Fix expected and actual types in ErrorFromAddingTypeEquation message and extended diagnostic data (#18915)

Author: DedSec256

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

@@ -35,6 +35,7 @@
 * Checker: fix declaring type for abbreviated types extensions ([PR #18909](https://github.com/dotnet/fsharp/pull/18909))
 * Caches: type subsumption cache key perf regression ([Issue #18925](https://github.com/dotnet/fsharp/issues/18925) [PR #18926](https://github.com/dotnet/fsharp/pull/18926))
 * Ensure that line directives are applied to source identifiers (issue [#18908](https://github.com/dotnet/fsharp/issues/18908), PR [#18918](https://github.com/dotnet/fsharp/pull/18918))
+* Fix expected and actual types in ErrorFromAddingTypeEquation message and extended diagnostic data. ([PR #18915](https://github.com/dotnet/fsharp/pull/18915))
 
 ### Changed
 * Use `errorR` instead of `error` in `CheckDeclarations.fs` when possible. ([PR #18645](https://github.com/dotnet/fsharp/pull/18645))

src/Compiler/Driver/CompilerDiagnostics.fs

@@ -733,21 +733,15 @@ type Exception with
             if m.StartLine <> m2.StartLine then
                 os.AppendString(SeeAlsoE().Format(stringOfRange m))
 
-        | ConstraintSolverTypesNotInEqualityRelation(denv, (TType_measure _ as ty1), (TType_measure _ as ty2), m, m2, _) ->
-            // REVIEW: consider if we need to show _cxs (the type parameter constraints)
-            let ty1, ty2, _cxs = NicePrint.minimalStringsOfTwoTypes denv ty1 ty2
-
-            os.AppendString(ConstraintSolverTypesNotInEqualityRelation1E().Format ty1 ty2)
-
-            if m.StartLine <> m2.StartLine then
-                os.AppendString(SeeAlsoE().Format(stringOfRange m))
-
         | ConstraintSolverTypesNotInEqualityRelation(denv, ty1, ty2, m, m2, contextInfo) ->
             // REVIEW: consider if we need to show _cxs (the type parameter constraints)
-            let ty1, ty2, _cxs = NicePrint.minimalStringsOfTwoTypes denv ty1 ty2
+            let ty1str, ty2str, _cxs = NicePrint.minimalStringsOfTwoTypes denv ty1 ty2
 
-            OutputTypesNotInEqualityRelationContextInfo contextInfo ty1 ty2 m os (fun _ ->
-                os.AppendString(ConstraintSolverTypesNotInEqualityRelation2E().Format ty1 ty2))
+            match ty1, ty2 with
+            | TType_measure _, TType_measure _ -> os.AppendString(ConstraintSolverTypesNotInEqualityRelation1E().Format ty1str ty2str)
+            | _ ->
+                OutputTypesNotInEqualityRelationContextInfo contextInfo ty1str ty2str m os (fun _ ->
+                    os.AppendString(ConstraintSolverTypesNotInEqualityRelation2E().Format ty1str ty2str))
 
             if m.StartLine <> m2.StartLine then
                 os.AppendString(SeeAlsoE().Format(stringOfRange m))
@@ -816,11 +810,26 @@ type Exception with
                     os.AppendString(SeeAlsoE().Format(stringOfRange m1))
 
         | ErrorFromAddingTypeEquation(g, denv, ty1, ty2, e, _) ->
-            if not (typeEquiv g ty1 ty2) then
-                let ty1, ty2, tpcs = NicePrint.minimalStringsOfTwoTypes denv ty1 ty2
+            let e =
+                if not (typeEquiv g ty1 ty2) then
+                    let ty1, ty2, tpcs = NicePrint.minimalStringsOfTwoTypes denv ty1 ty2
 
