Assertion failure `it != implicitTypes.end()` in wasm::WATParser::ParseModuleTypesCtx::makeTypeUse at contexts.h:1474 (main branch @3ef8d19)

[HackC0der]

Summary

An assertion failure is triggered in wasm::WATParser::ParseModuleTypesCtx::makeTypeUse() when parsing malformed WebAssembly input with wasm-emscripten-finalize.
The assertion it != implicitTypes.end() fails when looking up an expected implicit type.

Affected Version

• Branch: main
• Commit: 3ef8d19 (2026-04-21 latest)
• Tool: wasm-emscripten-finalize
• Build type: Debug + AddressSanitizer

Reproduction

mkdir build && cd build

cmake \
  -DCMAKE_C_COMPILER=/home/AFLplusplus/afl-clang-fast \
  -DCMAKE_CXX_COMPILER=/home/AFLplusplus/afl-clang-fast++ \
  -DCMAKE_C_FLAGS="-O0 -g -fno-omit-frame-pointer -fsanitize=address -Wno-error" \
  -DCMAKE_CXX_FLAGS="-std=c++20 -O0 -g -fno-omit-frame-pointer -fsanitize=address -Wno-bitwise-instead-of-logical -Wno-ambiguous-reversed-operator -Wno-error" \
  -DCMAKE_EXE_LINKER_FLAGS="-fsanitize=address -no-pie -pthread -ldl -lm" \
  -DCMAKE_BUILD_TYPE=Debug \
  -DBUILD_TESTS=OFF \
  -DBUILD_TOOLS=ON \
  -DENABLE_BYESAN=OFF \
  ..

make -j$(nproc)

./wasm-emscripten-finalize -n poc

Error Log

wasm-emscripten-finalize: /home/new-cases/binaryen/latestest-04.21/binaryen-main/src/parser/contexts.h:1474: Result<wasm::WATParser::ParseModuleTypesCtx::TypeUseT> wasm::WATParser::ParseModuleTypesCtx::makeTypeUse(wasm::Index, std::optional<HeapTypeT>, wasm::WATParser::TypeParserCtx<wasm::WATParser::ParseModuleTypesCtx>::ParamsT *, wasm::WATParser::TypeParserCtx<wasm::WATParser::ParseModuleTypesCtx>::ResultsT *): Assertion `it != implicitTypes.end()' failed.

Root Cause

The method makeTypeUse() at contexts.h:1474 expects to find a type entry in implicitTypes,
but the input causes a lookup of a non-existent type, leading to an assertion failure.

Call chain:

wasm-emscripten-finalize parsing → ParseModuleTypesCtx::makeTypeUse() → Assertion it != implicitTypes.end() failed

PoC

PoC