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Bugfix :: Nullness :: Bug selection - #19262

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T-Gro merged 22 commits into
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nullness-bugs
Feb 18, 2026
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Bugfix :: Nullness :: Bug selection#19262
T-Gro merged 22 commits into
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nullness-bugs

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@T-GroT-Gro commented Feb 8, 2026•
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Description

Five nullness-related bug fixes and two regression tests for the F# compiler's nullable reference type checking.

Bug Fixes

  1. UoM ToString returns string for value types (fixesNullness issue - ToString on an integer with UoM takes nullness information from ValueType #17539)

    • int<kg>.ToString() now returns string instead of string | null
    • Added isMeasureableValueType helper in TypedTreeOps.fs
    • Extended the ToString return-type override in MethodCalls.fs to cover measureable value types

  2. Pipe operator nullness warning location (fixesNullness issue - Error shows on incorrect symbol when using |> #18013)

    • Nullness warnings from piped expressions (e.g. bar |> foo "mr") now point at the nullable argument (bar) instead of the pipe operator
    • Added NullnessCheckOfCapturedArg context to ConstraintSolver, propagated through CheckExpressions.fs

  3. AllowNullLiteral false positive on non-null instances (fixesNullness issue - No way to silence warning in e.g. ServiceCollection.AddSingleton if type allows null #18021)

    • MyClass() where MyClass has [<AllowNullLiteral>] no longer triggers a false nullness warning
    • Rewrote TypeNullIsExtraValueNew to check explicit nullness annotation first, only falling back to AllowNullLiteral when nullness is ambivalent

  4. not null constraint on type extensions (fixesNullness issue – Invalid type constraint resolution on ILookup #18334)

    • type ILookup<'Key, 'Value when 'Key : not null> with ... no longer errors
    • Added typarsAEquivWithAddedNotNullConstraintsAllowed for extension checks
    • Fixed letrec binding fixup to strip/restore extra not-null constraints during CheckTypars

  5. Tuple null elimination over-inference (fixesNullness issue - type check in multi-match expressions doesn't eliminate null #19042)

    • match x, y with | null, _ -> ... | s, 5 -> test s no longer over-infers non-null when non-wild patterns restrict non-nullable elements
    • Simplified the tuple null elimination condition

Regression Tests (no code changes, tests only)

Changed Files (13 total)

Source (10 files):

  • src/Compiler/Checking/CheckDeclarations.fs
  • src/Compiler/Checking/ConstraintSolver.fs + .fsi
  • src/Compiler/Checking/Expressions/CheckExpressions.fs
  • src/Compiler/Checking/MethodCalls.fs
  • src/Compiler/Driver/CompilerDiagnostics.fs
  • src/Compiler/Symbols/FSharpDiagnostic.fs
  • src/Compiler/TypedTree/TypedTreeOps.fs + .fsi

Tests (3 files):

  • tests/FSharp.Compiler.ComponentTests/Language/Nullness/NullableReferenceTypesTests.fs (704 lines added)
  • 3 BSL files updated for changed error locations/baselines

Release notes:

  • docs/release-notes/.FSharp.Compiler.Service/10.0.300.md

@T-Gro
T-Gro requested a review from a team as a code ownerFebruary 8, 2026 18:36
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✅ No release notes required

