Fudge code to get rid of warnings from GCC

GCC is generating some spurious warnings:

* The OSMObject::user() functions returns a pointer outside the
  OSMObject. That's okay, because we know we put the data there, but GCC
  thinks this is fishy. We can not disable the stringop-overread warning
  inside the library, because it is only produced in the users code. But
  the magic [[gnu::noipa]]attribute stops GCC from looking through the
  user() call and the warning doesn't happen. Because older GCCs and
  other compilers don't understand the attribute, we have to check for
  the compiler version.
* The Tag::after_null() call similarly triggers an array-bounds warning,
  which the changes code doesn't for some reason.

.github/workflows/ci.yml

@@ -26,7 +26,6 @@ jobs:
             # Uses gcc 12.2.0, clang 15.0.7, cmake 3.24.2
             image: "ubuntu:22.04"
             ubuntu: 22
-            CXXFLAGS: -Wno-stringop-overread
           - name: Debian-10
             # Uses gcc 8.3.0, clang 7.0.1, cmake 3.13.4
             image: "debian:buster"
@@ -65,15 +64,12 @@ jobs:
           - name: Debian-12
             # Uses gcc 12.2.0, clang 15.0.6, cmake 3.25.1
             image: "debian:bookworm"
-            CXXFLAGS: -Wno-stringop-overread
           - name: Debian-12
             image: "debian:bookworm"
             cpp_version: c++17
-            CXXFLAGS: -Wno-stringop-overread
           - name: Debian-12
             image: "debian:bookworm"
             cpp_version: c++20
-            CXXFLAGS: -Wno-stringop-overread
           - name: Debian-12
             image: "debian:bookworm"
             c_compiler: clang
@@ -91,7 +87,6 @@ jobs:
           - name: Debian-12
             image: "debian:bookworm"
             build_type: RelWithDebInfo
-            CXXFLAGS: -Wno-stringop-overread
           - name: Debian-12
             image: "debian:bookworm"
             c_compiler: clang
@@ -100,34 +95,29 @@ jobs:
             LDFLAGS: -fsanitize=address,undefined
           - name: Debian-Testing
             image: "debian:testing"
-            CXXFLAGS: -Wno-stringop-overread -Wno-dangling-reference
           - name: Debian-Testing
             image: "debian:testing"
             c_compiler: clang
             cpp_compiler: clang++
           - name: Debian-Experimental
             image: "debian:experimental"
-            CXXFLAGS: -Wno-stringop-overread -Wno-dangling-reference
           - name: Debian-Experimental
             image: "debian:experimental"
             c_compiler: clang
             cpp_compiler: clang++
           - name: Fedora-35
             # Uses gcc 11.2.1, clang 12.0.1, cmake 3.20.5
             image: "fedora:35"
-            CXXFLAGS: -Wno-stringop-overread
           - name: Fedora-36
             # Uses gcc 12.2.0, clang 14.0.5, cmake 3.24.2
             image: "fedora:36"
-            CXXFLAGS: -Wno-stringop-overread
           - name: Fedora-37
             # Uses gcc 12.3.1, clang 15.0.7, cmake 3.26.4
             image: "fedora:37"
-            CXXFLAGS: -Wno-stringop-overread
           - name: Fedora-38
             # Uses gcc 13.0.1, clang 16.0.5, cmake 3.26.4
             image: "fedora:38"
-            CXXFLAGS: -Wno-stringop-overread -Wno-dangling-reference
+            CXXFLAGS: -Wno-dangling-reference
     container:
       image: ${{ matrix.image }}
       env:

include/osmium/memory/buffer.hpp

@@ -480,6 +480,9 @@ namespace osmium {
              *          buffer.
              */
             template <typename T>
+#if defined(__GNUC__) && (__GNUC__ > 11)
+            [[gnu::noipa]] // avoids dangling-reference warning from GNU compiler
+#endif
             T& get(const std::size_t offset) const {
                 assert(m_data && "This must be a valid buffer");
                 assert(offset % alignof(T) == 0 && "Wrong alignment");

include/osmium/osm/object.hpp

@@ -316,6 +316,9 @@ namespace osmium {
         }
 
         /// Get user name for this object.
+#if defined(__GNUC__) && (__GNUC__ > 11)
+        [[gnu::noipa]] // avoids stringop-overread warning from GNU compiler
+#endif
         const char* user() const noexcept {
             return reinterpret_cast<const char*>(data() + sizeof_object());
         }

include/osmium/osm/tag.hpp

@@ -51,11 +51,15 @@ namespace osmium {
         friend class osmium::memory::CollectionIterator;
 
         static unsigned char* after_null(unsigned char* ptr) noexcept {
-            return reinterpret_cast<unsigned char*>(std::strchr(reinterpret_cast<char*>(ptr), 0) + 1);
+            while (*ptr) { ++ptr; }
+            ++ptr;
+            return ptr;
         }
 
         static const unsigned char* after_null(const unsigned char* ptr) noexcept {
-            return reinterpret_cast<const unsigned char*>(std::strchr(reinterpret_cast<const char*>(ptr), 0) + 1);
+            while (*ptr) { ++ptr; }
+            ++ptr;
+            return ptr;
         }
 
         unsigned char* next() noexcept {