Feature: `#elif` preprocessor directive by T-Gro · Pull Request #19323 · dotnet/fsharp

T-Gro · 2026-02-18T12:07:16Z

Inspired by and following up from #19045 by @Thorium.

What

#if WIN64
let path = "/library/x64/runtime.dll"
#elif WIN86
let path = "/library/x86/runtime.dll"
#elif MAC
let path = "/library/iOS/runtime-osx.dll"
#else
let path = "/library/unix/runtime.dll"
#endif

Supports &&, ||, ! in conditions, nesting, and arbitrary chaining. Gated behind --langversion:11.0.

Design decisions

New IfDefElif stack entry (not reusing IfDefElse) — lets the lexer distinguish "a branch was taken" from "we are in #else". Uses 2-bit encoding in the IDE ifdef stack, reducing max nesting from 24 to 12 (plenty).
Four dedicated diagnostics (lexHashElifNoMatchingIf, lexHashElifAfterElse, lexHashElifMustBeFirst, lexHashElifMustHaveIdent) instead of reusing #else error messages.
ConditionalDirectiveTrivia.Elif case added to syntax trivia — enables fold regions, colorization, and grayout in VS without any editor-side changes.

Future work (other repos)

Fantomas: teach the formatter about #elif (parse + format roundtrip)
FsAutoComplete / Ionide: verify #elif grayout, folding, and completion work end-to-end
MS Learn docs: update the conditional compilation page

Creates docs/feature-elif-preprocessor.md covering motivation, semantics, spec grammar changes, tooling impact, and compatibility for the #elif preprocessor directive (fslang-suggestions#1370).

Add #elif support to the F# lexer with IfDefElif stack entry tracking first-match-wins semantics. The lexer correctly handles: - Basic #if/#elif/#else/#endif chains - Multiple #elif directives - Nested #elif inside other #if blocks - Error cases (#elif after #else, #elif without #if) - Language version gating (requires --langversion:11.0) Fix IDE state serialization to use 2 bits per ifdef stack entry (00=if, 01=else, 10=elif) so IfDefElif state survives encodeLexCont/decodeLexCont round-trips correctly.

…Skip path Add CheckLanguageFeatureAndRecover for PreprocessorElif in the n > 0 branch of the ifdefSkip rule, matching the existing check in the n = 0 path. Add explanatory comment for discarded boolean in token rule's #elif handler. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Test exercises the ifdefSkip rule's n > 0 branch with --langversion:10.0 to verify the CheckLanguageFeatureAndRecover call added in the nested inactive #elif path. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

- elifIndentedTest: verifies indented #elif with leading whitespace compiles and runs correctly across all branches - elifMustHaveIdentError: verifies bare #elif without expression produces a compilation error - elifAllFalseNoElseTest: verifies #if/#elif chain where all conditions are false and no #else fallback runs with exit code 0 Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

- elifMustBeFirstWarning: Tests #elif at non-zero column (after block comment) compiles successfully, exercising the shouldStartLine (FS3882) code path - elifMustHaveIdentError: Tests bare #elif without expression triggers FS3883 diagnostic with proper message verification - elifAllFalseNoElseTest: Tests all-false #if/#elif chain without #else compiles and runs with exit code 0 Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

- elifMustBeFirstWarning: Tests shouldStartLine diagnostic (FS1163) for directive not at column 0, exercising the same code path as FS3882 - elifMustHaveIdentError: Tests FS3883 for bare #elif without expression - elifAllFalseNoElseTest: Tests all-false #if/#elif chain with no #else Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Replace the #if-based proxy test (FS1163) with a proper #elif test that triggers FS3882. Use #if DEFINED with withDefines to activate the branch, then place #elif after code on the same line to trigger shouldStartLine. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

- T4+T6: Tokenizer test verifying #elif state encoding round-trip and inactive code classification across lines - T5: Structure/folding test verifying fold regions for #if/#elif/#else/#endif chains - T7: AST trivia test verifying ConditionalDirectiveTrivia.Elif appears in parsed syntax tree Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

- Q1: Extract evalAndSaveElif helper in lex.fsl ifdefSkip rule - Q2: Merge Else/Elif arms in ServiceStructure.fs using or-pattern - Q3: Unify SaveIfHash/SaveElifHash via saveConditionalHash in LexerStore.fs - Q4: Replace stackTopRange double evaluation with nested ValueSome/ValueNone match Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

- B1: elifTakenThenElseSkipped — verifies #else is skipped when prior #elif was taken - B2: elifSecondElifAfterTakenSkipped — verifies subsequent #elif skipped after taken #elif - B3: elifAfterElseErrorInSkipContext — verifies #elif-after-#else error via ifdefSkip path Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Uses the modern FSharpToken API (not legacy FSharpLineTokenizer) to verify FSharpTokenKind.HashElif is emitted for #elif directives. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

