Fix directive @warning("-102") not working

[cknitt]

Fixes #8160.

Symptom

The reported repro was:

let x = (a, b) => {
  @warning("-102") (a->Pair.second > b->Pair.second)
}

With warning 102 enabled, this still produced:

Warning number 102
Polymorphic comparison introduced (maybe unsafe)

Root Cause

Warning 102 (Bs_polymorphic_comparison) was emitted too late, in compiler/core/lam_compile_primitive.ml.

By the time that code ran, local warning scopes created by source attributes such as @warning("-102") had already been restored. As a result, file-wide warning settings still worked, but expression-level suppression did not affect warning 102.

Fix

The warning emission was moved into typedtree-to-lambda translation in compiler/ml/translcore.ml, where it still runs under the relevant source-level warning scopes.

Concretely:

• warning 102 is emitted when the polymorphic comparison primitive is selected during translation
• the late warning emission was removed from compiler/core/lam_compile_primitive.ml
• translation re-enters warning scopes so local source attributes apply at the warning site

This keeps warning 102 enabled by default, while making local suppression behave correctly.

Tests

Regression tests were added under tests/build_tests/super_errors/:

• warning102_expr_attribute.res
• warning102_structure_attribute.res
• warning102_value_attribute.res

These cover:

• expression-level @warning("-102")
• file-level @@warning("-102")
• direct value use such as let v = @warning("-102") compare

All corresponding snapshots are empty, because successful suppression means there is no warning output.

Result

After the fix:

• @warning("-102") works for local polymorphic comparison expressions
• @@warning("-102") still works
• warning 102 still appears normally when no suppression is present

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commit: 1654767

[cknitt]

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[chatgpt-codex-connector]

Preserve source-order scope for @@warning directives

Calling Builtin_attributes.warning_attribute here mutates the global warning state, but transl_structure translates rem before translating many current items (for example Tstr_value and Tstr_module), so a @@warning placed mid-structure can incorrectly apply to declarations that appear earlier in source order. In practice this can wrongly suppress or promote warnings (including @@warnerror) on the wrong bindings, which changes build outcomes. The warning setting should be applied with proper scoping/restoration that matches source order instead of mutating global state in this branch.

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[cknitt]

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Reviewed commit: 165476741a

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[chatgpt-codex-connector]

Do not warn on null/undefined equality primitives

Emitting Bs_polymorphic_comparison for every Pobjcomp here regresses the previous behavior for null/undefined checks: primitives like %equal_null / %equal_undefined / %equal_nullable (and ==/!= cases with a null/undefined constant) used to bypass warning 102 in lam_compile_primitive because they compile to dedicated null checks, but this early unconditional warning now fires before arguments are inspected. In projects using -warn-error +102, valid null/undefined comparisons can now fail compilation.

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