parser: allow constant usage in assoc

vlib/compiler/fn.v

@@ -70,6 +70,20 @@ fn (a []TypeInst) str() string {
 	return r.str()
 }
 
+fn (p mut Parser) find_var_or_const(name string) ?Var {
+	if p.known_var(name) {
+		return p.find_var(name)
+	}
+	if p.table.known_const(name) {
+		return p.table.find_const(name)
+	}
+	modname := p.prepend_mod(name)
+	if p.table.known_const(modname) {
+		return p.table.find_const(modname)
+	}
+	return none
+}
+
 fn (p &Parser) find_var(name string) ?Var {
 	for i in 0 .. p.var_idx {
 		if p.local_vars[i].name == name {

vlib/compiler/parser.v

@@ -2150,11 +2150,13 @@ fn (p mut Parser) assoc() string {
 	// println('assoc()')
 	p.next()
 	name := p.check_name()
-	var := p.find_var(name) or {
+	var := p.find_var_or_const(name) or {
 		p.error('unknown variable `$name`')
 		exit(1)
 	}
-	p.mark_var_used(var)
+	if !var.is_const {
+		p.mark_var_used(var)
+	}
 	p.check(.pipe)
 	p.gen('($var.typ){')
 	mut fields := []string// track the fields user is setting, the rest will be copied from the old object
@@ -2177,7 +2179,7 @@ fn (p mut Parser) assoc() string {
 		if f in fields {
 			continue
 		}
-		p.gen('.$f = ${name}.$f,')
+		p.gen('.$f = ${var.name}.$f,')
 	}
 	p.check(.rcbr)
 	p.gen('}')

vlib/compiler/tests/struct_test.v

@@ -134,3 +134,23 @@ fn test_default_vals() {
 	assert d2.b == 20
 }
 
+fn test_assoc_with_vars() {
+	def2 := Def { a: 12 }
+	merged := { def2 | a: 42 }
+	assert merged.a == 42
+	assert merged.b == 7
+}
+
+const (
+	const_def = Def { a: 100 }
+)
+fn test_assoc_with_constants() {
+	merged := { const_def | a: 42 }
+	assert merged.a == 42
+	assert merged.b == 7
+
+	again := { const_def | b: 22 }
+	assert again.a == 100
+	assert again.b == 22
+}
+