Fix exhaustive match on tuples of union types#4939
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The subtype checker didn't recognize that a tuple of unions is a subtype of the equivalent union of tuples. For example, ((A | None), (A | None)) was not recognized as a subtype of (A, (A|None)) | ((A|None), A) | (None, None), even though every possible value of the tuple is covered by at least one member of the union. This caused the exhaustiveness checker to reject valid exhaustive matches on tuples of union types when concrete types (like None) were used instead of don't-care patterns. The fix expands a tuple of unions into the equivalent union of tuples via cross-product distribution, then checks if every expanded alternative is covered. A cap of 256 alternatives prevents pathological blowup. Closes #4937
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The subtype checker didn't recognize that a tuple of unions is a subtype of the equivalent union of tuples. This caused the exhaustiveness checker to reject valid
match \exhaustive\on tuples of union types when concrete types were used instead of don't-care patterns.The fix expands a tuple of unions into the equivalent union of tuples via cross-product distribution in
is_x_sub_union, then checks if every expanded alternative is covered. This addresses the long-standing TODO at line 555 of subtype.c. A cap of 256 alternatives prevents pathological blowup.Closes #4937