[jishnub]

Space promotion in promoteplus produces operator types that depend on the values of the spaces. For example, Conversion(Chebyshev(), Ultraspherical(λ)) is a ConcreteConversion for λ = 1 but a ConversionWrapper for λ = 2. That union used to reach the PlusOperator constructor, so + inferred as a 3-way union.

Changes:

• promoteplus now keeps the concrete operator type only if promotion preserves it, and otherwise stores the operators as Operator{T}. The bandwidths are computed from the promoted tuple before the operators are converted to Operator{T}, so the bandwidth type parameters still infer.
• promotedomainspace/promoterangespace on a Conversion return the operator unchanged if the space already matches, instead of rebuilding it.

Inferred return type of +, with C1 = Conversion(Chebyshev(), Ultraspherical(1)), C2 = Conversion(Chebyshev(), Ultraspherical(2)), D = Derivative(Chebyshev()):

Sum	Before	After
C1 + C2	3-way union	concrete
C1 + C1, D + D	3-way union	2-way union
D + C1, Multiplication + D	—	concrete

Tradeoff: sums whose spaces genuinely need promotion now always store Vector{Operator{T}}.

The ApproxFunBase and ApproxFunOrthogonalPolynomials test suites pass locally against this branch. The PR adds no new inference test, because none of the spaces in ApproxFunBase reproduce this case.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.47%. Comparing base (023b1a4) to head (e4bab05).
⚠️ Report is 57 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #708      +/-   ##
==========================================
+ Coverage   75.20%   76.47%   +1.27%     
==========================================
  Files          79       81       +2     
  Lines        8445     8761     +316     
==========================================
+ Hits         6351     6700     +349     
+ Misses       2094     2061      -33     

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