Fix isinplace detection for Basis types (#554)

[ChrisRackauckas-Claude]

Summary

This PR fixes issue #554 where creating an ODEProblem from a Basis with controls would throw a MethodError because SciMLBase incorrectly detected the basis as supporting in-place evaluation.

Problem

When a Basis{false, true} (no implicit variables, with controls) was passed to ODEProblem, SciMLBase's isinplace check would return true, causing it to call the basis with a 4-argument signature (du, u, p, t). However, Basis{false, CTRLS} only supports out-of-place signatures like (u, p, t) or (u, p, t, c).

The IMPL type parameter determines whether a basis supports in-place evaluation:

• IMPL=false: Out-of-place only (returns result)
• IMPL=true: In-place (mutates first argument du)

Solution

Added DiffEqBase.isinplace method for Basis types:

DiffEqBase.isinplace(::Basis{IMPL, CTRLS}, n) where {IMPL, CTRLS} = IMPL

This ensures isinplace correctly returns false for Basis{false, CTRLS} and true for Basis{true, CTRLS}, based solely on whether the basis has implicit variables.

Testing

• All existing tests pass (252 tests)
• Verified that isinplace now returns correct value for both Basis{false, false} and Basis{false, true}
• Confirmed ODEProblem can now be created from Basis without the MethodError

Fixes #554

🤖 Generated with Claude Code

Co-Authored-By: Claude noreply@anthropic.com

[ChrisRackauckas-Claude]

Investigation Summary

Root Cause Analysis

The error occurred because there was no SciMLBase.isinplace method defined for Basis. When SciMLBase tried to determine if a function supports in-place calls, it likely used a heuristic that detected the 4-argument signature (u, p, t, c) on Basis{false, true} and incorrectly concluded it was an in-place function.

However, the actual signatures are:

• Basis{false, false}: Out-of-place with no controls: (u, p, t)
• Basis{false, true}: Out-of-place with controls: (u, p, t, c) ← This was being mistaken for in-place!
• Basis{true, false}: In-place with no controls: (du, u, p, t)
• Basis{true, true}: In-place with controls: (du, u, p, t, c)

The IMPL type parameter is what determines in-place vs out-of-place, not the number of arguments.

Implementation Details

The fix is minimal and elegant - just one line:

DiffEqBase.isinplace(::Basis{IMPL, CTRLS}, n) where {IMPL, CTRLS} = IMPL

This leverages the existing type parameter to provide the correct answer without any runtime checks.

Testing Verification

Ran the full test suite (Pkg.test("DataDrivenDiffEq")) - all 252 tests pass with the fix in place.