-                if ty1 <> ty2 + tpcs then
-                    os.AppendString(ErrorFromAddingTypeEquation2E().Format ty1 ty2 tpcs)
+                    if ty1 <> ty2 + tpcs then
+                        os.AppendString(ErrorFromAddingTypeEquation2E().Format ty1 ty2 tpcs)
+
+                    e
+
+                else
+                    // Fix for https://github.com/dotnet/fsharp/issues/18905
+                    // If ty1 = ty2 after the type solving, then ty2 holds an actual type.
+                    // The order of expected and actual types in ConstraintSolverTypesNotInEqualityRelation can be arbitrary
+                    // due to type solving logic.
+                    // If ty1 = ty2 = ty2b, it means ty2b is also an actual type, and it needs to be swapped with ty1b
+                    // to be correctly used in the type mismatch error message based on ConstraintSolverTypesNotInEqualityRelation
+                    match e with
+                    | ConstraintSolverTypesNotInEqualityRelation(env, ty1b, ty2b, m, m2, contextInfo) when typeEquiv g ty2 ty2b ->
+                        ConstraintSolverTypesNotInEqualityRelation(env, ty2b, ty1b, m, m2, contextInfo)
+                    | _ -> e
 
             e.Output(os, suggestNames)
 

src/Compiler/Symbols/FSharpDiagnostic.fs

@@ -176,6 +176,7 @@ type FSharpDiagnostic(m: range, severity: FSharpDiagnosticSeverity, message: str
             | Some symbolEnv ->
 
             match diagnostic.Exception with
+            | ErrorFromAddingConstraint(displayEnv, ConstraintSolverTypesNotInEqualityRelation(_, actualType, expectedType, _, _, contextInfo), _)
             | ErrorFromAddingTypeEquation(_, displayEnv, expectedType, actualType, ConstraintSolverTupleDiffLengths(contextInfo = contextInfo), _)
             | ErrorsFromAddingSubsumptionConstraint(_, displayEnv, expectedType, actualType, _, contextInfo, _) ->
                let context = DiagnosticContextInfo.From(contextInfo)
@@ -187,6 +188,8 @@ type FSharpDiagnostic(m: range, severity: FSharpDiagnosticSeverity, message: str
                        ty1, ty2
                    elif not (typeEquiv g ty1 ty2) then
                        ty1, ty2
+                   elif typeEquiv g ty2 ty2b then
+                       ty1b, ty2b
                    else ty2b, ty1b
 
                let context = DiagnosticContextInfo.From(contextInfo)

tests/FSharp.Compiler.ComponentTests/Conformance/Types/RecordTypes/RecordTypes.fs

@@ -116,7 +116,7 @@ module RecordTypes =
             (Warning 464, Line 15, Col 22, Line 15, Col 28, "This code is less generic than indicated by its annotations. A unit-of-measure specified using '_' has been determined to be '1', i.e. dimensionless. Consider making the code generic, or removing the use of '_'.")
             (Warning 464, Line 15, Col 35, Line 15, Col 42, "This code is less generic than indicated by its annotations. A unit-of-measure specified using '_' has been determined to be '1', i.e. dimensionless. Consider making the code generic, or removing the use of '_'.")
             (Error 5, Line 17, Col 1, Line 17, Col 5, "This field is not mutable")
-            (Error 1, Line 17, Col 16, Line 17, Col 22, "The type 'decimal<Kg>' does not match the type 'float<Kg>'")
+            (Error 1, Line 17, Col 16, Line 17, Col 22, "The type 'float<Kg>' does not match the type 'decimal<Kg>'")
             (Error 5, Line 18, Col 1, Line 18, Col 5, "This field is not mutable")
             (Error 1, Line 18, Col 16, Line 18, Col 21, "This expression was expected to have type\n    'float'    \nbut here has type\n    'decimal'    ")
         ]

tests/FSharp.Compiler.ComponentTests/Conformance/Types/TypeConstraints/IWSAMsAndSRTPs/IWSAMsAndSRTPsTests.fs

@@ -1145,7 +1145,7 @@ module StaticAbstractBug =
         |> compile
         |> shouldFail
         |> withDiagnostics [
-            (Error 1, Line 14, Col 41, Line 14, Col 42, "The type 'bool' does not match the type 'int'")
+            (Error 1, Line 14, Col 41, Line 14, Col 42, "The type 'int' does not match the type 'bool'")
             (Error 1, Line 16, Col 32, Line 16, Col 33, "This expression was expected to have type
     'bool'    
 but here has type