Fix CI: MSVC C4127 in the view builder and the single-header test build

- msvc (Win32, /W4 /WX) reported C4127 (conditional expression is
  constant) for `TrailingCommas && cur() == ']'` and the like when the
  option is off. Route the template arguments through a static enabled()
  function, as json.hpp's nesting_depth_exhausted() does.
- ci_test_single_header compiled unit-json_view_builder.cpp against
  single_include/, which does not contain the internal
  nlohmann/detail/view headers. Build that test only with the multiple
  headers.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/view/builder.hpp

@@ -418,6 +418,14 @@ class builder
         return w;
     }
 
+    /// a compile-time option as a runtime condition: testing the template
+    /// argument directly makes a condition like `TrailingCommas && c == ']'`
+    /// constant when the option is off, which MSVC reports as C4127
+    static NLOHMANN_VIEW_ALWAYS_INLINE bool enabled(bool option) noexcept
+    {
+        return option;
+    }
+
     /// The parse state and the parser proper. The cursor is a local object of
     /// run() whose address never escapes (everything it calls out of line is a
     /// member of the builder and gets the positions it needs), so that the
@@ -547,7 +555,7 @@ arr_next:
                 {
                     return false;
                 }
-                if (TrailingCommas && cur() == ']')
+                if (enabled(TrailingCommas) && cur() == ']')
                 {
                     ++p;
                     goto close_container;
@@ -614,7 +622,7 @@ obj_next:
                 {
                     return false;
                 }
-                if (TrailingCommas && cur() == '}')
+                if (enabled(TrailingCommas) && cur() == '}')
                 {
                     ++p;
                     goto close_container;
@@ -756,7 +764,7 @@ indent_done:
                 {
                     ++p;
                 }
-                if (Comments && cur() == '/')
+                if (enabled(Comments) && cur() == '/')
                 {
                     const unsigned char* const q = cold.comment(p);
                     if (q == nullptr)

tests/CMakeLists.txt

@@ -279,6 +279,10 @@ if(json_32bit_test_only)
 elseif(NOT json_32bit_test)
     list(FILTER files EXCLUDE REGEX src/unit-32bit.cpp)
 endif()
+if(NOT JSON_MultipleHeaders)
+    # the internal headers of json_view are not part of a single header yet
+    list(FILTER files EXCLUDE REGEX src/unit-json_view_builder.cpp)
+endif()
 
 foreach(file ${files})
     json_test_add_test_for(${file} MAIN test_main CXX_STANDARDS ${test_cxx_standards} ${test_force})