Missing validation error

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#8,785 opened on May 28, 2026 1 comment 0 reactions 0 assignees View on GitHub

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Assessment

Difficulty: 4/5
Estimated time: 3-5 days
Newbie friendliness: 45/100

Issue type: Bug
Clarity: Mostly clear
Activity status: Quiet
Tech stack: wasm
Domain: compilers

Research direction

Unpack jj.tar.gz and reproduce the V8 validation error with pkg/dart2wasm/bin/run_wasm.js, jj.mjs, and jj.wasm. Compare wasm-opt's handling of the shown unreachable WAT sequence with V8's validation, and add or update a regression test so the tools agree on whether this module is valid.

Written by the indexing model from the issue text.

Description

See jj.tar.gz.

Unpack via:

% tar xvzf jj.tar.gz
...

When running with V8 we get a validation error:

% d8 $PWD/pkg/dart2wasm/bin/run_wasm.js  -- $PWD/jj.mjs $PWD/jj.wasm   
run_wasm.js:346: CompileError: WebAssembly.compile(): Compiling function #181:"main" failed: not enough arguments on the stack for drop (need 1, got 0) @+25996
  }
  ^
CompileError: WebAssembly.compile(): Compiling function #181:"main" failed: not enough arguments on the stack for drop (need 1, got 0) @+25996

1 pending unhandled Promise rejection(s) detected.

Though optimizing with wasm-opt works fine and the result runs:

% wasm-opt --enable-gc --enable-reference-types --enable-multivalue --enable-exception-handling --enable-nontrapping-float-to-int --enable-sign-ext --enable-bulk-memory --enable-threads --enable-simd '--no-inline=*<noInline>*' --traps-never-happen -g -Os --gufa -Os -Os jj.wasm -o jj.wasm  
% d8 $PWD/pkg/dart2wasm/bin/run_wasm.js  -- $PWD/jj.mjs $PWD/jj.wasm
%

Is wasm-opt not supposed to issue a validation error here as well? (even though the corresponding code is unreachable)

        local.get $reporter
        drop
        drop
        block $label2
          unreachable
        end
        drop

Dominant language: WebAssembly
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