checker, cgen: fix go call fn using map value (#15665)

vlib/v/checker/fn.v

@@ -617,6 +617,8 @@ pub fn (mut c Checker) fn_call(mut node ast.CallExpr, mut continue_check &bool)
 			if elem_sym.info is ast.FnType {
 				func = elem_sym.info.func
 				found = true
+				node.is_fn_var = true
+				node.fn_var_type = sym.info.elem_type
 			} else {
 				c.error('cannot call the element of the array, it is not a function',
 					node.pos)
@@ -626,6 +628,8 @@ pub fn (mut c Checker) fn_call(mut node ast.CallExpr, mut continue_check &bool)
 			if value_sym.info is ast.FnType {
 				func = value_sym.info.func
 				found = true
+				node.is_fn_var = true
+				node.fn_var_type = sym.info.value_type
 			} else {
 				c.error('cannot call the value of the map, it is not a function', node.pos)
 			}
@@ -634,6 +638,8 @@ pub fn (mut c Checker) fn_call(mut node ast.CallExpr, mut continue_check &bool)
 			if elem_sym.info is ast.FnType {
 				func = elem_sym.info.func
 				found = true
+				node.is_fn_var = true
+				node.fn_var_type = sym.info.elem_type
 			} else {
 				c.error('cannot call the element of the array, it is not a function',
 					node.pos)

vlib/v/gen/c/fn.v

@@ -1722,6 +1722,17 @@ fn (mut g Gen) go_expr(node ast.GoExpr) {
 			g.gen_anon_fn_decl(mut expr.left)
 			name = expr.left.decl.name
 		}
+	} else if expr.left is ast.IndexExpr {
+		if expr.is_fn_var {
+			fn_sym := g.table.sym(expr.fn_var_type)
+			func := (fn_sym.info as ast.FnType).func
+			fn_var := g.fn_var_signature(func.return_type, func.params.map(it.typ), tmp_fn)
+			g.write('\t$fn_var = ')
+			g.expr(expr.left)
+			g.writeln(';')
+			name = fn_sym.cname
+			use_tmp_fn_var = true
+		}
 	}
 	name = util.no_dots(name)
 	if g.pref.obfuscate && g.cur_mod.name == 'main' && name.starts_with('main__') {

vlib/v/tests/go_call_fn_using_map_value_test.v

@@ -0,0 +1,16 @@
+module main
+
+fn test_go_call_fn_using_map_value() {
+	sum := fn (x int, y int) int {
+		return x + y
+	}
+
+	mut fns := map[string]fn (int, int) int{}
+	fns['sum'] = sum
+
+	g := go fns['sum'](2, 3)
+	x := g.wait()
+
+	println('$x')
+	assert x == 5
+}