fix(flambda2-types): Prevent duplicate equations in add_equation #3306
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The new meet is cautious about keeping track of whether types are refined during the meet, and uses that information to avoid duplicating equations. However, this check is only done when performing a meet between an alias and a non-alias type, but not when equations are added through `Typing_env.add_XXX` functions. In particular, equations added when joining environments (through `add_env_extension_from_level`) are not deduplicated, which can (theoretically) cause the same equation to appear at multiple levels. This patch exposes the `meet_return_value` type to the typing env and uses it to perform duplication directly in `add_equation`. Since we now deduplicate in `add_equation`, we don't need to do it during the meet, which also eliminates a double meet.
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The new meet is cautious about keeping track of whether types are refined during the meet, and uses that information to avoid duplicating equations.
However, this check is only done when performing a meet between an alias and a non-alias type, but not when equations are added through
Typing_env.add_XXXfunctions. In particular, equations added when joining environments (throughadd_env_extension_from_level) are not deduplicated, which can (theoretically) cause the same equation to appear at multiple levels.This patch exposes the
meet_return_valuetype to the typing env and uses it to perform duplication directly inadd_equation. Since we now deduplicate inadd_equation, we don't need to do it during the meet, which also eliminates a double meet.I suggest hiding whitespace changes to review the changes in the typing env.