Improve active pattern error reporting (#17666)

* Improve active pattern error reporting * release notes.
dotnet · Sep 6, 2024 · 7bfdd73 · 7bfdd73
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diff --git a/docs/release-notes/.FSharp.Compiler.Service/9.0.100.md b/docs/release-notes/.FSharp.Compiler.Service/9.0.100.md
@@ -45,6 +45,7 @@
 * Better error reporting for unions with duplicated fields. ([PR #17521](https://github.com/dotnet/fsharp/pull/17521))
 * Better error reporting for let bindings. ([PR #17601](https://github.com/dotnet/fsharp/pull/17601))
 * Optimize ILTypeDef interface impls reading from metadata. ([PR #17382](https://github.com/dotnet/fsharp/pull/17382))
+* Better error reporting for active patterns. ([PR #17666](https://github.com/dotnet/fsharp/pull/17666))
 
 
 ### Breaking Changes
diff --git a/src/Compiler/Checking/Expressions/CheckExpressions.fs b/src/Compiler/Checking/Expressions/CheckExpressions.fs
@@ -8674,14 +8674,14 @@ and TcUnionCaseOrExnCaseOrActivePatternResultItemThen (cenv: cenv) overallTy env
     let ucaseAppTy = NewInferenceType g
     let mkConstrApp, argTys, argNames =
         match item with
-        | Item.ActivePatternResult(apinfo, _apOverallTy, n, _) ->
+        | Item.ActivePatternResult(apinfo, _apOverallTy, n, m) ->
             let aparity = apinfo.ActiveTags.Length
             match aparity with
             | 0 | 1 ->
                 let mkConstrApp _mArgs = function [arg] -> arg | _ -> error(InternalError("ApplyUnionCaseOrExn", mItem))
                 mkConstrApp, [ucaseAppTy], [ for s, m in apinfo.ActiveTagsWithRanges -> mkSynId m s ]
             | _ ->
-                let ucref = mkChoiceCaseRef g mItem aparity n
+                let ucref = mkChoiceCaseRef g m aparity n
                 let _, _, tinst, _ = FreshenTyconRef2 g mItem ucref.TyconRef
                 let ucinfo = UnionCaseInfo (tinst, ucref)
                 ApplyUnionCaseOrExnTypes mItem cenv env ucaseAppTy (Item.UnionCase(ucinfo, false))
@@ -11059,7 +11059,7 @@ and TcNormalizedBinding declKind (cenv: cenv) env tpenv overallTy safeThisValOpt
             else rhsExprChecked
 
         match apinfoOpt with
-        | Some (apinfo, apOverallTy, _) ->
+        | Some (apinfo, apOverallTy, m) ->
             let activePatResTys = NewInferenceTypes g apinfo.ActiveTags
             let _, apReturnTy = stripFunTy g apOverallTy
             let apRetTy =
@@ -11080,7 +11080,7 @@ and TcNormalizedBinding declKind (cenv: cenv) env tpenv overallTy safeThisValOpt
                 checkLanguageFeatureError g.langVersion LanguageFeature.StructActivePattern mBinding
             | ActivePatternReturnKind.RefTypeWrapper -> ()
 
-            UnifyTypes cenv env mBinding (apinfo.ResultType g rhsExpr.Range activePatResTys apRetTy) apReturnTy
+            UnifyTypes cenv env mBinding (apinfo.ResultType g m activePatResTys apRetTy) apReturnTy
 
         | None ->
             if isStructRetTy then

diff --git a/tests/FSharp.Compiler.ComponentTests/Conformance/PatternMatching/Named/E_LargeActivePat01.fs b/tests/FSharp.Compiler.ComponentTests/Conformance/PatternMatching/Named/E_LargeActivePat01.fs
@@ -5,3 +5,14 @@
 
 let (|One|Two|Three|Four|Five|Six|Seven|Eight|) x = One
 
+let (|A|B|C|D|E|F|G|H|) x =
+    match x with
+    | 0 -> A
+    | 1 -> B
+    | 2 -> C
+    | 3 -> D
+    | 4 -> E
+    | 5 -> F
+    | 6 -> G
+    | _ -> H
+
diff --git a/tests/FSharp.Compiler.ComponentTests/Conformance/PatternMatching/Named/Named.fs b/tests/FSharp.Compiler.ComponentTests/Conformance/PatternMatching/Named/Named.fs
@@ -470,8 +470,10 @@ but here has type
         |> withOptions ["--test:ErrorRanges"]
         |> typecheck
         |> shouldFail
-        |> withSingleDiagnostic (Error 265, Line 6, Col 53, Line 6, Col 56, "Active patterns cannot return more than 7 possibilities")
-
+        |> withDiagnostics [
+            (Error 265, Line 6, Col 6, Line 6, Col 47, "Active patterns cannot return more than 7 possibilities")
+            (Error 265, Line 8, Col 6, Line 8, Col 23, "Active patterns cannot return more than 7 possibilities")
+        ]
 
     // This test was automatically generated (moved from FSharpQA suite - Conformance/PatternMatching/Named)
     [<Theory; Directory(__SOURCE_DIRECTORY__, Includes=[|"E_MulticasePartialNotAllowed01.fs"|])>]