Check dynamic widht/precision id at compile time (#1614)

include/fmt/core.h

@@ -8,6 +8,7 @@
 #ifndef FMT_CORE_H_
 #define FMT_CORE_H_
 
+#include <climits>
 #include <cstdio>  // std::FILE
 #include <cstring>
 #include <functional>
@@ -562,14 +563,24 @@ class basic_format_parse_context : private ErrorHandler {
  private:
   basic_string_view<Char> format_str_;
   int next_arg_id_;
+  int num_args_;
+
+  FMT_CONSTEXPR int do_check_arg_id(int id) {
+    if (id >= num_args_) on_error("argument not found");
+    return id;
+  }
 
  public:
   using char_type = Char;
   using iterator = typename basic_string_view<Char>::iterator;
 
   explicit FMT_CONSTEXPR basic_format_parse_context(
-      basic_string_view<Char> format_str, ErrorHandler eh = ErrorHandler())
-      : ErrorHandler(eh), format_str_(format_str), next_arg_id_(0) {}
+      basic_string_view<Char> format_str, int num_args = INT_MAX,
+      ErrorHandler eh = {})
+      : ErrorHandler(eh),
+        format_str_(format_str),
+        next_arg_id_(0),
+        num_args_(num_args) {}
 
   /**
     Returns an iterator to the beginning of the format string range being
@@ -594,7 +605,7 @@ class basic_format_parse_context : private ErrorHandler {
     the next argument index and switches to the automatic indexing.
    */
   FMT_CONSTEXPR int next_arg_id() {
-    if (next_arg_id_ >= 0) return next_arg_id_++;
+    if (next_arg_id_ >= 0) return do_check_arg_id(next_arg_id_++);
     on_error("cannot switch from manual to automatic argument indexing");
     return 0;
   }
@@ -603,11 +614,13 @@ class basic_format_parse_context : private ErrorHandler {
     Reports an error if using the automatic argument indexing; otherwise
     switches to the manual indexing.
    */
-  FMT_CONSTEXPR void check_arg_id(int) {
-    if (next_arg_id_ > 0)
+  FMT_CONSTEXPR void check_arg_id(int id) {
+    if (next_arg_id_ > 0) {
       on_error("cannot switch from automatic to manual argument indexing");
-    else
-      next_arg_id_ = -1;
+      return;
+    }
+    do_check_arg_id(id);
+    next_arg_id_ = -1;
   }
 
   FMT_CONSTEXPR void check_arg_id(basic_string_view<Char>) {}

include/fmt/format.h

@@ -2632,19 +2632,17 @@ class format_string_checker {
   explicit FMT_CONSTEXPR format_string_checker(
       basic_string_view<Char> format_str, ErrorHandler eh)
       : arg_id_(-1),
-        context_(format_str, eh),
+        context_(format_str, num_args, eh),
         parse_funcs_{&parse_format_specs<Args, parse_context_type>...} {}
 
   FMT_CONSTEXPR void on_text(const Char*, const Char*) {}
 
   FMT_CONSTEXPR void on_arg_id() {
     arg_id_ = context_.next_arg_id();
-    check_arg_id();
   }
   FMT_CONSTEXPR void on_arg_id(int id) {
     arg_id_ = id;
     context_.check_arg_id(id);
-    check_arg_id();
   }
   FMT_CONSTEXPR void on_arg_id(basic_string_view<Char>) {
     on_error("compile-time checks don't support named arguments");
@@ -2665,10 +2663,6 @@ class format_string_checker {
   using parse_context_type = basic_format_parse_context<Char, ErrorHandler>;
   enum { num_args = sizeof...(Args) };
 
-  FMT_CONSTEXPR void check_arg_id() {
-    if (arg_id_ >= num_args) context_.on_error("argument not found");
-  }
-
   // Format specifier parsing function.
   using parse_func = const Char* (*)(parse_context_type&);
 

test/format-test.cc

@@ -2493,6 +2493,8 @@ TEST(FormatTest, FormatStringErrors) {
   EXPECT_ERROR("{:+}", "format specifier requires signed argument", unsigned);
   EXPECT_ERROR("{:-}", "format specifier requires signed argument", unsigned);
   EXPECT_ERROR("{: }", "format specifier requires signed argument", unsigned);
+  EXPECT_ERROR("{:{}}", "argument not found", int);
+  EXPECT_ERROR("{:.{}}", "argument not found", 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", bool);