TypeMismatchDiagnosticExtendedData: fix expected type calculation (#18851)

Author: DedSec256

docs/release-notes/.FSharp.Compiler.Service/10.0.100.md

@@ -21,6 +21,7 @@
 
 * Fix SRTP nullness constraint resolution for types imported from older assemblies. AmbivalentToNull types now use legacy F# nullness rules instead of always satisfying `'T : null` constraints. ([Issue #18390](https://github.com/dotnet/fsharp/issues/18390), [Issue #18344](https://github.com/dotnet/fsharp/issues/18344))
 * Fix Show XML doc for enum fields in external metadata ([Issue #17939](https://github.com/dotnet/fsharp/issues/17939#issuecomment-3137410105), [PR #18800](https://github.com/dotnet/fsharp/pull/18800))
+* TypeMismatchDiagnosticExtendedData: fix expected and actual types calculation. ([Issue ](https://github.com/dotnet/fsharp/pull/18851))
 
 ### Changed
 * Use `errorR` instead of `error` in `CheckDeclarations.fs` when possible. ([PR #18645](https://github.com/dotnet/fsharp/pull/18645))

src/Compiler/Symbols/FSharpDiagnostic.fs

@@ -170,11 +170,21 @@ type FSharpDiagnostic(m: range, severity: FSharpDiagnosticSeverity, message: str
 
             match diagnostic.Exception with
             | ErrorFromAddingTypeEquation(_, displayEnv, expectedType, actualType, ConstraintSolverTupleDiffLengths(contextInfo = contextInfo), _)
-            | ErrorFromAddingTypeEquation(_, displayEnv, expectedType, actualType, ConstraintSolverTypesNotInEqualityRelation(_, _, _, _, _, contextInfo), _)
             | ErrorsFromAddingSubsumptionConstraint(_, displayEnv, expectedType, actualType, _, contextInfo, _) ->
                let context = DiagnosticContextInfo.From(contextInfo)
                Some(TypeMismatchDiagnosticExtendedData(symbolEnv, displayEnv, expectedType, actualType, context))
 
+            | ErrorFromAddingTypeEquation(g, displayEnv, ty1, ty2, ConstraintSolverTypesNotInEqualityRelation(_, ty1b, ty2b, _, _, contextInfo), _) ->
+               let expectedType, actualType =
+                   if typeEquiv g ty1 ty1b && typeEquiv g ty2 ty2b then
+                       ty1, ty2
+                   elif not (typeEquiv g ty1 ty2) then
+                       ty1, ty2
+                   else ty2b, ty1b
+
+               let context = DiagnosticContextInfo.From(contextInfo)
+               Some(TypeMismatchDiagnosticExtendedData(symbolEnv, displayEnv, expectedType, actualType, context))
+
             | ErrorFromAddingTypeEquation(_, displayEnv, expectedType, actualType, _, _)->
                Some(TypeMismatchDiagnosticExtendedData(symbolEnv, displayEnv, expectedType, actualType, DiagnosticContextInfo.NoContext))
 

tests/FSharp.Compiler.ComponentTests/ErrorMessages/ExtendedDiagnosticDataTests.fs

@@ -4,7 +4,6 @@
 open FSharp.Compiler.Text
 open FSharp.Compiler.Diagnostics
 open FSharp.Compiler.Diagnostics.ExtendedData
-open FSharp.Test
 open FSharp.Test.Compiler
 open Xunit
 
@@ -144,6 +143,50 @@ if true then 1 else "a"
         Assert.Equal("int", typeMismatch.ExpectedType.Format(displayContext))
         Assert.Equal("string", typeMismatch.ActualType.Format(displayContext)))
 
+[<Fact>]
+let ``TypeMismatchDiagnosticExtendedData 08`` () =
+    FSharp """
+type R = { Field1: int }
+let f (x: R) = "" + x.Field1
+"""
+    |> typecheckResults
+    |> checkDiagnostic
+       (1, "The type 'int' does not match the type 'string'")
+       (fun (typeMismatch: TypeMismatchDiagnosticExtendedData) ->
+        let displayContext = typeMismatch.DisplayContext
+        Assert.Equal(DiagnosticContextInfo.NoContext, typeMismatch.ContextInfo)
+        Assert.Equal("string", typeMismatch.ExpectedType.Format(displayContext))
+        Assert.Equal("int", typeMismatch.ActualType.Format(displayContext)))
+
+[<Fact>]
+let ``TypeMismatchDiagnosticExtendedData 09`` () =
+    FSharp """
+let x: string = 1
+"""
+    |> typecheckResults
+    |> checkDiagnostic
+       (1, "This expression was expected to have type\n    'string'    \nbut here has type\n    'int'    ")
+       (fun (typeMismatch: TypeMismatchDiagnosticExtendedData) ->
+        let displayContext = typeMismatch.DisplayContext
+        Assert.Equal(DiagnosticContextInfo.NoContext, typeMismatch.ContextInfo)
+        Assert.Equal("string", typeMismatch.ExpectedType.Format(displayContext))
+        Assert.Equal("int", typeMismatch.ActualType.Format(displayContext)))
+
+[<Fact>]
+let ``TypeMismatchDiagnosticExtendedData 10`` () =
+    FSharp """
+let f1 (x: outref<'T>) = 1
+let f2 (x: inref<'T>) = f1 &x
+"""
+    |> typecheckResults
+    |> checkDiagnostic
+       (1, "Type mismatch. Expecting a\n    'outref<'T>'    \nbut given a\n    'inref<'T>'    \nThe type 'ByRefKinds.Out' does not match the type 'ByRefKinds.In'")
+       (fun (typeMismatch: TypeMismatchDiagnosticExtendedData) ->
+        let displayContext = typeMismatch.DisplayContext
+        Assert.Equal(DiagnosticContextInfo.NoContext, typeMismatch.ContextInfo)
+        Assert.Equal("outref<'T>", typeMismatch.ExpectedType.Format(displayContext))
+        Assert.Equal("inref<'T>", typeMismatch.ActualType.Format(displayContext)))
+
 [<Theory>]
 [<InlineData true>]
 [<InlineData false>]