Type checker: recover function type unification in method checking

docs/release-notes/.FSharp.Compiler.Service/11.0.0.md

@@ -9,6 +9,8 @@
 
 ### Added
 
+* Type checker: recover function type unification in method checking ([#19036](https://github.com/dotnet/fsharp/pull/19036#pullrequestreview-3392846258))
+
 ### Changed
 
 * Parallel compilation stabilised and enabled by default ([PR #18998](https://github.com/dotnet/fsharp/pull/18998))

src/Compiler/Checking/Expressions/CheckExpressions.fs

@@ -689,6 +689,20 @@ let UnifyFunctionType extraInfo (cenv: cenv) denv mFunExpr ty =
         | Some argm -> error (NotAFunction(denv, ty, mFunExpr, argm))
         | None -> error (FunctionExpected(denv, ty, mFunExpr))
 
+let UnifyFunctionTypeAndRecover extraInfo (cenv: cenv) denv mFunExpr ty =
+    match UnifyFunctionTypeUndoIfFailed cenv denv mFunExpr ty with
+    | ValueSome res -> res
+    | ValueNone ->
+        match extraInfo with
+        | Some argm -> errorR (NotAFunction(denv, ty, mFunExpr, argm))
+        | None -> errorR (FunctionExpected(denv, ty, mFunExpr))
+
+        let g = cenv.g
+        let domainTy = NewInferenceType g
+        let resultTy = NewInferenceType g
+        domainTy, resultTy
+
+
 let ReportImplicitlyIgnoredBoolExpression denv m ty expr =
     let checkExpr m expr =
         match stripDebugPoints expr with
@@ -9988,7 +10002,7 @@ and TcMethodApplication_UniqueOverloadInference
         // type we assume the number of arguments is just "1".
         | None, _ ->
 
-            let domainTy, returnTy = UnifyFunctionType None cenv denv mMethExpr exprTy.Commit
+            let domainTy, returnTy = UnifyFunctionTypeAndRecover None cenv denv mMethExpr exprTy.Commit
             let argTys = if isUnitTy g domainTy then [] else tryDestRefTupleTy g domainTy
             // Only apply this rule if a candidate method exists with this number of arguments
             let argTys =
@@ -10068,7 +10082,7 @@ and TcMethodApplication_CheckArguments
                 let curriedArgTys, returnTy = UnifyMatchingSimpleArgumentTypes cenv env exprTy.Commit calledMeth mMethExpr mItem
                 curriedArgTys, paramNamesIfFeatureEnabled g calledMeth, MustEqual returnTy
             | _ ->
-                let domainTy, returnTy = UnifyFunctionType None cenv denv mMethExpr exprTy.Commit
+                let domainTy, returnTy = UnifyFunctionTypeAndRecover None cenv denv mMethExpr exprTy.Commit
                 let argTys = if isUnitTy g domainTy then [] else tryDestRefTupleTy g domainTy
                 // Only apply this rule if a candidate method exists with this number of arguments
                 let argTys, argNames =

tests/FSharp.Compiler.Service.Tests/TypeChecker/TypeCheckerRecoveryTests.fs

@@ -1,9 +1,20 @@
 module FSharp.Compiler.Service.Tests.TypeChecker.TypeCheckerRecoveryTests
 
 open FSharp.Compiler.Service.Tests
+open FSharp.Compiler.Text
 open FSharp.Test.Assert
 open Xunit
 
+let assertHasSymbolUsageAtCaret name source =
+    let context, checkResults = Checker.getCheckedResolveContext source
+
+    getSymbolUses checkResults
+    |> Seq.exists (fun symbolUse ->
+        Range.rangeContainsPos symbolUse.Range context.Pos &&
+        symbolUse.Symbol.DisplayNameCore = name
+    )
+    |> shouldEqual true
+
 [<Fact>]
 let ``Let 01`` () =
     let _, checkResults = getParseAndCheckResults """
@@ -49,4 +60,42 @@ Math.Max(a,b,)
         "(4,0--4,14)", 503
     ]
 
-    assertHasSymbolUsages ["Max"] checkResults
+    assertHasSymbolUsages ["Max"] checkResults
+
+module Expressions =
+    [<Fact>]
+    let ``Method type 01`` () =
+        assertHasSymbolUsageAtCaret "ToString" """
+if true then
+    "".ToString{caret}
+"""
+
+
+    [<Fact>]
+    let ``Method type 02`` () =
+        assertHasSymbolUsageAtCaret "M" """
+type T =
+    static member M() = ""
+
+if true then
+    T.M{caret}
+"""
+
+    [<Fact>]
+    let ``Method type 03`` () =
+        assertHasSymbolUsageAtCaret "M" """
+type T =
+    static member M(i: int) = ""
+    static member M(s: string) = ""
+
+if true then
+    T.M{caret}
+"""
+
+    [<Fact>]
+    let ``Method type 04`` () =
+        assertHasSymbolUsageAtCaret "GetHashCode" """
+let o: obj = null
+if true then
+    o.GetHashCode{caret}
+"""