Fix boolalpha extraction to be case-sensitive again

stl/inc/xlocale

@@ -3242,8 +3242,11 @@ protected:
 };
 
 // FUNCTION TEMPLATE _Getloctxt
-template <class _Elem, class _InIt>
-int __CRTDECL _Getloctxt(_InIt& _First, _InIt& _Last, size_t _Numfields, const _Elem* _Ptr) {
+enum class _Case_sensitive : bool { _No, _Yes };
+
+template <class _InIt, class _Elem>
+int __CRTDECL _Getloctxt(
+    _InIt& _First, _InIt& _Last, size_t _Numfields, const _Elem* _Ptr, const _Case_sensitive _Matching) {
     // find field at _Ptr that matches longest in [_First, _Last)
     for (size_t _Off = 0; _Ptr[_Off] != _Elem{}; ++_Off) {
         if (_Ptr[_Off] == _Ptr[0]) {
@@ -3271,7 +3274,10 @@ int __CRTDECL _Getloctxt(_InIt& _First, _InIt& _Last, size_t _Numfields, const _
                        || _Ptr[_Off] == _Elem{}) { // matched all of field, save as possible answer
                 _Str[_Field] = static_cast<char>(_Column < 127 ? _Column : 127); // save skip count if small enough
                 _Ans         = static_cast<int>(_Field); // save answer
-            } else if (_First == _Last || _CType.tolower(_Ptr[_Off]) != _CType.tolower(static_cast<_Elem>(*_First))) {
+            } else if (_First == _Last
+                       || (_Matching == _Case_sensitive::_Yes
+                               ? _Ptr[_Off] != *_First
+                               : _CType.tolower(_Ptr[_Off]) != _CType.tolower(static_cast<_Elem>(*_First)))) {
                 _Str[_Field] = static_cast<char>(_Column < 127 ? _Column : 127); // no match, just save skip count
             } else {
                 _Prefix = true; // still a valid prefix

stl/inc/xlocnum

@@ -369,7 +369,7 @@ protected:
             _Str += _Punct_fac.falsename();
             _Str.push_back(_Elem{});
             _Str += _Punct_fac.truename(); // construct "\0false\0true"
-            switch (_Getloctxt(_First, _Last, 2, _Str.c_str())) {
+            switch (_Getloctxt(_First, _Last, 2, _Str.c_str(), _Case_sensitive::_Yes)) {
             case 0:
                 _Val = false;
                 break;

stl/inc/xloctime

@@ -326,7 +326,7 @@ protected:
 
     virtual _InIt __CLR_OR_THIS_CALL do_get_weekday(_InIt _First, _InIt _Last, ios_base&, ios_base::iostate& _State,
         tm* _Pt) const { // get weekday from [_First, _Last) into _Pt
-        int _Num = _Getloctxt(_First, _Last, 0, _Days);
+        int _Num = _Getloctxt(_First, _Last, 0, _Days, _Case_sensitive::_No);
         if (_Num < 0) {
             _State |= ios_base::failbit;
         } else {
@@ -338,7 +338,7 @@ protected:
 
     virtual _InIt __CLR_OR_THIS_CALL do_get_monthname(_InIt _First, _InIt _Last, ios_base&, ios_base::iostate& _State,
         tm* _Pt) const { // get month from [_First, _Last) into _Pt
-        int _Num = _Getloctxt(_First, _Last, 0, _Months);
+        int _Num = _Getloctxt(_First, _Last, 0, _Months, _Case_sensitive::_No);
 
         if (_Num < 0) {
             _State |= ios_base::failbit;
@@ -444,7 +444,7 @@ protected:
             break;
 
         case 'p':
-            _Ans = _Getloctxt(_First, _Last, 0, ":AM:am:PM:pm");
+            _Ans = _Getloctxt(_First, _Last, 0, ":AM:am:PM:pm", _Case_sensitive::_No);
             if (_Ans < 0) {
                 _State |= ios_base::failbit;
             } else if (1 < _Ans) {

tests/std/test.lst

@@ -172,6 +172,7 @@ tests\GH_001103_countl_zero_correctness
 tests\GH_001105_custom_streambuf_throws
 tests\GH_001123_random_cast_out_of_range
 tests\GH_001411_core_headers
+tests\GH_001541_case_sensitive_boolalpha
 tests\LWG2597_complex_branch_cut
 tests\LWG3018_shared_ptr_function
 tests\P0019R8_atomic_ref

tests/std/tests/GH_001541_case_sensitive_boolalpha/env.lst

@@ -0,0 +1,4 @@
+# Copyright (c) Microsoft Corporation.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+RUNALL_INCLUDE ..\usual_matrix.lst

tests/std/tests/GH_001541_case_sensitive_boolalpha/test.cpp

@@ -0,0 +1,41 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <cassert>
+#include <ios>
+#include <sstream>
+using namespace std;
+
+enum class Parse { Success, Failure };
+
+struct TestCase {
+    const char* str;
+    ios_base::fmtflags flags;
+    Parse expected;
+    bool result;
+};
+
+// clang-format off
+constexpr TestCase test_cases[] = {
+    {"0",     ios_base::fmtflags{}, Parse::Success, false},
+    {"1",     ios_base::fmtflags{}, Parse::Success, true },
+    {"2",     ios_base::fmtflags{}, Parse::Failure, true }, // N4868 [facet.num.get.virtuals]/6
+    {"WOOF",  ios_base::fmtflags{}, Parse::Failure, false}, // N4868 [facet.num.get.virtuals]/3.6
+    {"false", ios_base::boolalpha,  Parse::Success, false},
+    {"true",  ios_base::boolalpha,  Parse::Success, true },
+    {"WOOF",  ios_base::boolalpha,  Parse::Failure, false}, // N4868 [facet.num.get.virtuals]/7
+    {"FALSE", ios_base::boolalpha,  Parse::Failure, false}, // GH-1541
+    {"TRUE",  ios_base::boolalpha,  Parse::Failure, false}, // GH-1541
+};
+// clang-format on
+
+int main() {
+    for (const auto& test : test_cases) {
+        bool val = !test.result;
+        istringstream iss(test.str);
+        iss.setf(test.flags);
+        iss >> val;
+        assert(iss.fail() == (test.expected == Parse::Failure));
+        assert(val == test.result);
+    }
+}