<chrono>: ambiguous_local_time's message is slightly incorrect

[JMazurkiewicz]

Bug description

The message of ambiguous_local_time exception class should be (per [time.zone.exception.ambig]/4):

Effects: Initializes the base class with a sequence of char equivalent to that produced by os.str() initialized as shown below:

ostringstream os;
os << tp << " is ambiguous.  It could be\n"
   << tp << ' ' << i.first.abbrev << " == "
   << tp - i.first.offset << " UTC or\n"
   << tp << ' ' << i.second.abbrev  << " == "
   << tp - i.second.offset  << " UTC";

Repro:

#include <cassert>
#include <chrono>
#include <iostream>
#include <string>

int main() {
  const std::string_view required_start =
      "2016-11-06 01:30:00 is ambiguous.  It could be";

  // Example 1, [time.zone.exception.ambig]/4
  using namespace std::chrono;
  try {
    auto zt = zoned_time{"America/New_York",
                         local_days{Sunday[1] / November / 2016} + 1h + 30min};
  } catch (const ambiguous_local_time &e) {
    std::cout << e.what() << '\n';
    assert(std::string_view{e.what()}.starts_with(required_start));
  }
}

Build:

PS D:\stl-playground> clang -std=c++20 .\test.cpp
PS D:\stl-playground> .\a.exe
2016-11-06 01:30:00 is ambiguous. It could be
2016-11-06 01:30:00 GMT-4 == 2016-11-06 05:30:00 UTC or
2016-11-06 01:30:00 GMT-5 == 2016-11-06 06:30:00 UTC   
Assertion failed: std::string_view{e.what()}.starts_with(required_start), file .\test.cpp, line 17

Summary:

• There is one extra space (0x20) missing between "ambiguous." and "It",
• Compiler explorer (GCC): https://godbolt.org/z/E36xP4x3z.

"Good first issue" tips

It should say is ambiguous. It could be (two spaces) in those places:

• STL/stl/inc/chrono

  Line 6014 in e6a12f7

  _Os << _Tp << " is ambiguous. It could be\n"

• STL/tests/std/tests/P0355R7_calendars_and_time_zones_formatting/test.cpp

  Line 801 in e6a12f7

  == "2016-11-06 01:30:00 is ambiguous. It could be\n"

• STL/tests/std/tests/P0355R7_calendars_and_time_zones_formatting/test.cpp

  Line 805 in e6a12f7

  == "2016-11-06 01:30:00 is ambiguous. It could be\n"

[StephanTLavavej]

This is very interesting. It appears as 2 spaces in the WG21-N4944 PDF, copies as 1 space, but is 2 spaces in the original LaTeX:

https://github.com/cplusplus/draft/blob/24d7340146caa11227304b18ed57822f1bd53627/source/time.tex#L9181

It does seem that 2 spaces are mandated here.

When fixing this, please note that in addition to the product code in stl/inc/chrono, there are 2 lines of tests/std/tests/P0355R7_calendars_and_time_zones_formatting/test.cpp that will need to be updated.

[MattStephanson]

This is very interesting. It appears as 2 spaces in the WG21-N4944 PDF, copies as 1 space, but is 2 spaces in the original LaTeX:

Off-topic, but I couldn't stop myself from going down this rabbit hole. For os << tp << " is ambiguous. It could be\n", the corresponding PDF stream is:

[(os)-525(<<)-525(tp)-525(<<)-525(")-525(is)-525(ambiguous.)-1050(It)-525(could)-525(be\134n")]TJ

The TJ operator takes the preceding array and, if the entry is text, displays it. If the entry is a number, that amount is subtracted from the current position. You can see that most words are separated by a 525-unit space, but between "ambiguous." and "It", there's a single, double-width space.