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Fix #19596: unit-parameter member conformance with signature#19615

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Fix #19596: unit-parameter member conformance with signature#19615
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fix/unit-member-conformance

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@T-Gro T-Gro commented Apr 19, 2026 •
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Fixes #19596

Stacked on #19609.

member M(()) produces ValReprInfo [[]] (empty arg group), but sig member M: unit -> unit parses to [[unit_arg]]. Both produce identical IL (.method instance void M()). The conformance checker rejected them because TotalArgCount differs (1 vs 2 including this).

Fix: Relax signature conformance matching for the unit-parameter case:

  • checkValInfo: allow empty impl group [] to match singleton sig group [_]
  • Overloaded matching: fallback using typeAEquivAux EraseAll + TotalArgCount diff=1 guard
  • Single-val [av],[fv] path: same relaxation

Safety:

  • IL verified identical via verifyILContains test
  • EraseAll matches the erasure mode used by valLinkageAEquiv in the primary matching path
  • TotalArgCount diff=1 guard restricts to unit-param case only
  • Existing E_StructConstructor01 conformance test still correctly fails
  • Both directions tested with consumer: sig+impl+consumer compiles correctly

@T-Gro T-Gro requested a review from a team as a code owner April 19, 2026 17:23
@T-Gro T-Gro changed the base branch from sig-roundtrip-sweep to main April 19, 2026 17:30
@T-Gro T-Gro changed the base branch from main to sig-roundtrip-sweep April 19, 2026 17:37
@T-Gro T-Gro force-pushed the fix/unit-member-conformance branch from e8e6743 to e4bc6ae Compare April 20, 2026 13:50
T-Gro added a commit that referenced this pull request Apr 20, 2026
@T-Gro
Fix release note PR reference and add trailing newline
9e5e4d3 
- Release note for #19596 now correctly references PR #19615 (not #19609)
- Add missing trailing newline to TypeTests.fs

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
@T-Gro T-Gro force-pushed the sig-roundtrip-sweep branch 2 times, most recently from 4b5e839 to 3608686 Compare April 20, 2026 18:38
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Copilot AI commented Apr 21, 2026

@copilot resolve the merge conflicts in this pull request

Merge conflicts resolved in commit 66e6da3. Here's what was done for each conflict:

  • NicePrint.fs: Took base branch's cleaner Option.defaultValue/IsNullOrEmpty null handling
  • PrettyNaming.fs: Took base branch's public EscapeActivePatternCases and removed the redundant second escape application from my branch's private escapeActivePatternCaseNames
  • TypeTests.fs: Kept my HEAD tests (the unit-param conformance fix tests) and removed the Skip attribute from the roundtrip test since the issue is now fixed
  • TooltipTests.fs: Took base branch's removal of the infeasible backtick tooltip test

Build verified: 0 warnings/errors.

@T-Gro T-Gro force-pushed the sig-roundtrip-sweep branch from 3608686 to 8923561 Compare April 28, 2026 09:15
@T-Gro T-Gro requested a review from abonie April 28, 2026 09:56
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Copilot AI commented Apr 28, 2026

@copilot resolve the merge conflicts in this pull request

Merge conflicts resolved in commit e1bf547:

  • PrettyNaming.fsi: Made EscapeActivePatternCases internal (matching the base branch review fix)
  • TypeTests.fs: Kept my unit-param conformance tests; removed stray <<<<<<< HEAD marker left from the previous merge; kept roundtrip test enabled (no Skip)
  • TooltipTests.fs: Included all new method/property tagging tooltip tests from the base branch

Build: 0 warnings/errors.

abonie
abonie reviewed Apr 28, 2026
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Comment thread tests/FSharp.Compiler.ComponentTests/Signatures/TypeTests.fs
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@T-Gro T-Gro added AI-reviewed PR reviewed by AI review council and removed AI-reviewed PR reviewed by AI review council labels May 7, 2026
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T-Gro commented May 12, 2026
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Review: Fix #19596 — unit-parameter member conformance

The core fix in SignatureConformance.fs is sound for the overloaded case. The three-part approach — relaxed arg group arity in checkValInfo, min-guarded List.take, and type-equivalence fallback matching — correctly handles the M(()) vs M: unit → unit discrepancy.

Key concern: single-val path gap (@abonie's comment is correct)

The [av],[fv] path at line 689 still uses valuesPartiallyMatch which requires strict TotalArgCount equality. A non-overloaded member M(()) with sig member M: unit → unit will fail here (TotalArgCount 1 vs 2). The relaxed matching only applies to the multi-val _ -> branch.

