From 48e0a59222f2e1200193fd83a7c55ea3879dd8dc Mon Sep 17 00:00:00 2001
From: Victor Zverovich <viz@fb.com>
Date: Thu, 9 Jun 2022 17:59:27 -0700
Subject: [PATCH] Implement compile-time checks for dynamic width/precision
 type

---
 include/fmt/core.h  | 24 +++++++++++++++++++++---
 test/core-test.cc   |  1 +
 test/format-test.cc |  1 +
 3 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/include/fmt/core.h b/include/fmt/core.h
index 1e8ecad1..c6ebef20 100644
--- a/include/fmt/core.h
+++ b/include/fmt/core.h
@@ -710,8 +710,8 @@ class basic_format_parse_context : private ErrorHandler {
     next_arg_id_ = -1;
     do_check_arg_id(id);
   }
-
   FMT_CONSTEXPR void check_arg_id(basic_string_view<Char>) {}
+  FMT_CONSTEXPR void check_dynamic_spec(int arg_id);
 
   FMT_CONSTEXPR void on_error(const char* message) {
     ErrorHandler::on_error(message);
@@ -738,7 +738,8 @@ class compile_parse_context
       ErrorHandler eh = {}, int next_arg_id = 0)
       : base(format_str, eh, next_arg_id), num_args_(num_args), types_(types) {}
 
-  constexpr int num_args() const { return num_args_; }
+  constexpr auto num_args() const -> int { return num_args_; }
+  constexpr auto arg_type(int id) const -> type { return types_[id]; }
 
   FMT_CONSTEXPR auto next_arg_id() -> int {
     int id = base::next_arg_id();
@@ -751,6 +752,11 @@ class compile_parse_context
     if (id >= num_args_) this->on_error("argument not found");
   }
   using base::check_arg_id;
+
+  FMT_CONSTEXPR void check_dynamic_spec(int arg_id) {
+    if (arg_id < num_args_ && types_ && !is_integral_type(types_[arg_id]))
+      this->on_error("width/precision is not integer");
+  }
 };
 FMT_END_DETAIL_NAMESPACE
 
@@ -766,6 +772,15 @@ basic_format_parse_context<Char, ErrorHandler>::do_check_arg_id(int id) {
   }
 }
 
+template <typename Char, typename ErrorHandler>
+FMT_CONSTEXPR void
+basic_format_parse_context<Char, ErrorHandler>::check_dynamic_spec(int arg_id) {
+  if (detail::is_constant_evaluated()) {
+    using context = detail::compile_parse_context<Char, ErrorHandler>;
+    static_cast<context*>(this)->check_dynamic_spec(arg_id);
+  }
+}
+
 template <typename Context> class basic_format_arg;
 template <typename Context> class basic_format_args;
 template <typename Context> class dynamic_format_arg_store;
@@ -2243,11 +2258,14 @@ class dynamic_specs_handler
 
   FMT_CONSTEXPR auto make_arg_ref(int arg_id) -> arg_ref_type {
     context_.check_arg_id(arg_id);
+    context_.check_dynamic_spec(arg_id);
     return arg_ref_type(arg_id);
   }
 
   FMT_CONSTEXPR auto make_arg_ref(auto_id) -> arg_ref_type {
-    return arg_ref_type(context_.next_arg_id());
+    int arg_id = context_.next_arg_id();
+    context_.check_dynamic_spec(arg_id);
+    return arg_ref_type(arg_id);
   }
 
   FMT_CONSTEXPR auto make_arg_ref(basic_string_view<char_type> arg_id)
diff --git a/test/core-test.cc b/test/core-test.cc
index ce0816c6..96cde1ed 100644
--- a/test/core-test.cc
+++ b/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; }
diff --git a/test/format-test.cc b/test/format-test.cc
index 4ec7c838..5d8f19f3 100644
--- a/test/format-test.cc
+++ b/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);