parser: fix struct field fn type with default value (fix #19099) (#19106)

vlib/v/parser/parse_type.v

@@ -424,7 +424,7 @@ fn (mut p Parser) parse_type() ast.Type {
 		is_required_field := p.inside_struct_field_decl && p.tok.kind == .lsbr
 			&& p.peek_tok.kind == .name && p.peek_tok.lit == 'required'
 
-		if p.tok.line_nr > line_nr || p.tok.kind in [.comma, .rpar] || is_required_field {
+		if p.tok.line_nr > line_nr || p.tok.kind in [.comma, .rpar, .assign] || is_required_field {
 			mut typ := ast.void_type
 			if is_option {
 				typ = typ.set_flag(.option)

vlib/v/tests/struct_field_default_fn_type_value_init_test.v

@@ -0,0 +1,25 @@
+struct Flip {
+	name    string  = 'NULL'
+	execute fn () ! = unsafe { nil }
+}
+
+fn (flip Flip) exec() ! {
+	if isnil(flip.execute) {
+		return
+	}
+
+	println('Executing ${flip.name}')
+	flip.execute()!
+}
+
+fn test_struct_field_default_fn_type_value() {
+	fl := Flip{
+		name: 'a function'
+		execute: fn () ! {
+			println('Hello, World!')
+		}
+	}
+
+	fl.exec()!
+	assert true
+}