Fix internal error when missing measure attribute in an unsolved measure typar (#18234)

Author: edgarfgp

docs/release-notes/.FSharp.Compiler.Service/9.0.300.md

@@ -1,6 +1,7 @@
 ### Fixed
 
 * Fix Realsig+ generates nested closures with incorrect Generic  ([Issue #17797](https://github.com/dotnet/fsharp/issues/17797), [PR #17877](https://github.com/dotnet/fsharp/pull/17877))
+* Fix internal error when missing measure attribute in an unsolved measure typar. ([Issue #7491](https://github.com/dotnet/fsharp/issues/7491), [PR #18234](https://github.com/dotnet/fsharp/pull/18234)==
 * Set `Cancellable.token` from async computation ([Issue #18235](https://github.com/dotnet/fsharp/issues/18235), [PR #18238](https://github.com/dotnet/fsharp/pull/18238))
 
 ### Added

src/Compiler/FSComp.txt

@@ -1789,5 +1789,6 @@ featureUseTypeSubsumptionCache,"Use type conversion cache during compilation"
 3872,tcPartialActivePattern,"Multi-case partial active patterns are not supported. Consider using a single-case partial active pattern or a full active pattern."
 featureDontWarnOnUppercaseIdentifiersInBindingPatterns,"Don't warn on uppercase identifiers in binding patterns"
 3873,chkDeprecatePlacesWhereSeqCanBeOmitted,"This construct is deprecated. Sequence expressions should be of the form 'seq {{ ... }}'"
+3874,tcExpectedTypeParamMarkedWithUnitOfMeasureAttribute,"Expected unit-of-measure type parameter must be marked with the [<Measure>] attribute."
 featureDeprecatePlacesWhereSeqCanBeOmitted,"Deprecate places where 'seq' can be omitted"
 featureSupportValueOptionsAsOptionalParameters,"Support ValueOption as valid type for optional member parameters"

src/Compiler/TypedTree/TypedTreeOps.fs

@@ -243,6 +243,9 @@ and remapMeasureAux tyenv unt =
           | Some tpTy -> 
               match tpTy with
               | TType_measure unt -> unt
+              | TType_var(typar= typar) when tp.Kind = TyparKind.Measure ->
+                    // This is a measure typar that is not yet solved, so we can't remap it
+                    error(Error(FSComp.SR.tcExpectedTypeParamMarkedWithUnitOfMeasureAttribute(), typar.Range))
               | _ -> failwith "remapMeasureAux: incorrect kinds"
           | None -> unt
        | Some (TType_measure unt) -> remapMeasureAux tyenv unt