Inline the getResultCapture helper into header.

We still keep the error check in the function, but hide it in an
outlined function inside a .cpp file, to promote inlining of the
retrieval part.

In the future, we should explore two things

1) Skipping over the context retrieval here, allowing direct access.
   I currently do not see a way to do this while keeping the
   "greppability" of mutable vs immutable accesses that is there now,
   but it would help a lot when inlining is not enabled.
2) Removing the error check, to make the function trivially inlinable,
   and without branches.

**runtime difference**

| --------- | Debug | Release |
|:----------|------:|--------:|
| Slow path |  0.98 |    1.07 |
| Fast path |  1.04 |    1.08 |

We lost bit of performance on the assertion slow path in debug mode,
but together with the previous commit, it comes out at net zero.
For other combinations, we see 5-10% perf improvement across the
two commits.

.clang-tidy

@@ -78,5 +78,5 @@ WarningsAsErrors: >-
   readability-duplicate-include,
 HeaderFilterRegex: '.*\.(c|cxx|cpp)$'
 FormatStyle:     none
-CheckOptions: {}
+CheckOptions: []
 ...

src/catch2/interfaces/catch_interfaces_capture.cpp

@@ -7,7 +7,14 @@
 // SPDX-License-Identifier: BSL-1.0
 
 #include <catch2/interfaces/catch_interfaces_capture.hpp>
+#include <catch2/internal/catch_enforce.hpp>
 
 namespace Catch {
+    namespace Detail {
+        void missingCaptureInstance() {
+            CATCH_INTERNAL_ERROR( "No result capture instance" );
+        }
+    } // namespace Detail
+
     IResultCapture::~IResultCapture() = default;
-}
+} // namespace Catch

src/catch2/interfaces/catch_interfaces_capture.hpp

@@ -111,8 +111,6 @@ namespace Catch {
         } else {
             Detail::missingCaptureInstance();
         }
-
-
     }
 
 }

src/catch2/internal/catch_context.hpp

@@ -32,7 +32,6 @@ namespace Catch {
             m_resultCapture = resultCapture;
         }
         constexpr void setConfig( IConfig const* config ) { m_config = config; }
-
     };
 
     Context& getCurrentMutableContext();

src/catch2/internal/catch_fatal_condition_handler.cpp

@@ -86,23 +86,27 @@ namespace Catch {
         { EXCEPTION_INT_DIVIDE_BY_ZERO, "Divide by zero error" },
     };
 
+    // Since we do not support multiple instantiations, we put these
+    // into global variables and rely on cleaning them up in outlined
+    // constructors/destructors
+    static LPTOP_LEVEL_EXCEPTION_FILTER previousTopLevelExceptionFilter = nullptr;
+
+
     static LONG CALLBACK topLevelExceptionFilter(PEXCEPTION_POINTERS ExceptionInfo) {
         for (auto const& def : signalDefs) {
             if (ExceptionInfo->ExceptionRecord->ExceptionCode == def.id) {
                 reportFatal(def.name);
             }
         }
-        // If its not an exception we care about, pass it along.
+        // If a filter was previously registered, invoke it
+        if (previousTopLevelExceptionFilter) {
+            return previousTopLevelExceptionFilter(ExceptionInfo);
+        }
+        // Otherwise, pass along all exceptions.
         // This stops us from eating debugger breaks etc.
         return EXCEPTION_CONTINUE_SEARCH;
     }
 
-    // Since we do not support multiple instantiations, we put these
-    // into global variables and rely on cleaning them up in outlined
-    // constructors/destructors
-    static LPTOP_LEVEL_EXCEPTION_FILTER previousTopLevelExceptionFilter = nullptr;
-
-
     // For MSVC, we reserve part of the stack memory for handling
     // memory overflow structured exception.
     FatalConditionHandler::FatalConditionHandler() {

src/catch2/internal/catch_reusable_string_stream.cpp

@@ -12,6 +12,7 @@
 
 #include <cstdio>
 #include <sstream>
+#include <tuple>
 #include <vector>
 
 namespace Catch {
@@ -23,16 +24,16 @@ namespace Catch {
         std::ostringstream m_referenceStream; // Used for copy state/ flags from
         Detail::Mutex m_mutex;
 
-        auto add() -> std::size_t {
+        auto add() -> std::pair<std::size_t, std::ostringstream*> {
             Detail::LockGuard _( m_mutex );
             if( m_unused.empty() ) {
                 m_streams.push_back( Detail::make_unique<std::ostringstream>() );
-                return m_streams.size()-1;
+                return { m_streams.size()-1, m_streams.back().get() };
             }
             else {
                 auto index = m_unused.back();
                 m_unused.pop_back();
-                return index;
+                return { index, m_streams[index].get() };
             }
         }
 
@@ -46,10 +47,10 @@ namespace Catch {
         }
     };
 
-    ReusableStringStream::ReusableStringStream()
-    :   m_index( Singleton<StringStreams>::getMutable().add() ),
-        m_oss( Singleton<StringStreams>::getMutable().m_streams[m_index].get() )
-    {}
+    ReusableStringStream::ReusableStringStream() {
+        std::tie( m_index, m_oss ) =
+            Singleton<StringStreams>::getMutable().add();
+    }
 
     ReusableStringStream::~ReusableStringStream() {
         static_cast<std::ostringstream*>( m_oss )->str("");

src/catch2/internal/catch_run_context.cpp

@@ -21,7 +21,6 @@
 #include <catch2/internal/catch_assertion_handler.hpp>
 #include <catch2/internal/catch_test_failure_exception.hpp>
 #include <catch2/internal/catch_result_type.hpp>
-#include <catch2/interfaces/catch_interfaces_capture.hpp>
 
 #include <cassert>
 #include <algorithm>
@@ -840,10 +839,4 @@ namespace Catch {
         return getCurrentContext().getConfig()->rngSeed();
     }
 
-
-    namespace Detail {
-        void missingCaptureInstance() {
-            CATCH_INTERNAL_ERROR( "No result capture instance" );
-        }
-    } // namespace Detail
 }