Reject non-function bindings for single-case and partial active pattern names (dotnet#19763)

Author: Copilot

docs/release-notes/.FSharp.Compiler.Service/11.0.100.md

@@ -1,5 +1,6 @@
 ### Fixed
 
+* Reject non-function bindings for single-case and partial active pattern names with FS1209, matching the existing multi-case behavior. ([PR #19763](https://github.com/dotnet/fsharp/pull/19763))
 * Fix FS0421 "The address of the variable cannot be used at this point" incorrectly raised for the discard pattern `let _ = &expr` when `let x = &expr` compiles. ([Issue #18841](https://github.com/dotnet/fsharp/issues/18841), [PR #19811](https://github.com/dotnet/fsharp/pull/19811))
 * Honor `--nowarn` and `--warnaserror` for warnings emitted during command-line option parsing ([Issue #19576](https://github.com/dotnet/fsharp/issues/19576), [PR #19776](https://github.com/dotnet/fsharp/pull/19776))
 * Fix `[<return: X>]` prefix attributes being silently dropped on class members, and fix false-positive `AllowMultiple=false` errors when `[<X>]` and `[<return: X>]` are applied to the same binding. ([Issue #17904](https://github.com/dotnet/fsharp/issues/17904), [Issue #19020](https://github.com/dotnet/fsharp/issues/19020), [PR #19738](https://github.com/dotnet/fsharp/pull/19738))

src/Compiler/Checking/PostInferenceChecks.fs

@@ -2089,10 +2089,12 @@ and CheckBinding cenv env alwaysCheckNoReraise ctxt (TBind(v, bindRhs, _) as bin
 
     let env = { env with external = env.external || ValHasWellKnownAttribute g WellKnownValAttributes.DllImportAttribute v }
 
-    // Check that active patterns don't have free type variables in their result
+    // Check active pattern shape/type constraints
     match TryGetActivePatternInfo vref with
-    | Some _apinfo when _apinfo.ActiveTags.Length > 1 ->
-        if doesActivePatternHaveFreeTypars g vref then
+    | Some apinfo ->
+        let hasFreeTypars = doesActivePatternHaveFreeTypars g vref
+
+        if apinfo.ActiveTags.Length > 1 && hasFreeTypars then
            errorR(Error(FSComp.SR.activePatternChoiceHasFreeTypars(v.LogicalName), v.Range))
     | _ -> ()
 

tests/FSharp.Compiler.ComponentTests/Conformance/LexicalFiltering/OffsideExceptions/RelaxWhitespace2.fs

@@ -237,7 +237,7 @@ module ActivePatterns =
     |) _ = (|
         A
     |)
-    let (|C|) =
+    let (|C|) _ =
         if true then ignore (|
             A
         |)
@@ -255,7 +255,7 @@ module ActivePatterns =
         F
         |
         _
-    |) as f = Some (|
+    |) _ = Some (|
         C
     |)
     let (|
@@ -1265,7 +1265,7 @@ type ActivePatterns() =
     |) = (|
         A
     |)
-    let (|C|) =
+    let (|C|) _ =
         if true then ignore (|
             A
         |)
@@ -1283,7 +1283,7 @@ type ActivePatterns() =
         F
         |
         _
-    |) as f = Some (|
+    |) _ = Some (|
         C
     |)
     let (|
@@ -1305,7 +1305,7 @@ type ActivePatterns() =
     |) = (|
         A_
     |)
-    static let (|C_|) =
+    static let (|C_|) _ =
         if true then ignore (|
             A_
         |)
@@ -1323,7 +1323,7 @@ type ActivePatterns() =
         F_
         |
         _
-    |) as f_ = Some (|
+    |) _ = Some (|
         C_
     |)
     static let (|
@@ -2593,7 +2593,7 @@ let ActivePatterns<'a> =
     |) _ = (|
         A
     |)
-    let (|C|) =
+    let (|C|) _ =
         if true then ignore (|
             A
         |)
@@ -2611,7 +2611,7 @@ let ActivePatterns<'a> =
         F
         |
         _
-    |) as f = Some (|
+    |) _ = Some (|
         C
     |)
     let (|

tests/FSharp.Compiler.ComponentTests/Conformance/PatternMatching/Named/E_ActivePatternNotAFunction.fs

@@ -1,5 +1,9 @@
 // #Regression #Conformance #PatternMatching #ActivePatterns 
 // Regression test for FSHARP1.0:5590
-//<Expects status="error" span="(5,6-5,11)" id="FS1209">Active pattern '|A|B|' is not a function$</Expects>
+//<Expects status="error" span="(7,6-7,11)" id="FS1209">Active pattern '|A|B|' is not a function$</Expects>
+//<Expects status="error" span="(8,6-8,9)" id="FS1209">Active pattern '|C|' is not a function$</Expects>
+//<Expects status="error" span="(9,6-9,11)" id="FS1209">Active pattern '|D|_|' is not a function$</Expects>
 
 let (|A|B|) = failwith "" : Choice<int,int>
+let (|C|) = 3
+let (|D|_|) = None

tests/FSharp.Compiler.ComponentTests/Conformance/PatternMatching/Named/Named.fs

@@ -184,7 +184,11 @@ module Named =
         |> withOptions ["--test:ErrorRanges"]
         |> typecheck
         |> shouldFail
-        |> withSingleDiagnostic (Error 1209, Line 5, Col 6, Line 5, Col 11, "Active pattern '|A|B|' is not a function")
+        |> withDiagnostics [
+            (Error 1209, Line 7, Col 6, Line 7, Col 11, "Active pattern '|A|B|' is not a function")
+            (Error 1209, Line 8, Col 6, Line 8, Col 9, "Active pattern '|C|' is not a function")
+            (Error 1209, Line 9, Col 6, Line 9, Col 11, "Active pattern '|D|_|' is not a function")
+        ]
 
     // This test was automatically generated (moved from FSharpQA suite - Conformance/PatternMatching/Named)
     [<Theory; FileInlineData("E_ActivePatterns01.fs")>]
@@ -621,4 +625,4 @@ but here has type
         |> asFs
         |> withOptions ["--test:ErrorRanges"]
         |> typecheck
-        |> shouldSucceed
+        |> shouldSucceed