Championed proposal: dotnet/csharplang#7700
Speclet: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-13.0/params-collections.md
Feature branch: https://github.com/dotnet/roslyn/tree/features/ParamsCollections

• LangVersion (LanguageVersion_01_Declaration, LanguageVersion_02_CallSite)
• Public API changes are approved
  • Add ArgumentKind.ParamCollection #72222
  • Add IParameterSymbol.IsParamCollection property  #71816
• Open ParamCollectionAttribute API proposal for BCL - [API Proposal]: ParamCollectionAttribute runtime#99285
• Add a test against an official definition of ParamCollectionAttribute once we can test against an SDK with it.
• Cloned all param array specific IDE unit-tests that I could find (Clone some IDE tests for params collections #72228) opened three issues.
• Test new betterness rules when two candidates applicable in expanded form and same arguments used in params collection:
  • (assume T implicitly convertible to TBase)
  • ReadOnlySpan<T> vs Span<TBase> (ROS is better)
  • Span/ReadOnlySpan<T> vs TBase[] (Span/ROS is better)
  • Span/ReadOnlySpan<T> vs IEnumerable<TBase> (Span/ROS is better)
  • T[] vs IEnumerable<T> (Neither is better)
  • ICollection<T> vs IEnumerable<TBase> (ICollection<T> is better if covariant conversion exists to IEnumerable<TBase>, otherwise neither is better)
  • ICollection<T> vs IEnumerable<T> (ICollection<T> is better)
• Test usage of all "collection type kinds"
  • Span<T>
  • ReadOnlySpan<T>
  • Create method
  • Collection initializer.
    • Require constructor and Add on the type itself (ImplementsIEnumerableT_04_MissingConstructor, ImplementsIEnumerableT_07_MissingAdd)
  • List interface
• Test usage in attribute (invalid) (Span_InAttribute, CreateMethod_02_InAttribute)
• Test accessibility of constructor+Add (require at least as 'params' method) (ImplementsIEnumerableT_05_InaccessibleConstructor, ImplementsIEnumerableT_08_InaccessibleAdd)
• Test extension Add does not satisfy requirement (ImplementsIEnumerableT_21_AddIsNotAnExtension)
• Test when expected members are Obsolete (incl. collection type itself) (incl. suppression thru obsolete context)
• Test accessibility of Create method (require at least as 'params' method)
• Test dynamic (error when no static resolution and may be applicable in expanded form) (DynamicInvocation_)
• Test lambda parameter with params modifier (attribute not emitted) (WRN_ParamsArrayInLambdaOnly_01)
• Test local function parameter with params modifier (attribute not emitted) (ParamCollectionInLocalFunctionOnly)
• Test better params collection type (BetterConversionFromExpression_)
• Test better element type (BetterNess_01_ElementType)
• Test different sets of arguments as params arguments (ambiguous) (BetterOverload_02_NotSameCollectionElements)
• Ref safety (ParameterRefSafetyScope_)
• params is inherited by override but override is not implicitly scoped (ParameterRefSafetyScope_13_Mismatch)
• Cannot use params collection in expression tree (ExpressionTree)
• Test cycles through expected members, e.g. CollectionType(params CollectionType t). (Cycle_).
• Collection type has required member (error at params declaration site) (CollectionWithRequiredMember_)
• Type inference (GenericInference) (e.g. void M<T>(params IEnumerable<T> e))
• Nullable analysis (NullableAnalysis_)
• Disallowed modifier combinations (ref/in/out) (scoped is permitted for ref structs)
• params difference between overloads (NoOverloadingOnParams_)
• Consuming from metadata, including when metadata has attribute combinations not possible in source (MetadataImport_)
• Evaluation order of invocations
• Well-known members for using etc. which have params collections
• constructor/Add has optional unsafe parameters (overload resolution not affected)
• NoPIA (ParamArrayAttribute is transferred when embedding a related API, ParamCollectionAttribute is not)
• heads up to F# re: interop
• test that VB can consume params collections APIs in normal form
• add to compiler test plan

Manual IDE Test Pass

• params classification (shows up as blue)
• params completion void M(p$$), void M($$ IEnumerable<int> e)
• hovering over a parameter declaration
• hitting enter at reasonable points within the construct indent properly
• params modifier of params collection parameter is carried through when generating an override or interface implementation, similar to params array
• analyzer (nice to have) user has M(new List<int> { ... }) but could do M(...)
• fixer (nice to have) from void M(params int[] i) which forwards to M((IEnumerable<int>)i) then change to M(params IEnumerable<int> i) and potentially remove void M(params int[] i)
• analyzer (nice to have) consider changing from void M(Span<int> s) to void M(params Span<int> s)