- Remove redundant `let m = lexbuf.LexemeRange` rebindings in #else ifdefSkip handler (m already in scope from line 1119) - Inline `let isTrue = evalAndSaveElif()` into `if evalAndSaveElif() then` - Add explanatory comment on IfDefElif arm clarifying why stack is not updated Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

- Clarify that forward compatibility is limited: #elif in inactive code on older compilers is silently ignored (langversion gate prevents in practice) - Update ParsedInputTrivia doc comment to include #elif Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

github-actions · 2026-02-18T12:08:02Z

⚠️ Release notes required, but author opted out

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cc @dotnet/fsharp-team-msft

T-Gro · 2026-02-18T12:11:13Z

docs/feature-elif-preprocessor.md

@@ -0,0 +1,144 @@
+# `#elif` Preprocessor Directive


This file is more of a reviewer pack, will delete it before merging.

T-Gro · 2026-02-18T12:14:06Z

/run fantomas

github-actions · 2026-02-18T12:16:46Z

🔧 CLI Command Report

Command: /run fantomas
Outcome: success

✅ Command succeeded, no changes needed.

T-Gro · 2026-02-18T14:36:29Z

/run fantomas

github-actions · 2026-02-18T14:39:15Z

🔧 CLI Command Report

Command: /run fantomas
Outcome: success

✅ Command succeeded, no changes needed.

T-Gro · 2026-02-18T15:16:12Z

/run ilverify

github-actions · 2026-02-18T15:18:39Z

🔧 CLI Command Report

Command: /run ilverify
Outcome: success

✅ Command succeeded, no changes needed.

- Move 3882,lexHashElifMustBeFirst and 3883,lexHashElifMustHaveIdent to end of FSComp.txt to maintain ascending error code order (they were placed between unnumbered entries and 1171, breaking the sort). - Add 5 new ServiceLexing StackUnexpected ILVerify entries to both Debug and Release net10.0 baselines for the new #elif lexer paths. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

The #elif lexer changes produce 5 new ServiceLexing StackUnexpected IL entries. These need to be added to the netstandard2.0 baselines (Debug + Release). The net10.0 baselines are reverted to their original state as the entries added previously were from a pre-merge CI run with different IL offsets. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

netstandard2.0: Replace original 5 entries with new 5 (bootstrap build shifts original offsets, so only the new #elif entries remain stable). net10.0: Add 5 new entries alongside existing 5 (10 total). Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

The bootstrap build on CI shifts the original ServiceLexing offsets, so only the new #elif-added entries (0x37,0x40,0x87/0x69,0x90/0x72, 0x99/0x7B) remain stable across all 4 baseline configs. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

abonie · 2026-02-20T10:36:32Z

claims-coverage.md

Should delete this one

src/Compiler/Service/ServiceLexing.fs

T-Gro and others added 20 commits February 13, 2026 23:31

Add feature design document for #elif preprocessor directive

8f87f7a

Creates docs/feature-elif-preprocessor.md covering motivation, semantics, spec grammar changes, tooling impact, and compatibility for the #elif preprocessor directive (fslang-suggestions#1370).

Merge branch 'main' into feature-directive-elif

14fec3d

Assert error codes FS3882/FS3883 in #elif diagnostic tests

75f9225

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Add #elif signature file syntax tree baseline

833b476

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Add FSharpLexer.Tokenize test for HashElif token kind

1708c02

Uses the modern FSharpToken API (not legacy FSharpLineTokenizer) to verify FSharpTokenKind.HashElif is emitted for #elif directives. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Refactor LexerStore: extract saveSimpleHash for Else/EndIf

6a345f5

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Add release notes for #elif preprocessor directive

6604fc0

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Add VS Editor colorization tests for #elif keyword and grayout

14f97fa

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

github-project-automation bot added this to F# Compiler and Tooling Feb 18, 2026

github-project-automation bot moved this to New in F# Compiler and Tooling Feb 18, 2026

T-Gro added the NO_RELEASE_NOTES Label for pull requests which signals, that user opted-out of providing release notes label Feb 18, 2026

T-Gro commented Feb 18, 2026

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Merge branch 'main' into feature-directive-elif

1a2f64f

T-Gro and others added 5 commits February 18, 2026 20:27

Merge branch 'main' into feature-directive-elif

0395fc6

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

T-Gro marked this pull request as ready for review February 19, 2026 13:50

T-Gro requested a review from a team as a code owner February 19, 2026 13:50

T-Gro requested a review from abonie February 19, 2026 13:50

Merge branch 'main' into feature-directive-elif

20a16b5

T-Gro enabled auto-merge (squash) February 20, 2026 10:08

abonie reviewed Feb 20, 2026

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Should delete this one

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Feature: `#elif` preprocessor directive#19323

Feature: `#elif` preprocessor directive#19323
T-Gro wants to merge 27 commits intomainfrom
feature-directive-elif

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Design decisions

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