parser: add errors for invalid anonymous function (#7786)

vlib/v/parser/fn.v

@@ -443,12 +443,24 @@ fn (mut p Parser) anon_fn() ast.AnonFn {
 			is_arg: true
 		})
 	}
+	mut same_line := p.tok.line_nr == p.prev_tok.line_nr
 	mut return_type := table.void_type
+	// lpar: multiple return types
+	if same_line {
 		if p.tok.kind.is_start_of_type() {
 			return_type = p.parse_type()
+		} else if p.tok.kind != .lcbr {
+			p.error_with_pos('expected return type, not $p.tok for anonymous function',
+				p.tok.position())
+		}
 	}
 	mut stmts := []ast.Stmt{}
 	no_body := p.tok.kind != .lcbr
+	same_line = p.tok.line_nr == p.prev_tok.line_nr
+	if no_body && same_line {
+		p.error_with_pos('unexpected `$p.tok.kind` after anonymous function signature, expecting `{`',
+			p.tok.position())
+	}
 	if p.tok.kind == .lcbr {
 		stmts = p.parse_block_no_scope(false)
 	}

vlib/v/parser/tests/anon_fn_return_type.out

@@ -0,0 +1,6 @@
+vlib/v/parser/tests/anon_fn_return_type.vv:7:22: error: expected return type, not string "hi" for anonymous function
+    5 | _ = fn (name string) flag.Flag
+    6 | _ = fn (name string) flag.Flag {return {}}
+    7 | _ = fn (name string) "hi" + name
+      |                      ~~~~
+    8 |

vlib/v/parser/tests/anon_fn_return_type.vv

@@ -0,0 +1,8 @@
+import flag
+
+_ = fn (name string) 
+_ = fn (name string) {}
+_ = fn (name string) flag.Flag
+_ = fn (name string) flag.Flag {return {}}
+_ = fn (name string) "hi" + name
+

vlib/v/token/token.v

@@ -437,7 +437,8 @@ pub fn (kind Kind) is_infix() bool {
 pub fn (tok &Token) can_start_type(builtin_type_names []string) bool {
 	match tok.kind {
 		.name { return tok.lit[0].is_capital() || tok.lit in builtin_type_names }
-		.amp, .lsbr, .question { return true }
+		// Note: return type (T1, T2) should be handled elsewhere
+		.amp, .key_fn, .lsbr, .question { return true }
 		else {}
 	}
 	return false