checker: disallow multiple return values in const declarations (#18273)

vlib/v/checker/checker.v

@@ -1531,6 +1531,13 @@ fn (mut c Checker) const_decl(mut node ast.ConstDecl) {
 			}
 			c.error('duplicate const `${field.name}`', name_pos)
 		}
+		if field.expr is ast.CallExpr {
+			sym := c.table.sym(c.check_expr_opt_call(field.expr, c.expr(field.expr)))
+			if sym.kind == .multi_return {
+				c.error('const declarations do not support multiple return values yet',
+					field.expr.pos)
+			}
+		}
 		c.const_names << field.name
 	}
 	for i, mut field in node.fields {

vlib/v/checker/tests/const_decl_multi_return_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/const_decl_multi_return_err.vv:6:6: error: const declarations do not support multiple return values yet
+    4 |
+    5 | const (
+    6 |     a = foo()
+      |         ~~~~~
+    7 | )
+    8 |

vlib/v/checker/tests/const_decl_multi_return_err.vv

@@ -0,0 +1,12 @@
+fn foo() (int, int, int) {
+	return 1, 2, 3
+}
+
+const (
+	a = foo()
+)
+
+
+fn main() {
+	println("$a")
+}