Implement compile-time checks for dynamic width/precision type

test/core-test.cc

@@ -584,6 +584,7 @@ struct test_parse_context {
 
   constexpr int next_arg_id() { return 11; }
   template <typename Id> FMT_CONSTEXPR void check_arg_id(Id) {}
+  FMT_CONSTEXPR void check_dynamic_spec(int) {}
 
   constexpr const char* begin() { return nullptr; }
   constexpr const char* end() { return nullptr; }

test/format-test.cc

@@ -2177,6 +2177,7 @@ TEST(format_test, format_string_errors) {
   EXPECT_ERROR("{: }", "format specifier requires signed argument", unsigned);
   EXPECT_ERROR("{:{}}", "argument not found", int);
   EXPECT_ERROR("{:.{}}", "argument not found", double);
+  EXPECT_ERROR("{:{}}", "width/precision is not integer", int, double);
   EXPECT_ERROR("{:.2}", "precision not allowed for this argument type", int);
   EXPECT_ERROR("{:s}", "invalid type specifier", int);
   EXPECT_ERROR("{:s}", "invalid type specifier", char);