Problem
TcMethodApplication resolves method calls in two phases. For single-candidate calls (the common case), Phase 2 re-runs constraint checks that Phase 1 already committed, producing duplicate warnings and wasted work.
// --checknulls --langversion:preview
let f (c: C | null) = c.P // FS3261 emitted TWICE
Root cause
Phase 1 (UnifyUniqueOverloading, ConstraintSolver.fs:4008): For exactly 1 candidate, runs CanMemberSigsMatchUpToCheck with NoTrace to guide argument type inference. Solutions committed permanently. Warnings emitted via CommitOperationResult (CheckExpressions.fs:10465).
Phase 2 (ResolveOverloadingForCall, CheckExpressions.fs:10487): Always called. For single candidate, skips ResolveOverloadingCore entirely (line 3654: | _, [calledMeth] when not isOpConversion -> Some calledMeth, CompleteD, NoTrace), but the final commit block (line 3733) unconditionally re-runs the full CanMemberSigsMatchUpToCheck. Warnings emitted via RaiseOperationResult (line 10527).
What's redundant vs. new in Phase 2's final commit
| Sub-step |
Redundant? |
Why |
TDC1: TypesEquiv (instantiation) |
Yes |
calledTyArgs = minst reused from Phase 1 (line 10455→10479). FreshenMethInfo in CalledMeth ctor only freshens property setters, not the method's type params. |
TDC2: TypesMustSubsume (obj-arg) |
Yes |
callerObjArgTys computed once (line 10394), shared by both phases. Source of duplicate FS3261/3262. |
TDC7: ReturnTypesMustSubsumeOrConvert |
Yes |
Same returnTy, same method signature. Source of duplicate FS3388. |
TDC3-6: ArgsMustSubsumeOrConvert + paramArray + named + setters |
No |
Phase 1 used synthetic mkSynUnit placeholders. Phase 2 has real typed Expr args and enforceNullableOptionalsKnownTypes=true. |
Affected warnings
FS3261 (nullness mismatch), FS3262 (strict-null-required), FS3388 (return-type TDC). Any warning from TDC1/2/7 inside CanMemberSigsMatchUpToCheck.
Fix
Add a CanMemberSigsMatchArgsOnly helper (or guarded branch in the final commit) that runs only TDC3-6. Thread alreadyValidatedByPhase1: bool (= existing uniquelyResolved value) through ResolveOverloadingForCall → ResolveOverloading.
Guards (conservative — exclude from fast path):
not isOpConversion — op_Implicit/op_Explicit have special return-type handlingcx.IsNone — SRTP trait resolution not run through Phase 1calledMethTrace = NoTrace — single-candidate path only
Why safe:
- Phase 1 warnings hit the logger before Phase 2 starts (
CommitOperationResult at line 10465)
- Multi-candidate path untouched (uses
WithTrace replay)
- Phase 1 failure → exception at 10465 →
alreadyValidatedByPhase1 never true on error path
- Arg checking (genuinely new work) still runs fully
Constraint on correctness: Missing a warning (0 instead of 1) is drastically worse than duplicating (2 instead of 1). This fix only skips checks whose results are already committed and whose warnings are already emitted. No warning can be lost.
Rollout
Consider gating behind LanguageFeature flag for one release. Validate with --times on large compilations. Update baselines containing duplicated warning lines via TEST_UPDATE_BSL=1.
Related
Problem
TcMethodApplicationresolves method calls in two phases. For single-candidate calls (the common case), Phase 2 re-runs constraint checks that Phase 1 already committed, producing duplicate warnings and wasted work.Root cause
Phase 1 (
UnifyUniqueOverloading,ConstraintSolver.fs:4008): For exactly 1 candidate, runsCanMemberSigsMatchUpToCheckwithNoTraceto guide argument type inference. Solutions committed permanently. Warnings emitted viaCommitOperationResult(CheckExpressions.fs:10465).Phase 2 (
ResolveOverloadingForCall,CheckExpressions.fs:10487): Always called. For single candidate, skipsResolveOverloadingCoreentirely (line 3654:| _, [calledMeth] when not isOpConversion -> Some calledMeth, CompleteD, NoTrace), but the final commit block (line 3733) unconditionally re-runs the fullCanMemberSigsMatchUpToCheck. Warnings emitted viaRaiseOperationResult(line 10527).What's redundant vs. new in Phase 2's final commit
TypesEquiv(instantiation)calledTyArgs = minstreused from Phase 1 (line 10455→10479).FreshenMethInfoinCalledMethctor only freshens property setters, not the method's type params.TypesMustSubsume(obj-arg)callerObjArgTyscomputed once (line 10394), shared by both phases. Source of duplicate FS3261/3262.ReturnTypesMustSubsumeOrConvertreturnTy, same method signature. Source of duplicate FS3388.ArgsMustSubsumeOrConvert+ paramArray + named + settersmkSynUnitplaceholders. Phase 2 has real typedExprargs andenforceNullableOptionalsKnownTypes=true.Affected warnings
FS3261 (nullness mismatch), FS3262 (strict-null-required), FS3388 (return-type TDC). Any warning from TDC1/2/7 inside
CanMemberSigsMatchUpToCheck.Fix
Add a
CanMemberSigsMatchArgsOnlyhelper (or guarded branch in the final commit) that runs only TDC3-6. ThreadalreadyValidatedByPhase1: bool(= existinguniquelyResolvedvalue) throughResolveOverloadingForCall→ResolveOverloading.Guards (conservative — exclude from fast path):
not isOpConversion— op_Implicit/op_Explicit have special return-type handlingcx.IsNone— SRTP trait resolution not run through Phase 1calledMethTrace = NoTrace— single-candidate path onlyWhy safe:
CommitOperationResultat line 10465)WithTracereplay)alreadyValidatedByPhase1never true on error pathConstraint on correctness: Missing a warning (0 instead of 1) is drastically worse than duplicating (2 instead of 1). This fix only skips checks whose results are already committed and whose warnings are already emitted. No warning can be lost.
Rollout
Consider gating behind
LanguageFeatureflag for one release. Validate with--timeson large compilations. Update baselines containing duplicated warning lines viaTEST_UPDATE_BSL=1.Related