What does this fix?
The following C code doesn't cause conversion warnings (nor GCC neither CLANG with -Wsign-conversion)

void test() {
	unsigned x = 1 << 5;
}

The issue is if you enable the -Wconversion flag, you will get warnings such as:

/home/girgias/Dev/php-src/Zend/zend_hash.h:1615:19: warning: implicit conversion changes signedness: 'int' to 'uint32_t' (aka 'unsigned int') [-Wsign-convers>
 1615 |                 HT_FLAGS(ht) &= ~HASH_FLAG_STATIC_KEYS;
      |                              ~~ ^~~~~~~~~~~~~~~~~~~~~~

because the signed bitmask flag is being assigned to an unsigned struct field. And because virtually all fields in Zend that are bitmask flags are of type uint32_t each individual flag also be unsigned IMHO.

I'll fix the description of the PR as it is indeed the wrong compiler flag I mentioned.

The issue is if you enable the -Wconversion flag, you will get warnings such as:

/home/girgias/Dev/php-src/Zend/zend_hash.h:1615:19: warning: implicit conversion changes signedness: 'int' to 'uint32_t' (aka 'unsigned int') [-Wsign-convers>
 1615 |                 HT_FLAGS(ht) &= ~HASH_FLAG_STATIC_KEYS;
      |                              ~~ ^~~~~~~~~~~~~~~~~~~~~~

because the signed bitmask flag is being assigned to an unsigned struct field. And because virtually all fields in Zend that are bitmask flags are of type uint32_t each individual flag also be unsigned IMHO.

You are right. May be changing the left operand of the shift is enough?

void test(unsigned *p) {
	*p &= ~(1U << 5);
}

It looks like Rust changes the mind of C compilers developers and we have to bloat the source code with more and more qualifications... :)