Jand42 commented 4 years ago

Description

The F# compiler can give a Type mismatch when building 'expr' error on two types which should be the same during the running of a TP. Found when building https://github.com/dotnet-websharper/ui and this is a breaking change introduced by #332, works with the previous state (e9d2c93).

Example of error:

 The type provider 'WebSharper.UI.Templating.TemplatingProvider' reported an error in the context of provided type 'WebSharper.UI.Templating.Template,pathOrHtml="index.html,template.html",clientLoad="2",legacyMode="3"+template', member 'NewDescription'. The error: Type mismatch when building 'expr': the expression has the wrong type. Expected 'WebSharper.UI.Var`1[System.String]', but received type 'WebSharper.UI.Var`1[System.String]'.Parameter name: receivedType

The WebSharper.UI.Templating type provider uses expression splicing extensively, probably the issue is related to that, I am experimenting to find a minimal failing test case and will update this description if found.

Repro steps

Checkout repo at this commit: https://github.com/dotnet-websharper/ui/commit/18932b446aec091cbdafa1c0a196299ea5aaf11e
Run build WS-Update
Run build WS-BuildRelease

Expected behavior

WebSharper.UI.Templating.Tests project should build as before, now fails on erased TP use.

Actual behavior

Build errors.

Known workarounds

Use earlier state of FSharp.TypeProviders.SDK.

Related information

Operating system: Win10Pro 64bit
Branch: master
.NET Runtime, CoreCLR or Mono Version: .NET Framework 4.6.1, .NET Standard 2.0

sergey-tihon commented 4 years ago

cc @thinkbeforecoding

thinkbeforecoding commented 4 years ago

332 was actually done to build methods that have parameters or return types that are RuntimeType generics (like Task<'t> or Async<'t>) of generated type.

Previously, the handling of method parameters used t.MakeGeneric which returns such generic that doesn't work then for generation.. I may have broken your usecase in the process. A repro would ease the fix