Fix fstream.seekp(0, ios::cur) (microsoft#3773)

Co-authored-by: Stephan T. Lavavej <[email protected]>
Co-authored-by: Casey Carter <[email protected]>

Author: 3 people

stl/inc/__msvc_filebuf.hpp

@@ -651,8 +651,7 @@ class basic_filebuf : public basic_streambuf<_Elem, _Traits> { // stream buffer
             _Off -= static_cast<off_type>(sizeof(_Elem)); // back up over _Elem bytes
         }
 
-        if (!_Myfile || !_Endwrite()
-            || ((_Off != 0 || _Way != ios_base::cur) && _CSTD _fseeki64(_Myfile, _Off, _Way) != 0)
+        if (!_Myfile || !_Endwrite() || _CSTD _fseeki64(_Myfile, _Off, _Way) != 0
             || _CSTD fgetpos(_Myfile, &_Fileposition) != 0) {
             return pos_type{off_type{-1}}; // report failure
         }

tests/std/test.lst

@@ -223,6 +223,7 @@ tests\GH_003105_piecewise_densities
 tests\GH_003119_error_category_ctor
 tests\GH_003246_cmath_narrowing
 tests\GH_003570_allocate_at_least
+tests\GH_003572_fstream_seekp_0_cur
 tests\GH_003617_vectorized_meow_element
 tests\GH_003676_format_large_hh_mm_ss_values
 tests\GH_003735_char_traits_signatures

tests/std/tests/GH_003572_fstream_seekp_0_cur/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_003572_fstream_seekp_0_cur/test.cpp

@@ -0,0 +1,47 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <cassert>
+#include <cstring>
+#include <fstream>
+#include <ios>
+
+int main() {
+    using namespace std;
+
+    fstream f("test.txt", ios::in | ios::out | ios::trunc);
+
+    f << "123";
+
+    auto check = [&f](const char(&content)[4]) {
+        char buffer[4]{};
+        f.seekg(0);
+        f.read(buffer, 3);
+        assert(f);
+        assert(memcmp(buffer, content, 4) == 0);
+    };
+
+    f.seekg(0);
+    (void) f.get();
+    f.seekp(f.tellp());
+    f << "*";
+    check("1*3");
+
+    f.seekg(0);
+    (void) f.get();
+    f.seekp(0, ios::cur);
+    f << "!";
+    check("1!3");
+
+    f.seekg(0);
+    (void) f.get();
+    f.seekp(1, ios::cur);
+    f << "!";
+    check("1!!");
+
+    f.seekg(0);
+    (void) f.get();
+    f.seekp(-1, ios::cur);
+    f << "!";
+    check("!!!");
+}