CHECK_EQUAL: fix array != array compare under C++26#1886
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C++26 [over.match.oper]/p10 makes array == array (and array != array)
ill-formed. CHECK_EQUAL_LOCATION expanded its two side-effect-detection
probes literally as (operand) != (operand). As soon as either operand
was an array (most often a string literal in CHECK_EQUAL("text", x)),
the inner self-compare became a forbidden array-vs-array compare. The
same pattern is also a -Wdeprecated-array-compare error under C++20+
when -Werror is on.
Add a CppUTestPrivate::checkEqualSelfNotEqual<T>(T, T) helper that
takes both arguments by value, so any array decays to a pointer
before the comparison runs. The two side-effect probes
((actual) != (actual) and (expected) != (expected)) are routed
through it. Both arguments come from the same operand expression, so
T is identical on both sides; -Wsign-compare cannot fire.
The outer (expected) != (actual) is intentionally left at the macro
call site. Routing it through a function template would lose
-Wsign-compare's constant-fits exception that lets idioms like
CHECK_EQUAL(0, some_unsigned) build clean today.
Behavior is unchanged for every non-array operand. For arrays, the
resulting pointer-address self-compare matches the historical C++03
behavior (always false, since the same array's decayed pointer
equals itself).
Add a regression test exercising the array-operand path. Without the
helper, the test TU is ill-formed under C++26 and emits
-Wdeprecated-array-compare under C++20+; with the helper it compiles
at every standard from C++98 through C++26.
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Problem
C++26 [over.match.oper]/p10 makes
array == array(andarray != array) ill-formed.CHECK_EQUAL_LOCATIONexpanded its two side-effect-detection probes literally as(operand) != (operand). As soon as either operand was an array — most commonly a string literal inCHECK_EQUAL("text", x)— the inner self-compare became a forbidden array-vs-array compare.The same pattern is also a
-Wdeprecated-array-compareerror under C++20+ when-Werroris on. CppUTest's existing tests don't exercise the array-operand path, so this never showed up in CI; it surfaces immediately in any consuming project that does.Fix
A small
CppUTestPrivate::checkEqualSelfNotEqual<T>(T, T)helper that takes both arguments by value, so any array decays to a pointer before the comparison runs. The two side-effect probes ((actual) != (actual)and(expected) != (expected)) route through it. Both arguments come from the same operand expression, soTis identical on both sides —-Wsign-comparecannot fire.The outer
(expected) != (actual)is intentionally left at the macro call site. Routing it through a function template would lose-Wsign-compare's constant-fits exception that lets idioms likeCHECK_EQUAL(0, some_unsigned)build clean today.Behavior is unchanged for every non-array operand. For arrays, the resulting pointer-address self-compare matches the historical C++03 behavior (always false, since the same array's decayed pointer equals itself).
Test
Added
CHECK_EQUAL_compilesWithArrayOperandinTestUTestMacro.cpp. Without the fix, the test TU emits-Wdeprecated-array-compareunder C++20+ with-Werror, and is ill-formed at C++26 regardless of warning flags. With the fix it compiles at every standard from C++98 through C++26.CI coverage
Verified by the existing
-Wall -Werrorjobs (Make Defaults, CMake Linux GNU 17/20/23, etc.) —-Wdeprecated-array-compareis on by default at C++20+ and gates the relevant condition. A future PR could add a C++26 job once compiler C++26 support stabilizes; this fix doesn't depend on that addition.Compatibility
Works at C++98 through C++26. No new headers included. The helper is a plain function template — no
<type_traits>, noif constexpr, no forwarding refs.