test: check immutability of inline tables

According to toml-lang/toml:36d3091b3 "Clarify that inline tables are
immutable", check if it adds key-value pair to an inline table.
  This is one of the unreleased (after-0.5.0) toml feature. But this is
marked as "Clarify", so TOML-lang intended that inline tables are
immutable in all version.

tests/test_parse_inline_table.cpp

@@ -46,3 +46,19 @@ BOOST_AUTO_TEST_CASE(test_inline_table_value)
         TOML11_TEST_PARSE_EQUAL_VALUE(parse_value<toml::value>, "{type.name = \"pug\"}", value(t));
     }
 }
+
+BOOST_AUTO_TEST_CASE(test_inline_table_immutability)
+{
+    {
+        std::istringstream stream(std::string(
+                "a = {b = 1}\n"
+                "a.c = 2\n"));
+        BOOST_CHECK_THROW(toml::parse(stream), toml::syntax_error);
+    }
+    {
+        std::istringstream stream(std::string(
+                "a = {b = {c = 1}}\n"
+                "a.b.d = 2\n"));
+        BOOST_CHECK_THROW(toml::parse(stream), toml::syntax_error);
+    }
+}

toml/parser.hpp

@@ -1331,7 +1331,7 @@ insert_nested_key(typename Value::table_type& root, const Value& v,
             tab->insert(std::make_pair(k, v));
             return ok(true);
         }
-        else
+        else // k is not the last one, we should insert recursively
         {
             // if there is no corresponding value, insert it first.
             // related: you don't need to write
@@ -1347,6 +1347,27 @@ insert_nested_key(typename Value::table_type& root, const Value& v,
             // type checking...
             if(tab->at(k).is_table())
             {
+                // According to toml-lang/toml:36d3091b3 "Clarify that inline
+                // tables are immutable", check if it adds key-value pair to an
+                // inline table.
+                //   This is one of the unreleased (after-0.5.0) toml feature.
+                // But this is marked as "Clarify", so TOML-lang intended that
+                // inline tables are immutable in all version.
+                {
+                    // here, if the value is a (multi-line) table, the region
+                    // should be something like `[table-name]`.
+                    if(get_region(tab->at(k)).front() == '{')
+                    {
+                        throw syntax_error(format_underline(concat_to_string(
+                            "toml::insert_value: inserting to an inline table (",
+                            format_dotted_keys(first, std::next(iter)),
+                            ") but inline tables are immutable"), {
+                                {std::addressof(get_region(tab->at(k))),
+                                 "inline tables are immutable"},
+                                {std::addressof(get_region(v)), "inserting this"}
+                            }), v.location());
+                    }
+                }
                 tab = std::addressof((*tab)[k].as_table());
             }
             else if(tab->at(k).is_array()) // inserting to array-of-tables?

toml/region.hpp

@@ -41,6 +41,7 @@ struct region_base
     region_base& operator=(region_base&&     ) = default;
 
     virtual bool is_ok()           const noexcept {return false;}
+    virtual char front()           const noexcept {return '\0';}
 
     virtual std::string str()      const {return std::string("unknown region");}
     virtual std::string name()     const {return std::string("unknown file");}
@@ -89,6 +90,7 @@ struct location final : public region_base
     ~location() = default;
 
     bool is_ok() const noexcept override {return static_cast<bool>(source_);}
+    char front() const noexcept override {return *iter_;}
 
     // this const prohibits codes like `++(loc.iter())`.
     const const_iterator iter()  const noexcept {return iter_;}
@@ -240,6 +242,7 @@ struct region final : public region_base
     }
 
     bool is_ok() const noexcept override {return static_cast<bool>(source_);}
+    char front() const noexcept override {return *first_;}
 
     std::string str()  const override {return make_string(first_, last_);}
     std::string line() const override
@@ -476,8 +479,9 @@ inline std::string format_underline(const std::string& message,
         const region_base* const reg = iter->first;
         const std::string&   comment = iter->second;
 
-        retval << ' ' << std::setw(line_num_width) << color::bold << color::blue
-               << reg->line_num() << " | "  << color::reset << reg->line() << '\n';
+        retval << ' ' << color::bold << color::blue << std::setw(line_num_width)
+               << std::right << reg->line_num() << " | "  << color::reset
+               << reg->line() << '\n';
 
         retval << make_string(static_cast<std::size_t>(line_num_width + 1), ' ')
                << color::bold << color::blue << " | " << color::reset