checker: disallow static fn call when receiver type is unknown (#21970)

Author: Delta456

vlib/v/ast/ast.v

@@ -558,6 +558,7 @@ pub:
 	receiver_pos          token.Pos   // `(u User)` in `fn (u User) name()` position
 	is_method             bool
 	is_static_type_method bool      // true for `fn Foo.bar() {}`
+	static_type_pos       token.Pos // `Foo` in `fn Foo.bar() {}`
 	method_type_pos       token.Pos // `User` in ` fn (u User)` position
 	method_idx            int
 	rec_mut               bool // is receiver mutable

vlib/v/checker/fn.v

@@ -312,6 +312,13 @@ fn (mut c Checker) fn_decl(mut node ast.FnDecl) {
 				&& node.name.after_char(`.`) in reserved_type_names {
 				c.error('top level declaration cannot shadow builtin type', node.pos)
 			}
+			if _ := node.name.index('__static__') {
+				if sym := c.table.find_sym(node.name.all_before('__static__')) {
+					if sym.kind == .placeholder {
+						c.error('unknown type `${sym.name}`', node.static_type_pos)
+					}
+				}
+			}
 		}
 	}
 	if node.return_type != ast.Type(0) {

vlib/v/checker/tests/static_fn_call_no_struct_decl_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/static_fn_call_no_struct_decl_err.vv:5:4: error: unknown type `Mystery`
+    3 | }
+    4 | 
+    5 | fn Mystery.hey() {
+      |    ~~~~~~~
+    6 |     println('hey')
+    7 | }

vlib/v/checker/tests/static_fn_call_no_struct_decl_err.vv

@@ -0,0 +1,7 @@
+fn main() {
+	Mystery.hey()
+}
+
+fn Mystery.hey() {
+	println('hey')
+}

vlib/v/parser/fn.v

@@ -308,13 +308,15 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 	mut name := ''
 	mut type_sym := p.table.sym(rec.typ)
 	mut name_pos := p.tok.pos()
+	mut static_type_pos := p.tok.pos()
 	if p.tok.kind == .name {
 		mut check_name := ''
 		// TODO: high order fn
 		is_static_type_method = p.tok.lit.len > 0 && p.tok.lit[0].is_capital()
 			&& p.peek_tok.kind == .dot && language == .v // `fn Foo.bar() {}`
 		if is_static_type_method {
 			type_name := p.tok.lit // "Foo"
+			static_type_pos = p.tok.pos()
 			rec.typ = p.parse_type()
 			p.check(.dot)
 			check_name = p.check_name()
@@ -647,6 +649,7 @@ run them via `v file.v` instead',
 		receiver_pos: rec.pos
 		is_method: is_method
 		is_static_type_method: is_static_type_method
+		static_type_pos: static_type_pos
 		method_type_pos: rec.type_pos
 		method_idx: type_sym_method_idx
 		rec_mut: rec.is_mut