Make type_name2type_identifier retain leading underscores

Because the resulting partial identifiers are considered to be internal,
it is fine for them to start with an underscore.

Author: thomasspriggs

src/ansi-c/type2name.cpp

@@ -308,15 +308,15 @@ std::string type_name2type_identifier(const std::string &name)
       static const std::regex non_alpha_numeric{"[^A-Za-z0-9\x80-\xff]+"};
       return std::regex_replace(identifier, non_alpha_numeric, "_");
     };
-  const auto strip_leading_non_letters = [](const std::string &identifier) {
+  const auto strip_leading_digits = [](const std::string &identifier) {
     static const std::regex identifier_regex{
-      "[A-Za-z\x80-\xff][A-Za-z0-9_\x80-\xff]*"};
+      "[A-Za-z\x80-\xff_][A-Za-z0-9_\x80-\xff]*"};
     std::smatch match_results;
     bool found = std::regex_search(identifier, match_results, identifier_regex);
     POSTCONDITION(found);
     return match_results.str(0);
   };
-  return strip_leading_non_letters(replace_invalid_characters_with_underscore(
+  return strip_leading_digits(replace_invalid_characters_with_underscore(
     replace_special_characters(name)));
 }
 

unit/ansi-c/type2name.cpp

@@ -50,18 +50,19 @@ TEST_CASE(
     R"(abcdefghijklmnopqrstuvwxyz{|}~)");
   CHECK(
     type_name2type_identifier(printable_characters) ==
-    "ptr_0123456789_ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz_"
+    "_ptr_0123456789_ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz_"
     "start_sub_end_sub_");
 }
 
 TEST_CASE(
-  "type_name2type_identifier leading digits",
+  "type_name2type_identifier leading characters",
   "[core][ansi-c][type_name2type_identifier]")
 {
   CHECK(
-    type_name2type_identifier(
-      "0123456789_banana_0123456789_split_0123456789") ==
+    type_name2type_identifier("0123456789banana_0123456789_split_0123456789") ==
     "banana_0123456789_split_0123456789");
+  CHECK(type_name2type_identifier("0123456789_banana") == "_banana");
+  CHECK(type_name2type_identifier("_0123456789") == "_0123456789");
 }
 
 TEST_CASE(