Fix -Wextra-semi-stmt at its actual source, not assert()

clang_flags.cmake blamed the global -Wno-extra-semi-stmt on assert(),
but assert() expands to an expression under glibc and libc++ and does
not trigger this warning. unit-assert_macro.cpp overrides JSON_ASSERT
with "{if (!(x)) ++assert_counter; }", a bare block followed by a
semicolon at every JSON_ASSERT(...) call site in the library; that
was the actual source of 151 of the 208 -Wextra-semi-stmt sites found
in a Clang 22 -Weverything sweep of the test suite with the flag
removed. Switched to the standard do/while(false) macro idiom, which
does not expand to a statement-plus-semicolon, and corrected the
comment to name the remaining source instead: vendored Doctest's
CAPTURE(x) shim, which already ends in a semicolon.

Verified with clang++ -Wextra-semi-stmt (plus the file's other CI
ignores) that unit-assert_macro.cpp now compiles without any
-Wextra-semi-stmt diagnostic.

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

#5725 item 8 (step 1 of 2; step 2 covers the CAPTURE() call sites)

cmake/clang_flags.cmake

@@ -2,7 +2,8 @@
 # -Wno-c++98-compat               The library targets C++11.
 # -Wno-c++98-compat-pedantic      The library targets C++11.
 # -Wno-deprecated-declarations    The library contains annotations for deprecated functions.
-# -Wno-extra-semi-stmt            The library uses assert which triggers this warning.
+# -Wno-extra-semi-stmt            Vendored Doctest's CAPTURE() macro expands to a
+#                                 statement followed by a semicolon at every call site.
 # -Wno-padded                     We do not care about padding warnings.
 # -Wno-covered-switch-default     All switches list all cases and a default case.
 # -Wno-c2y-extensions             Clang 22.1 diagnoses __COUNTER__ as a C2y extension, also in

tests/src/unit-assert_macro.cpp

@@ -18,7 +18,7 @@ DOCTEST_CLANG_SUPPRESS_WARNING("-Wstrict-overflow")
 static int assert_counter;
 
 /// set failure variable to true instead of calling assert(x)
-#define JSON_ASSERT(x) {if (!(x)) ++assert_counter; }
+#define JSON_ASSERT(x) do { if (!(x)) { ++assert_counter; } } while (false)
 
 #include <nlohmann/json.hpp>
 using nlohmann::json;