Highlights of the release

Make polymorphic functions more efficient and expressive #17548
SIP-56: Better foundations for match types #18262
Make SIP 54 (Multi-Source Extension Overloads) a standard feature #17441
Value parameter inference for polymorphic lambdas #18041
Add @publicInBinary annotation and -WunstableInlineAccessors linting flag #18402
Stabilize Quotes defn.PolyFunction #18480
Stabilize Quotes Flags.AbsOverride #18482
Add -experimental compiler flags #18571
Stabilize SIP-53 (quote pattern explicit type variable syntax) #18574
Add reflect TypeRepr.dealiasKeepOpaques #18583
Add attributes section to TASTy and use it for Stdlib TASTy #18599
Error when reading class file with unknown newer jdk version #18618
Add support for xsbti.compile.CompileProgress #18739
Improve type inference for functions like fold #18780
Improve error message for mismatched tasty versions, allow configuration of header unpickler #18828
In 3.4 make refutable patterns in a for comprehension an error #18842
Disallow use of PolyFunction in user code #18920
Store source file in TASTY attributes #18948
First step to pipelining support - enable reading Java symbols from TASTy #19074
Activate constrainResult fix in 3.4 #19253
Parallelise JVM backend - Scala 2 port #15392

Deprecation warnings for old syntax

_ type wildcards #18813
private[this] #18819
var x = _ #18821
with as a type operator #18837
xs: _* varargs #18872
trailing _ to force eta expansion #18926

Other changes and fixes

Backend

Count size of parameters for platform limit check #18464
Don't emit line number for synthetic unit value #18717
Avoid too eager transform of $outer for lhs & accessor rhs #18949
Make more anonymous functions static #19251
Fix deadlock in initialization of CoreBTypes using Lazy container #19298
Fix #18769: Allow HK type args in Java signatures. #18883
Loading symbols from TASTy files directly #17594
Use dedicated equals method for univerval equality of chars #18770

Erasure

Get generic signature of fields entered after erasure from their accessor #19207
Detect case where two alternatives are the same after widening ExprTypes #18787
Improve erased params logic #18433

Experimental: Capture Checking

Fix capture set variable installation in Setup #18885
Don't follow opaque aliases when transforming sym info for cc #18929
Reset comparersInUse to zero in ContextState.reset #18915
Special handling of experimental.captureChecking import #17427
Change handling of curried function types in capture checking #18131
Fix #18246: correctly compute capture sets in TypeComparer.glb #18254
New capture escape checking based on levels #18463
A more robust scheme for resetting denotations after Recheck #18534
A more flexible scheme for handling the universal capability #18699
Fix potential soundness hole when adding references to a mapped capture set #18758
Alternative scheme for cc encapsulation #18899
Make reach refinement shallow #19171

F-bounds

Don't check bounds of Java applications in Java units #18054

GADTs

Avoid embedding SelectionProtos in Conversions #17755
Freeze constraints while calculating GADT full bounds #18222

Implicits

Followup fix to transparent inline conversion #18130
Select local implicits over name-imported over wildcard imported #18203
Fix how implicit candidates are combined #18321
Improve error message about missing type of context function parameter #18788
Support implicit arguments before extractor method #18671
Tweak convertible implicits fix #18727

Incremental Compilation

Make incremental compilation aware of synthesized mirrors #18310

Inference

Honour hard unions in lubbing and param replacing #18680

Infrastructure

Use -Yscala2-library-tasty to add Scala 2 lib TASTY to scalac (internal only) #18613
Rename stdlib-bootstrapped-tasty to scala2-library-tasty #18615
Fix #19286: Freeze rubygems-update at < 3.5.0. #19288

Initialization

Fix #17997: Handle intersection type as this type of super type #18069
Add test for issue #17997 affecting the global object initialization checker #18141
Fix i18624 and add test case for it #18859
Treat new Array(0) as immutable #19192
Fix #18407: Ignore Quote/Slice in init checker #18848
Check safe initialization of static objects #16970
Pattern match support in checking global objects #18127
Fix crash in global object initialization checker when select target has no source #18627
Fix warning underlining in global init checker #18668
Fix i18629 #18839
I18628 #18841
Make safe init checker skip global objects #18906
Handle local lazy vals properly #18998

Inline

Fix regression: inline match crash when rhs uses private inlined methods #18595
Add structural classes of dynamicApply before inlining #18766
Set missing expansion span for copied inlined node #18229
Fix callTrace of inlined methods #18738