@T-Gro
T-Groforce-pushed the nullness-bugs branch 8 times, most recently from ce5d2f0 to 052d236CompareFebruary 8, 2026 22:23
@T-Gro
Fix nullness bugs:#17539,#18013,#18021,#18334,#19042
48baf1b
Fix#17539: UoM ToString on value types returns string instead of string|null
Fix#18013: Pipe operator nullness warning location points to nullable argument
Fix#18021: No false positive nullness warning for non-null AllowNullLiteral instances
Fix#18334: Allow 'not null' constraint on type extensions
Fix#19042: Multi-match tuple null elimination with restricting patterns
Also adds regression tests for #17727 (Option.toObj inference) and
#18034 (FSharpPlus monad CE with nullness).
@T-GroT-Gro changed the title Fix nullness bugs: #17539, #18013, #18021, #18334, #19042Bugfix :: Nullness :: Bug selectionFeb 9, 2026
T-Gro added 14 commits February 9, 2026 17:12
@T-Gro
Add KnownFromConstructor DU case to Nullness and update constraint so…
24ebb80
…lver
@T-Gro
Revert TypeNullIsExtraValueNew to sound AllowNullLiteral logic and up…
ef447ce
…date tests
- Delete explicitNullnessOfTy helper (unsound)
- Revert TypeNullIsExtraValueNew to main branch logic using stripTyparEqns,
tryTcrefOfAppTy, TypeHasAllowNull, nullnessOfTy, GetTyparTyIfSupportsNull
- Update 6 AllowNullLiteral tests (B1-B6) for new diagnostics
- Remove GitHub issue URL comments from test code
@T-Gro
Fix code quality: restore docs, add comments, remove meaningless test
5b86a7d
@T-Gro
Fixup: deduplicate ILookup test source, remove redundant comments
95ae81e
@T-Gro
Fixup: narrow context passing to nullness-only in TcExprFlex and Unif…
f17771d
…yOverallType
Filter env.eContextInfo to only forward NullnessCheckOfCapturedArg context
to the constraint solver, passing NoContext for all other cases. This
prevents unintended side effects on non-nullness error formatting (e.g.
duplicated parse errors and expanded type mismatch messages in neg83.vsbsl).
@T-Gro
Fixup: extract nullnessContextOnly helper, remove test-only public AP…
6f50b3d
…I (IsFromConstructor, KnownWithoutNullFromCtor)
@T-Gro
Fixup: remove dead KnownFromConstructor DU case and clean up cosmetic…
ff1ddef
… issues
- Remove Nullness.KnownFromConstructor (dead code: never constructed in production)
- Remove all handling branches in ConstraintSolver.fs, TypedTree.fs, TypedTreeBasics.fs
- Replace KnownFromConstructor-specific tests with equivalent Known WithoutNull tests
- Remove redundant comments in TypedTreeOps.fs
- Clean up blank line artifacts in .fsi files
@T-Gro
Fixup: use ListSet.isSubsetOf in typarsAEquivWithFilter instead of in…
23c6dd2
…lined subset check
@T-Gro
Fixup: remove leftover NullnessInternalsTests, restore TypeNullIsExtr…
82a0bc3
…aValueNew comments
@T-Gro
Add sprint file 28_Fixup_3.md for HONEST-ASSESSMENT scope verification
dbab7a8
@T-Gro
AllowNullLiteral not warn if used right after .ctor call
e6828bf
@T-Gro
After assignment, strip KnownFromConstructor
dc43a02
@T-Gro
Fix FS1182: move 'let g' back inside #if !NO_TYPEPROVIDERS in TcCtorI…
95de804
…temThen
The proto/bootstrap build (netstandard2.0 with NO_TYPEPROVIDERS defined) fails
because 'let g = cenv.g' was moved outside the #if block but is only used
inside #if !NO_TYPEPROVIDERS code paths within TcCtorItemThen, causing
error FS1182 (unused value treated as error via --warnaserror).
@T-Gro
Extract shared AllowNullLiteral+consumeNonNull test preamble to reduc…
d33b220
…e duplication
@T-Gro
T-Gro requested a review from abonieFebruary 13, 2026 09:57
abonie
abonie reviewed Feb 18, 2026
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Comment threadsrc/Compiler/Checking/Expressions/CheckExpressions.fs Outdated
@T-Gro@abonie
Update src/Compiler/Checking/Expressions/CheckExpressions.fs
729a840
Co-authored-by: Adam Boniecki <20281641+abonie@users.noreply.github.com>
@T-Gro
T-Gro requested a review from abonieFebruary 18, 2026 14:54
@T-Gro
T-Gro enabled auto-merge (squash) February 18, 2026 14:54
@github-project-automationgithub-project-automationBot moved this from New to In Progress in F# Compiler and ToolingFeb 18, 2026
@T-Gro
T-Gro merged commit 176e33d into mainFeb 18, 2026
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@github-project-automationgithub-project-automationBot moved this from In Progress to Done in F# Compiler and ToolingFeb 18, 2026
This was referenced Feb 19, 2026
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@T-Gro
T-Gro deleted the nullness-bugs branch February 26, 2026 11:04
@dotnet-botdotnet-bot mentioned this pull request Mar 2, 2026
Closed
T-Gro added a commit that referenced this pull request Mar 2, 2026
@T-Gro
Fixup#1: Restore deleted release notes, add trailing newline
5c94a60
- Restore docs/release-notes/.Language/11.0.md (DIM hierarchy notes from PR #19241)
- Restore docs/release-notes/.FSharp.Compiler.Service/10.0.300.md entries
(Nullness fixes PR #19262, graph-based checking PR #19280, semantic
classification PR #19311, type checker recovery PR #19314, overload
resolution cache)
- Add trailing newline to Operators.fs
- Remove untracked docs/RFC_Review.md review artifact
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
@dotnet-botdotnet-bot mentioned this pull request Mar 14, 2026
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@github-actionsgithub-actionsBot mentioned this pull request Mar 20, 2026
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