Scope the base_macros.h warning disables to base.h

base_macros.h disables C5046, C4268, C4499 and C4630 without a matching
#pragma warning(push)/(pop) pair. base.h includes it near the top and never
restores the warning state, so those four warnings stay disabled for the rest
of every translation unit that includes base.h, not just for the C++/WinRT
declarations they were added for.

Consumers silently lose the warnings in their own code:

    namespace { struct S { int x; }; }
    S f();
    int main() { f(); }

That warns C5046 on its own, and stops warning as soon as <winrt/base.h> is
included ahead of it, at both /W4 and /Wall.

Open the scope before the include and close it at the end of base.h. The
disables still cover everything C++/WinRT declares, and the warning state is
handed back to the consumer unchanged. The set of warnings reported from
within the C++/WinRT headers is unaffected.

Author: DanielJump

cppwinrt/code_writers.h

@@ -117,6 +117,33 @@ namespace cppwinrt
         return { w, write_close_file_guard };
     }
 
+    static void write_macro_warning_pop(writer& w)
+    {
+        auto format = R"(#ifdef _MSC_VER
+#pragma warning(pop)
+#endif // _MSC_VER
+)";
+
+        w.write(format);
+    }
+
+    // base_macros.h disables a handful of compiler warnings that C++/WinRT's own
+    // declarations would otherwise trigger. Those disables have to stay in effect
+    // for the rest of the file that includes it, so the enclosing scope is opened
+    // here rather than inside base_macros.h itself. Without the matching pop the
+    // disables would remain active for everything the consumer writes after the
+    // include, silently turning the warnings off for their code as well.
+    [[nodiscard]] static finish_with wrap_macro_warning_scope(writer& w)
+    {
+        auto format = R"(#ifdef _MSC_VER
+#pragma warning(push)
+#endif // _MSC_VER
+)";
+
+        w.write(format);
+        return { w, write_macro_warning_pop };
+    }
+
     [[nodiscard]] static finish_with wrap_lean_and_mean(writer& w, bool is_lean_and_mean = true)
     {
         if (is_lean_and_mean)

cppwinrt/file_writers.h

@@ -17,6 +17,7 @@ namespace cppwinrt
                 w.write(strings::base_detect_numerics);
                 w.write(strings::base_include_numerics);
             }
+            auto wrap_macro_warnings = wrap_macro_warning_scope(w);
             w.write_root_include("base_macros");
             w.write(strings::base_source_location);
             w.write(strings::base_types);

strings/base_macros.h

@@ -23,6 +23,11 @@
 #define WINRT_IMPL_SHIM(...) (*(abi_t<__VA_ARGS__>**)&static_cast<__VA_ARGS__ const&>(static_cast<D const&>(*this)))
 
 #ifdef _MSC_VER
+// These disables deliberately apply to the remainder of the file that includes
+// this header, because they cover declarations made throughout C++/WinRT. The
+// including file opens a #pragma warning(push) beforehand and pops it at the
+// end, which keeps them from escaping into consumer code.
+
 // Note: this is a workaround for a false-positive warning produced by the Visual C++ 15.9 compiler.
 #pragma warning(disable : 5046)