Linting

Keep tree of type ascriptions of quote pattern splices #18412
Fix false positive in WUnused for renamed path-dependent imports #18468
Fix false positive in WUnused for renamed path-dependent imports (2) #18617
Fix wunused false positive on CanEqual #18641
Implement -Xlint:private-shadow, type-parameter-shadow #17622
Fix: reversed wconf parsing order to mirror scala 2 #18503
Revert Fix false positive in WUnused for renamed path-dependent imports #18514

Macro Annotations

Enter missing symbols generated by the MacroAnnotation expansion #18826

Match Types

Allow Tuple.Head and Tuple.Tail to work with EmptyTuple #17189
Fix match type reduction with avoided types #18043
Strip LazyRef before calling simplified, in MT reduction #18218
Fix MT separate compilation bug #18398
Do not show deprecation warning for _ in type match case #18887

Nullability

Improve logic when to emit pattern type error #18093
Allow nullability flow typing even in presence of pattern match #18206
Fix #11967: flow typing nullability in pattern matches #18212
Fix #18282: consider Predef.eq/ne in nullability flow typing #18299
Make this.type nullable again (unless under -Yexplicit-nulls). #18399

Opaque Types

Type ascribe trees that require opaque type usage #18101

Parser

Fix selecting terms using root #18335
Tweak java getlitch not to skip zero #18491
Fix i18518 #18520
Only apply future patches on future-migration #18820
Parser simple expression error recovery change from null to ??? #19103

Pattern Matching

Fix syntax and parsing of vararg patterns #18055
Avoid over widening in SpaceEngine #18252
Fix regression in exhaustivity of HK types #18303
Fix missing case in isSubspace, which broke reachablility #18326
Unsuppress unchecked warnings #18377
Consider extension methods in Space isSameUnapply #18642
Fix unreachable warning in deeply nested sealed hierarchy #18706
Remove unnecessary and recursive Space decomposition #19216
Prioritise sequence-matches over product-sequence-matches #19260
Propagate constant in result of inline match #18455
Disable match anaylsis in inlined trees #19190
Teach provablyDisjoint about AnyKind #18510
Warn about unchecked type tests in primitive catch cases #19206
Reprioritise seq-match over product-seq-match #19277
Fix exhaustivity due to separate TypeVar lambdas #18616

Presentation Compiler

Support completions for extension definition parameter #18331
Fix: Don't collect map, flatMap, withFilter in for-comprehension #18430
Bugfix: Catch exception from the compiler for broken shadowed pickles #18502
Bugfix: highlight for enum type params #18528
Bugfix: No signature help for local methods #18594
Bugfix: add moduleClass imported symbols in IndexedContext #18620
Bugfix: Named args completions with default values #18633
Fix: match completions for type aliases #18667
Bugfix: add multiline comment completion #18703
Bugfix: Backticked named arguments #18704
Bugfix: [metals] Case completions for tuple type #18751
Completions should prepend, not replace as it is for Scala 2 #18803
Bugfix: rename end marker #18838
Presentation compiler: Bugfix for semantic tokens and synthetic decorations #18955
Show documentation for value forwarders in completions #19200
Bugfix: Document highlight on class constructors #19209
Bugfix: Completions for extension methods with name conflict #19225

Polyfunctions

Check user defined PolyFunction refinements #18457
Support polymorphic functions with erased parameters #18293
Use PolyFunction instead of ErasedFunction #18295

Quotes

Support type variable with bounds in quoted pattern #16910
Add new EXPLICITtpt to TASTy format #17298
Inhibit typer to insert contextual arguments when it is inside arguments of HOAS patterns #18040
Compile quote patterns directly into QuotePattern AST #18133
Add missing span to synthesized product mirror #18354
Improve non-static macro implementation error message #18405
Fix scala 2 macros in traits with type parameters #18663
Patch underlyingArgument to avoid mapping into modules #18923
Fallback erasing term references #18731
Fix ignored type variable bound warning in type quote pattern #18199
Splice hole with singleton captures #18357
Fix macros with erased arguments #18431
Deprecate 3-arg FunctionClass constructor #18472
Deprecate Quotes {MethodType,TermParamClause}.isErased #18479
Avoid crashes on missing positions #19250

