fix: coerce operand types in Interval mul/div/intersect/union/contains

[adriangb]

Which issue does this PR close?

• Closes #.

Rationale for this change

Interval::mul, Interval::div, Interval::intersect, Interval::union, and Interval::contains all asserted that both operands had identical data types. This causes internal errors during interval propagation for ordinary SQL queries that mix Decimal128 precisions/scales — for example dividing numeric (which DataFusion maps to Decimal128(38, 10)) by a BIGINT column or count(*) (coerced to Decimal128(20, 0)):

Internal error: Assertion failed: dt.clone() == rhs_type.clone()
  (left: Decimal128(38, 10), right: Decimal128(20, 0)):
  Intervals must have the same data type for division

Interval::add and Interval::sub already used BinaryTypeCoercer to find a common arithmetic type; this PR brings mul and div in line. Once mul/div coerce, the result of an arithmetic op fed into intersect (e.g. by the CP solver in cp_solver.rs) may have a different type than the child interval, so intersect/union/contains are also relaxed to coerce via comparison_coercion (matching the existing pattern in Interval::contains_value).

Reproducer (datafusion-cli, before this PR)

CREATE TABLE t(c1 BIGINT) AS VALUES (1::bigint), (2::bigint), (10::bigint);

SELECT
  c1,
  CASE WHEN c1 = 0 THEN 100.0
       ELSE ROUND((1.0 - (c1::numeric / c1)) * 100, 2)
  END AS rate
FROM t
WHERE (1.0 - (c1::numeric / c1)) * 100 < 95.0;

fails with the internal-error message above. After this PR the query returns rows.

What changes are included in this PR?

• Replace the type-equality asserts in Interval::mul and Interval::div with a new coerce_operands helper that uses BinaryTypeCoercer::get_result_type and casts both intervals to the common type when they differ.
• Replace the type-equality asserts in Interval::intersect, Interval::union, and Interval::contains with a new coerce_for_comparison helper that uses comparison_coercion and casts both intervals to the common type when they differ.
• Update the doc comments on the affected methods to document the new coercion behavior.

The same-type fast path is preserved (no allocation/cast when both intervals already share a type), so this should be a no-op for existing call sites.

Are these changes tested?

Yes:

• New unit test test_mul_div_mismatched_operand_types in interval_arithmetic.rs exercising Decimal128(38, 10) / Decimal128(20, 0) and Decimal128 × Int64 for both mul and div.
• New regression query in decimal.slt covering the numeric / bigint shape from the reproducer above through the optimizer's interval-propagation path. Without this fix the SLT query fails with an internal error; with it, it returns rows.
• Existing datafusion-expr-common, datafusion-physical-expr, datafusion-physical-plan, and full sqllogictests suites all pass.

Are there any user-facing changes?

Queries that previously hit Internal error: Intervals must have the same data type ... (or the equivalent for intersect/union/contains) during interval/stats propagation will now succeed. No public API changes — these methods kept their signatures; the documented preconditions are relaxed.

🤖 Generated with Claude Code

[adriangb]

@pepijnve @neilconway any interest in reviewing this fix?