checker: fix missing concrete type checking on a generic type specifier (#21614)

vlib/v/checker/tests/missing_concrete_type_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/missing_concrete_type_err.vv:4:6: error: missing concrete type on generic type
+    2 | 
+    3 | fn main() {
+    4 |     bar[Foo]()
+      |         ~~~
+    5 | }
+    6 |

vlib/v/checker/tests/missing_concrete_type_err.vv

@@ -0,0 +1,9 @@
+struct Foo[T] {}
+
+fn main() {
+	bar[Foo]()
+}
+
+fn bar[T]() {
+	
+}

vlib/v/parser/parse_type.v

@@ -542,6 +542,11 @@ fn (mut p Parser) parse_type() ast.Type {
 			return 0
 		}
 		sym := p.table.sym(typ)
+		if p.inside_fn_concrete_type && sym.info is ast.Struct {
+			if !typ.has_flag(.generic) && sym.info.generic_types.len > 0 {
+				p.error_with_pos('missing concrete type on generic type', option_pos.extend(p.prev_tok.pos()))
+			}
+		}
 		if is_option && sym.info is ast.SumType && sym.info.is_anon {
 			p.error_with_pos('an inline sum type cannot be an Option', option_pos.extend(p.prev_tok.pos()))
 		}

vlib/v/parser/parser.v

@@ -47,6 +47,7 @@ mut:
 	inside_for_expr           bool
 	inside_fn                 bool // true even with implicit main
 	inside_fn_return          bool
+	inside_fn_concrete_type   bool // parsing fn_name[concrete_type]() call expr
 	inside_call_args          bool // true inside f(  ....  )
 	inside_unsafe_fn          bool
 	inside_str_interp         bool
@@ -3369,6 +3370,10 @@ fn (mut p Parser) parse_concrete_types() []ast.Type {
 	if p.tok.kind !in [.lt, .lsbr] {
 		return types
 	}
+	p.inside_fn_concrete_type = true
+	defer {
+		p.inside_fn_concrete_type = false
+	}
 	end_kind := if p.tok.kind == .lt { token.Kind.gt } else { token.Kind.rsbr }
 	p.next() // `<`
 	mut first_done := false