Merge #18901: fuzz: use std::optional for sep_pos_opt variable

420fa07 fuzz: use std::optional for sep_pos variable (Harris)

Pull request description:

  This PR changes the original `size_t sep_pos` to `std::optional<size_t> sep_post_opt` to remove the warning when compiling fuzz tests.

  ```shell
  warning: variable 'sep_pos' may be uninitialized when used here [-Wconditional-uninitialized]
  ```

  Also, it adds `--enable-c++17` flag to CI fuzz scripts.

ACKs for top commit:
  practicalswift:
    ACK 420fa07
  MarcoFalke:
    ACK 420fa07

Tree-SHA512: e967d5d8ab8ee7394b243ff5b28bac72d30bd14774e4a206f8c87474fad22769da76e4ba4e03cbef83b8f60e5293e9d9293b613e2e2e59e187d4e59ae6b874ca

Author: MarcoFalke

ci/test/00_setup_env_native_fuzz.sh

@@ -14,4 +14,4 @@ export RUN_UNIT_TESTS=false
 export RUN_FUNCTIONAL_TESTS=false
 export RUN_FUZZ_TESTS=true
 export GOAL="install"
-export BITCOIN_CONFIG="--enable-fuzz --with-sanitizers=fuzzer,address,undefined CC=clang CXX=clang++"
+export BITCOIN_CONFIG="--enable-fuzz --with-sanitizers=fuzzer,address,undefined --enable-c++17 CC=clang CXX=clang++"

ci/test/00_setup_env_native_fuzz_with_valgrind.sh

@@ -15,4 +15,4 @@ export RUN_FUNCTIONAL_TESTS=false
 export RUN_FUZZ_TESTS=true
 export FUZZ_TESTS_CONFIG="--valgrind"
 export GOAL="install"
-export BITCOIN_CONFIG="--enable-fuzz --with-sanitizers=fuzzer CC=clang CXX=clang++"
+export BITCOIN_CONFIG="--enable-fuzz --with-sanitizers=fuzzer --enable-c++17 CC=clang CXX=clang++"

src/test/fuzz/asmap_direct.cpp

@@ -6,27 +6,27 @@
 #include <test/fuzz/fuzz.h>
 
 #include <cstdint>
+#include <optional>
 #include <vector>
 
 #include <assert.h>
 
 void test_one_input(const std::vector<uint8_t>& buffer)
 {
     // Encoding: [asmap using 1 bit / byte] 0xFF [addr using 1 bit / byte]
-    bool have_sep = false;
-    size_t sep_pos;
+    std::optional<size_t> sep_pos_opt;
     for (size_t pos = 0; pos < buffer.size(); ++pos) {
         uint8_t x = buffer[pos];
         if ((x & 0xFE) == 0) continue;
         if (x == 0xFF) {
-            if (have_sep) return;
-            have_sep = true;
-            sep_pos = pos;
+            if (sep_pos_opt) return;
+            sep_pos_opt = pos;
         } else {
             return;
         }
     }
-    if (!have_sep) return; // Needs exactly 1 separator
+    if (!sep_pos_opt) return; // Needs exactly 1 separator
+    const size_t sep_pos{sep_pos_opt.value()};
     if (buffer.size() - sep_pos - 1 > 128) return; // At most 128 bits in IP address
 
     // Checks on asmap