Fix warning C26818 about switch statements needing default cases (#4715)

Author: StephanTLavavej

stl/inc/chrono

@@ -4177,6 +4177,8 @@ namespace chrono {
             case '+':
                 ++_First;
                 break;
+            default:
+                break;
             }
 
             // For a regular offset hh[mm], simply read four digits, with the option of an EOF or non-digit after

stl/inc/format

@@ -276,6 +276,8 @@ _NODISCARD constexpr _Decode_result<char> _Decode_utf(const char* _First, const
             return {_First + 1, false};
         }
         break;
+    default:
+        break;
     }
 
     // mask out the "value bits" in the leading byte,
@@ -292,6 +294,9 @@ _NODISCARD constexpr _Decode_result<char> _Decode_utf(const char* _First, const
     case 4:
         _Val &= 0b111u;
         break;
+    default:
+        _STL_UNREACHABLE; // can't happen, see how the value of _Num_bytes is determined
+        break;
     }
 
     for (int _Idx = 1; _Idx < _Num_bytes; ++_Idx) {
@@ -1258,6 +1263,8 @@ _NODISCARD constexpr const _CharT* _Parse_align(
         case '^':
             _Parsed_align = _Fmt_align::_Center;
             break;
+        default:
+            break;
         }
 
         if (_Parsed_align != _Fmt_align::_None) {
@@ -2849,6 +2856,8 @@ _NODISCARD _OutputIt _Write_integral(
     case 'o':
         _Base = 8;
         break;
+    default:
+        break;
     }
 
     // long long -1 representation in binary is 64 bits + sign
@@ -4447,6 +4456,8 @@ _NODISCARD constexpr const _CharT* _Parse_range_specs(
         _Callbacks._On_type(_Maybe_type);
         ++_Begin;
         break;
+    default:
+        break;
     }
 
     return _Begin;
@@ -4534,6 +4545,10 @@ public:
             }
 
             break;
+
+        default:
+            _STL_UNREACHABLE; // can't happen, we've handled all possible outputs from _Parse_range_specs()
+            break;
         }
 
         if (_Specs._No_brackets) {

stl/inc/regex

@@ -3922,6 +3922,9 @@ void _Parser<_FwdIt, _Elem, _RxTraits>::_Trans() { // map character to meta-char
         }
 
         break;
+
+    default:
+        break;
     }
 }
 

stl/inc/xlocmon

@@ -567,7 +567,15 @@ private:
                     if (_Pattern.field[_Off] == money_base::space && !_Seen) {
                         _Bad = true; // fail if no space seen
                     }
-                } // parse optional space
+
+                    break;
+                }
+
+            default:
+                _STL_ASSERT(false, "Invalid money_base::pattern, see N4981 [locale.moneypunct.general]/1: "
+                                   "In the field member of a pattern object, each value symbol, sign, value, "
+                                   "and either space or none appears exactly once.");
+                break;
             } // switch
         }
 
@@ -787,6 +795,12 @@ private:
                 }
 
                 break;
+
+            default:
+                _STL_ASSERT(false, "Invalid money_base::pattern, see N4981 [locale.moneypunct.general]/1: "
+                                   "In the field member of a pattern object, each value symbol, sign, value, "
+                                   "and either space or none appears exactly once.");
+                break;
             }
         }
 
@@ -840,6 +854,13 @@ private:
                     _Dest      = _Rep(_Dest, _Fill, _Fillcount);
                     _Fillcount = 0;
                 }
+                break;
+
+            default:
+                _STL_ASSERT(false, "Invalid money_base::pattern, see N4981 [locale.moneypunct.general]/1: "
+                                   "In the field member of a pattern object, each value symbol, sign, value, "
+                                   "and either space or none appears exactly once.");
+                break;
             }
         }
 

stl/inc/xloctime

@@ -563,6 +563,7 @@ protected:
 
         default:
             _State |= ios_base::failbit; // unknown specifier
+            break;
         }
 
         if (_First == _Last) {

tests/std/rulesets/stl.ruleset

@@ -7,5 +7,7 @@
     <Rule Id="C26815" Action="Warning" />
     <!-- The pointer points to memory allocated on the stack (ES.65) -->
     <Rule Id="C26816" Action="Warning" />
+    <!-- Switch statement does not cover all cases. Consider adding a 'default' label (es.79). -->
+    <Rule Id="C26818" Action="Warning" />
   </Rules>
 </RuleSet>

tests/std/tests/GH_002094_cpp_core_guidelines/test.compile.pass.cpp

@@ -2,3 +2,50 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 #include <__msvc_all_public_headers.hpp>
+
+using namespace std;
+
+void instantiate_regex_machinery() {
+    const regex r{R"(.+meow.+)"};
+    assert(regex_match("homeowner", r));
+}
+
+#if _HAS_CXX20
+void instantiate_chrono_parse_machinery() {
+    istringstream iss{"10:02:07"};
+    chrono::seconds s{};
+    iss >> chrono::parse("%H:%M:%S", s);
+    assert(s == 36127s);
+}
+#endif // ^^^ _HAS_CXX20 ^^^
+
+#if _HAS_CXX23
+template <class T>
+struct WrappedVector : vector<T> {
+    using vector<T>::vector;
+};
+
+template <class T>
+struct std::formatter<WrappedVector<T>, char> {
+public:
+    template <class ParseContext>
+    constexpr auto parse(ParseContext& ctx) {
+        return underlying.parse(ctx);
+    }
+
+    template <class FormatContext>
+    auto format(const WrappedVector<T>& rng, FormatContext& ctx) const {
+        return underlying.format(rng, ctx);
+    }
+
+private:
+    range_formatter<T, char> underlying;
+};
+
+void instantiate_range_formatter_machinery() {
+    const WrappedVector<int> v{11, 22, 33, 44};
+    assert(format("{}", v) == "[11, 22, 33, 44]");
+    assert(format("{:}", v) == "[11, 22, 33, 44]");
+    assert(format("{:n}", v) == "11, 22, 33, 44");
+}
+#endif // ^^^ _HAS_CXX23 ^^^