This means the fix only works when M has overloads (so it enters the _ -> branch). A solo member M(()) = () paired with sig member M: unit → unit will still fail. This should be addressed — either with a type-equivalence fallback in the [av],[fv] path, or tracked as a follow-up issue.

Minor observations

  1. NicePrint.fs change is a separate concern — the SRTP constraint dedup in layoutNonMemberVal is unrelated to unit-param conformance. Consider splitting into its own commit for clearer attribution.

  2. FS0503 test documents a behavioral asymmetry — d.M() fails when impl is M(()) but succeeds when impl is M(), despite both having sig member M: unit → unit. The sig should define the API contract, so consumers shouldn’t see impl-level calling convention differences. This may be a pre-existing design constraint, but worth documenting in a comment or tracking separately.

  3. Test coverage for the overloaded path is excellent — both directions, consumer access, IL verification, and negative cases are covered.

Comment thread src/Compiler/Checking/SignatureConformance.fs
Comment thread src/Compiler/Checking/SignatureConformance.fs Outdated
Comment thread src/Compiler/Checking/NicePrint.fs Outdated
@T-Gro T-Gro added the AI-reviewed PR reviewed by AI review council label May 12, 2026
T-Gro and others added 5 commits May 13, 2026 09:58
@T-Gro
Fix #19596: overloaded member with unit parameter conformance check
f40348a 
SignatureConformance.fs: when matching overloaded members, add relaxed
fallback for unmatched pairs using type equivalence. This handles
member M(()) (argInfos=[[]]) vs sig member M: unit->unit (argInfos=[[unit_arg]])
where types are equivalent but TotalArgCount differs.

Also relax checkValInfo arg group check: empty impl group is compatible
with singleton sig group for unit-parameter members.

Tests: roundtrip test, handwritten sig+impl+consumer (both directions).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
@T-Gro
Update release notes: add #19596 fix, fix PR numbers
4e7a8b1 
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
@T-Gro
Add IL verification: M(()) and M() produce identical IL method signat…
5e4aec4 
…ures

Proves the conformance relaxation is safe at IL level — both member M(())
and member M() compile to '.method public hidebysig instance int32 M()',
confirming the representation difference is compile-time only.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
@T-Gro
Add inverse conformance tests for unit param overload
32fc9db 
- Consumer cannot call d.M() when impl is M(()) with overloads (FS0503 expected)
- Consumer can call d.M() when impl is M() with sig M: unit -> unit

Covers both directions of sig↔impl conformance with consumer validation.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
@T-Gro
Address review feedback: fix non-overloaded unit-param conformance
c94466e 
- Add type-equivalence fallback in [av],[fv] single-val path so
  non-overloaded member M(()) conforms to sig member M: unit -> unit
  (addresses abonie + T-Gro feedback)
- Use idiomatic implGroup.IsEmpty && sigGroup.Length = 1 (T-Gro nit)
- Add test for non-overloaded unit-param member conformance

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
@T-Gro T-Gro force-pushed the fix/unit-member-conformance branch from e1bf547 to c94466e Compare May 13, 2026 08:00
@T-Gro T-Gro changed the base branch from sig-roundtrip-sweep to main May 13, 2026 08:00
T-Gro added a commit that referenced this pull request May 13, 2026
@T-Gro
Address expert review: fix PR number, prevent double-consume, style f…
0f7bbfe 
…ixes

- Fix release notes PR number: #19609 -> #19615
- Prevent double-consume of impl vals in relaxed matching by using
  List.fold to track remaining available avs
- Use obj.ReferenceEquals instead of System.Object.ReferenceEquals
  for consistency with codebase style
- Add trailing newline to TypeTests.fs

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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github-actions Bot commented May 13, 2026

❗ Release notes required

✅ Found changes and release notes in following paths:

Change path Release notes path Description
src/Compiler docs/release-notes/.FSharp.Compiler.Service/11.0.100.md

@T-Gro
Address expert review: fix PR number, prevent double-consume, style f…
7ac3c70 
…ixes

- Fix release notes PR number: #19609 -> #19615
- Prevent double-consume of impl vals in relaxed matching by using
  List.fold to track remaining available avs
- Use obj.ReferenceEquals instead of System.Object.ReferenceEquals
  for consistency with codebase style
- Add trailing newline to TypeTests.fs

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
@T-Gro T-Gro force-pushed the fix/unit-member-conformance branch from 0f7bbfe to 7ac3c70 Compare May 13, 2026 14:40
@T-Gro T-Gro requested a review from abonie May 14, 2026 09:23
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Signature generation roundtrip issue: overloaded member with unit parameter

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