`<streambuf>`: fix seekpos and seekoff (#2121)

stl/inc/streambuf

@@ -357,12 +357,12 @@ protected:
     virtual pos_type __CLR_OR_THIS_CALL seekoff(
         off_type, ios_base::seekdir, ios_base::openmode = ios_base::in | ios_base::out) {
         // change position by offset, according to way and mode
-        return streampos(-1);
+        return pos_type(off_type(-1));
     }
 
     virtual pos_type __CLR_OR_THIS_CALL seekpos(pos_type, ios_base::openmode = ios_base::in | ios_base::out) {
         // change to specified position, according to mode
-        return streampos(-1);
+        return pos_type(off_type(-1));
     }
 
     virtual basic_streambuf* __CLR_OR_THIS_CALL setbuf(_Elem*, streamsize) {

tests/std/test.lst

@@ -179,6 +179,7 @@ tests\GH_001850_clog_tied_to_cout
 tests\GH_001914_cached_position
 tests\GH_002039_byte_is_not_trivially_swappable
 tests\GH_002058_debug_iterator_race
+tests\GH_002120_streambuf_seekpos_and_seekoff
 tests\LWG2597_complex_branch_cut
 tests\LWG3018_shared_ptr_function
 tests\P0019R8_atomic_ref

tests/std/tests/GH_002120_streambuf_seekpos_and_seekoff/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_002120_streambuf_seekpos_and_seekoff/test.compile.pass.cpp

@@ -0,0 +1,18 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <streambuf>
+#include <string>
+
+using namespace std;
+
+struct State {};
+
+struct CharTraits : char_traits<char> {
+    using state_type = State;
+    using pos_type   = fpos<State>;
+};
+
+struct Streambuf : basic_streambuf<char, CharTraits> {};
+
+int main() {} // COMPILE-ONLY