Reflection

Add reflect.ValOrDefDef #16974
Check New tree for ill-formed module instantiations #17553
Add reflect TypeLambda.paramVariances #17568
Make check flags for newMethod, newVal and newBind in Quotes API less restrictive #18217
Normalise mirrorType for mirror Synthesis #19199
Add reflect defn.FunctionClass overloads #16849
Stabilize reflect flag JavaAnnotation #19267
Stabilize reflect paramVariance #19268

Reporting

Take into account the result type of inline implicit conversions unless they are transparent #17924
Check if a fatal warning issued in typer is silenced, before converting it into an error #18089
Elide companion defs to a object extending AnyVal #18451
Add regression test for issue i18493 #18497
Add better explanation to error message #18665
Better error message when accessing private members #18690
Improve message for discarded pure non-Unit values #18723
Better error message when a pattern match extractor is not found. #18725
Give "did you mean ...?" hints also for simple identifiers #18747
Better error for definition followed by keyword #18752
Better explain message for 'pattern expected' #18753
Improve failure message of enum fromOrdinal/valueOf #19182
Fix type mismatch error confusion between types with same simple name #19204
Add hint for nested quotes missing staged Quotes #18755
Better error messages for missing commas and more #18785
Fix imported twice error messages #18102
Improve error message for inaccessible types #18406
Future migration warning for with type operator #18818
Improve assertion error message for Apply and TypeApply #18700
Shorten traces for TypeMismatch errors under -explain [#18742](https://gi
Improve with in type migration warning #18852
hub.com//pull/18742)
Future migration warning for alphanumeric infix operator #18908
Make sure that trace is shown correctly in the presence of invalid line numbers #18930
Add migration warning for XML literals in language future #19101
Avoid diagnostic message forcing crashing the compiler #19113
Make sure that the stacktrace is shown with -Ydebug-unpickling #19115
Improve asExprOf cast error formatting #19195
Do not warn on underscore wildcard type in pattern #19249

Scala-JS

Fix #18658: Handle varargs of generic types in JSExportsGen. #18659

Scaladoc

Fix incorrect comment parser used in nightly scaladoc #18523

SemanticDB

Export diagnostics (including unused warnings) to SemanticDB #17835
Bugfix: Incorrect semanticdb span on Selectable #18576
Bugfix: in semanticdb make synthetic apply disambiguator consistent w/ Scala 2 implicit #17341

Standard Library

Intrinsify constValueTuple and summonAll #18013
Fix #18609: Add language.3.4 and language.3.4-migration. #18610

TASTy format

Eliminate FromJavaObject from TASTy of Java sources #19259
Add new HOLETYPES to TASTy format #17225
Add capture checking attributes to TASTy #19033
Add TASTyInfo abstraction #19089
Add UTF8 abstraction in the TASTy format #19090

Tooling

Don't add explanation twice #18779
ExtractDependencies uses more efficient caching #18403
Introduce the SourceVersions 3.4 and 3.4-migration; make 3.4 the default. #18501
Bugfix: Completions for named args in wrong order #18702
Align unpickled Scala 2 accessors encoding with Scala 3 #18874
Reinterpret Scala 2 case accessors xyz$access$idx #18907
Presentation-compiler: Add synthetic decorations #18951
Add compilation unit info to ClassSymbol #19010
Make sure that patches for 3.0 are also applied in later versions #19018

Transform

Also consider @TargetNAME when checking private overrides #18361
Teach PostTyper to handle untupled context closures #17739
Properly dealias tuple types when specializing #18724
Fix condition in prefixIsElidable to prevent compiler crash #18924
Fix #18816: Transfer the span of rewired This nodes in fullyParameterizedDef. #18840
List(...) optimization to avoid intermediate array #17166
Make Array.apply an intrinsic #18537
Add missing span to extension method select #18557

Tuples

Handle TupleXXL in match analysis #19212
Add reverse method to NonEmptyTuple #13752
Refine handling of pattern binders for large tuples #19085
Introduce Tuple.ReverseOnto and use it in Tuple.reverse #19183

