parser: fix panic when single letter receiver parsed (#8381)

vlib/v/checker/checker.v

@@ -5296,7 +5296,7 @@ fn (mut c Checker) fn_decl(mut node ast.FnDecl) {
 			sym := c.table.get_type_symbol(arg.typ)
 			if sym.kind == .placeholder
 				|| (sym.kind in [table.Kind.int_literal, .float_literal] && !c.is_builtin_mod) {
-				c.error('unknown type `$sym.name`', node.pos)
+				c.error('unknown type `$sym.name`', node.receiver_pos)
 			}
 		}
 	}

vlib/v/checker/tests/receiver_unknown_type_single_letter.out

@@ -0,0 +1,3 @@
+vlib/v/checker/tests/receiver_unknown_type_single_letter.vv:1:5: error: unknown type `A`
+    1 | fn (p A) foo() {}
+      |     ~~~

vlib/v/checker/tests/receiver_unknown_type_single_letter.vv

@@ -0,0 +1 @@
+fn (p A) foo() {}

vlib/v/parser/fn.v

@@ -190,6 +190,7 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 	mut rec_mut := false
 	mut params := []table.Param{}
 	if p.tok.kind == .lpar {
+		p.is_parsing_receiver = true
 		lpar_pos := p.tok.position()
 		p.next() // (
 		is_method = true
@@ -246,6 +247,7 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 			typ: rec_type
 		}
 		p.check(.rpar)
+		p.is_parsing_receiver = false
 	}
 	mut name := ''
 	if p.tok.kind == .name {

vlib/v/parser/parse_type.v

@@ -373,7 +373,7 @@ pub fn (mut p Parser) parse_any_type(language table.Language, is_ptr bool, check
 					return table.int_literal_type
 				}
 				else {
-					if name.len == 1 && name[0].is_capital() {
+					if name.len == 1 && name[0].is_capital() && !p.is_parsing_receiver {
 						return p.parse_generic_template_type(name)
 					}
 					if p.peek_tok.kind == .lt {

vlib/v/parser/parser.v

@@ -71,6 +71,7 @@ mut:
 	cur_fn_name         string
 	in_generic_params   bool // indicates if parsing between `<` and `>` of a method/function
 	name_error          bool // indicates if the token is not a name or the name is on another line
+	is_parsing_receiver bool // indicates if parser is parsing receiver fn (x Xxx)
 }
 
 // for tests