition.vv:7:8: error: redefinition of `x`
+    5 | fn main() {
+    6 |     x := 1
+    7 |     if x := opt_fn() {
+      |        ^
+    8 |         println(x)
+    9 |     }

vlib/v/parser/tests/if_guard_redefinition.vv

@@ -0,0 +1,10 @@
+fn opt_fn() ?int {
+    return 2
+}
+
+fn main() {
+    x := 1
+    if x := opt_fn() {
+        println(x)
+    }
+}