Typeclass Derivation

Consider all parents when checking access to the children of a sum #19083

Typer

Fix logic when comparing var/def bindings with val refinements #18049
Fix variance checking in refinements #18053
Fix accessibleType for package object prefixes #18057
Refix avoid GADT casting with ProtoTypes #18085
Avoid shadowing by private definitions in more situations #18142
Refine infoDependsOnPrefix #18204
Fix spurious subtype check pruning when both sides have unions #18213
Reimplement support for type aliases in SAM types #18317
Fix adaptation of constants to constant type aliases #18360
Issue "positional after named argument" errors #18363
Deprecate ops.long.S #18426
Tweak selection from self types #18467
Use the unwidened type when casting structural calls #18527
Fix #18649: Use loBound of param types when materializing a context function. #18651
Identify structural trees on Match Type qualifiers #18765
Tweak approximation of type variables when computing default types #18798
Admit parametric aliases of classes in parent typing #18849
Also add privateWithin when creating constructor proxies #18893
Revert part of Simplify defn.FunctionOf.unapply #19012
Check @TargetNAME when subtyping Refined Types #19081
Record failures to adapt application arguments #18269
Improve handling of AndTypes on the LHS of subtype comparisons #18235
Allow inferred parameter types always, when eta-expanding #18771
Fix failing bounds check on default getter #18419
Use constructor's default getters in case class synthetic apply methods #18716
Keep qualifier of Ident when selecting setter #18714
Retract SynthesizeExtMethodReceiver mode when when going deeper in overloading resolution #18759
Constant fold all the number conversion methods #17446
Refine criterion when to widen types #17180
Run all MatchType reduction under Mode.Type #17937
Force consistent MT post-redux normalisation, disallow infinite match types #18073
Fix #17467: Limit isNullable widening to stable TermRefs; remove under explicit nulls. #17470
Disallow naming the root package, except for selections #18187
Contextual varargs parameters #18186
Encode the name of the attribute in Selectable.selectDynamic #18928
Remove linearization requirement for override ref checks from java classes #18953
Fix type inferencing (constraining) regressions #19189
Repeated params must correspond in override #16836
Convert SAM result types to function types #17740
Disallow infix objects #17966
Fix hasMatchingMember handling NoDenotation #17977
Fix: disallow toplevel infix definitions for vals, vars, givens, methods and implicits #17994
Curried methods are not valid SAM methods #18110
Fix #17115: Try to normalize while computing typeSize. #18386
Add default arguments to derived refined type #18435
Handle dependent context functions #18443
Fix variance loophole for private vars #18693
Avoid crash arising from trying to find conversions from polymorphic singleton types #18760
Allow inner classes of universal traits #18796
Prevent crash when extension not found #18830

Contributors

Thank you to all the contributors who made this release possible 🎉

According to git shortlog -sn --no-merges 3.3.1..3.4.0-RC1 these are:

   458 Martin Odersky
   291 Nicolas Stucki
   132 Fengyun Liu
   118 Dale Wijnand
    77 Jamie Thompson
    69 Sébastien Doeraene
    49 Paweł Marks
    32 Chris Kipp
    27 Guillaume Martres
    26 Rikito Taniguchi
    21 Yichen Xu
    19 EnzeXing
    14 Szymon Rodziewicz
    13 Lucas Leblanc
    12 Jakub Ciesluk
    12 Jędrzej Rochala
    12 Katarzyna Marek
    11 Carl
    10 David Hua
     9 Florian3k
     9 Wojciech Mazur
     8 Eugene Flesselle
     8 ghostbuster91
     7 Hamza Remmal
     7 Ondrej Lhotak
     7 Quentin Bernet
     6 Jan Chyb
     6 Julien Richard-Foy
     6 Kacper Korban
     6 Seth Tisue
     5 Lorenzo Gabriele
     5 Matt Bovel
     5 Som Snytt
     5 Yuito Murase
     5 dependabot[bot]
     3 David
     3 Lucas
     3 Pascal Weisenburger
     3 Tomasz Godzik
     2 Aleksander Rainko
     2 Decel
     2 Guillaume Raffin
     2 Ondřej Lhoták
     2 Oron Port
     2 danecek
     2 rochala
     1 Adam Dąbrowski
     1 Aleksey Troitskiy
     1 Arnout Engelen
     1 Ausmarton Zarino Fernandes
     1 Bjorn Regnell
     1 Daniel Esik
     1 Eugene Yokota
     1 François Monniot
     1 Jakub Cieśluk
     1 John Duffell
     1 John M. Higgins
     1 Justin Reardon
     1 Kai
     1 Kisaragi
     1 Lucas Nouguier
     1 Lukas Rytz
     1 LydiaSkuse
     1 Martin Kucera
     1 Martin Kučera
     1 Matthew Rooney
     1 Matthias Kurz
     1 Mikołaj Fornal
     1 Nicolas Almerge
     1 Preveen P
     1 Shardul Chiplunkar
     1 Stefan Wachter
     1 philippus
     1 q-ata
     1 slim