cgen,checker: fix builtin fn var resolve

vlib/v/checker/checker.v

@@ -3605,6 +3605,25 @@ struct ACFieldMethod {
 	typ  string
 }
 
+fn (mut c Checker) resolve_var_fn(func ast.Fn, mut node ast.Ident, name string) ast.Type {
+	mut fn_type := ast.new_type(c.table.find_or_register_fn_type(func, false, true))
+	if func.generic_names.len > 0 {
+		concrete_types := node.concrete_types.map(c.unwrap_generic(it))
+		if typ_ := c.table.resolve_generic_to_concrete(fn_type, func.generic_names, concrete_types) {
+			fn_type = typ_
+			if concrete_types.all(!it.has_flag(.generic)) {
+				c.table.register_fn_concrete_types(func.fkey(), concrete_types)
+			}
+		}
+	}
+	node.name = name
+	node.kind = .function
+	node.info = ast.IdentFn{
+		typ: fn_type
+	}
+	return fn_type
+}
+
 fn (mut c Checker) ident(mut node ast.Ident) ast.Type {
 	if c.pref.linfo.is_running {
 		// Mini LS hack (v -line-info "a.v:16")
@@ -3850,25 +3869,7 @@ fn (mut c Checker) ident(mut node ast.Ident) ast.Type {
 		}
 		// Non-anon-function object (not a call), e.g. `onclick(my_click)`
 		if func := c.table.find_fn(name) {
-			mut fn_type := ast.new_type(c.table.find_or_register_fn_type(func, false,
-				true))
-			if func.generic_names.len > 0 {
-				concrete_types := node.concrete_types.map(c.unwrap_generic(it))
-				if typ_ := c.table.resolve_generic_to_concrete(fn_type, func.generic_names,
-					concrete_types)
-				{
-					fn_type = typ_
-					if concrete_types.all(!it.has_flag(.generic)) {
-						c.table.register_fn_concrete_types(func.fkey(), concrete_types)
-					}
-				}
-			}
-			node.name = name
-			node.kind = .function
-			node.info = ast.IdentFn{
-				typ: fn_type
-			}
-			return fn_type
+			return c.resolve_var_fn(func, mut node, name)
 		}
 	}
 	if node.language == .c {

vlib/v/gen/c/cgen.v

@@ -4756,7 +4756,7 @@ fn (mut g Gen) ident(node ast.Ident) {
 		g.write(util.no_dots(node.name[2..]))
 		return
 	}
-	mut name := c_name(node.name)
+	mut name := if node.kind == .function { c_fn_name(node.name) } else { c_name(node.name) }
 	if node.kind == .constant {
 		if g.pref.translated && !g.is_builtin_mod
 			&& !util.module_is_builtin(node.name.all_before_last('.')) {

vlib/v/tests/buitlin_fn_var_test.v

@@ -0,0 +1,5 @@
+fn test_main() {
+	a := error
+	assert dump(a('foo')) == error('foo')
+	assert dump(error) == error
+}