checker: check types with $for/$if introspection (#8984)

vlib/v/checker/checker.v

@@ -3109,11 +3109,10 @@ fn (mut c Checker) stmt(node ast.Stmt) {
 			c.branch_stmt(node)
 		}
 		ast.CompFor {
-			if node.typ > table.void_type {
-				sym := c.table.get_type_symbol(node.typ)
-				if sym.kind == .placeholder {
-					c.error('unknown type `$sym.name`', node.typ_pos)
-				}
+			typ := c.unwrap_generic(node.typ)
+			sym := c.table.get_type_symbol(typ)
+			if sym.kind == .placeholder || typ.has_flag(.generic) {
+				c.error('unknown type `$sym.name`', node.typ_pos)
 			}
 			c.stmts(node.stmts)
 		}
@@ -4806,7 +4805,11 @@ pub fn (mut c Checker) if_expr(mut node ast.IfExpr) table.Type {
 			if branch.cond is ast.InfixExpr {
 				if branch.cond.op == .key_is {
 					left := branch.cond.left
-					got_type := (branch.cond.right as ast.Type).typ
+					got_type := c.unwrap_generic((branch.cond.right as ast.Type).typ)
+					sym := c.table.get_type_symbol(got_type)
+					if sym.kind == .placeholder || got_type.has_flag(.generic) {
+						c.error('unknown type `$sym.name`', branch.cond.right.position())
+					}
 					if left is ast.SelectorExpr {
 						comptime_field_name = left.expr.str()
 						c.comptime_fields_type[comptime_field_name] = got_type

vlib/v/checker/tests/comptime_field_selector_not_name_err.out

@@ -12,7 +12,7 @@ vlib/v/checker/tests/comptime_field_selector_not_name_err.vv:12:12: error: unkno
       |                     ~~
    13 |         }
    14 |     }
-vlib/v/checker/tests/comptime_field_selector_not_name_err.vv:15:10: error: compile time field access can only be used when iterating over `T.fields`
+vlib/v/checker/tests/comptime_field_selector_not_name_err.vv:15:10: error: undefined ident: `f`
    13 |         }
    14 |     }
    15 |     _ = t.$(f.name)

vlib/v/checker/tests/comptime_for.out

@@ -3,10 +3,53 @@ vlib/v/checker/tests/comptime_for.vv:2:12: error: unknown type `Huh`
     2 |     $for m in Huh.methods {}
       |               ~~~
     3 |     $for f in Huh.fields {}
-    4 | }
+    4 |     $for f in T.fields {
 vlib/v/checker/tests/comptime_for.vv:3:12: error: unknown type `Huh`
     1 | fn unknown() {
     2 |     $for m in Huh.methods {}
     3 |     $for f in Huh.fields {}
       |               ~~~
-    4 | }
+    4 |     $for f in T.fields {
+    5 |         $if f.typ is Huh {}
+vlib/v/checker/tests/comptime_for.vv:4:12: error: unknown type `T`
+    2 |     $for m in Huh.methods {}
+    3 |     $for f in Huh.fields {}
+    4 |     $for f in T.fields {
+      |               ^
+    5 |         $if f.typ is Huh {}
+    6 |         $if f.typ is T {}
+vlib/v/checker/tests/comptime_for.vv:5:16: error: unknown type `Huh`
+    3 |     $for f in Huh.fields {}
+    4 |     $for f in T.fields {
+    5 |         $if f.typ is Huh {}
+      |                      ~~~
+    6 |         $if f.typ is T {}
+    7 |     }
+vlib/v/checker/tests/comptime_for.vv:6:16: error: unknown type `T`
+    4 |     $for f in T.fields {
+    5 |         $if f.typ is Huh {}
+    6 |         $if f.typ is T {}
+      |                      ^
+    7 |     }
+    8 |     _ = m
+vlib/v/checker/tests/comptime_for.vv:8:6: error: undefined ident: `m`
+    6 |         $if f.typ is T {}
+    7 |     }
+    8 |     _ = m
+      |         ^
+    9 | }
+   10 |
+vlib/v/checker/tests/comptime_for.vv:14:16: error: unknown type `U`
+   12 |     $for f in T.fields {
+   13 |         $if f.typ is T {}
+   14 |         $if f.typ is U {}
+      |                      ^
+   15 |     }
+   16 |     _ = f
+vlib/v/checker/tests/comptime_for.vv:16:6: error: undefined ident: `f`
+   14 |         $if f.typ is U {}
+   15 |     }
+   16 |     _ = f
+      |         ^
+   17 | }
+   18 |

vlib/v/checker/tests/comptime_for.vv

@@ -1,4 +1,23 @@
 fn unknown() {
 	$for m in Huh.methods {}
 	$for f in Huh.fields {}
+	$for f in T.fields {
+		$if f.typ is Huh {}
+		$if f.typ is T {}
+	}
+	_ = m
 }
+
+fn gf<T>() {
+	$for f in T.fields {
+		$if f.typ is T {}
+		$if f.typ is U {}
+	}
+	_ = f
+}
+
+struct S1 {
+	i int
+}
+
+gf<S1>()

vlib/v/parser/comptime.v

@@ -227,6 +227,7 @@ fn (mut p Parser) comp_for() ast.CompFor {
 	p.check(.dot)
 	for_val := p.check_name()
 	mut kind := ast.CompForKind.methods
+	p.open_scope()
 	if for_val == 'methods' {
 		p.scope.register(ast.Var{
 			name: val_var
@@ -247,6 +248,7 @@ fn (mut p Parser) comp_for() ast.CompFor {
 	}
 	spos := p.tok.position()
 	stmts := p.parse_block()
+	p.close_scope()
 	return ast.CompFor{
 		val_var: val_var
 		stmts: stmts