Merge pull request fsharp#657 from dsyme/fix-656

Fix 656 - no access to the given key

Author: dsyme

CHANGELOG-fsharp.md

@@ -1,3 +1,6 @@
+4.0.1.21
+  * Fix [#656](https://github.com/fsharp/fsharp/issues/656)  - error FS0193: internal error: No access to the given key
+
 4.0.1.20
   * Fix #639 - Problems with F# scripts on Mono 
 

appveyor.ps1

@@ -1,6 +1,6 @@
 
 # the version under development, update after a release
-$version = '4.0.1.20'
+$version = '4.0.1.21'
 
 function isVersionTag($tag){
     $v = New-Object Version

src/fsharp/MSBuildReferenceResolver.fs

@@ -111,6 +111,7 @@ module internal Microsoft.FSharp.Compiler.MSBuildReferenceResolver
 
     /// Use MSBuild to determine the version of the highest installed framework.
     let HighestInstalledNetFrameworkVersion() =
+      try
 #if MSBUILD_AT_LEAST_14
         if box (ToolLocationHelper.GetPathToDotNetFramework(TargetDotNetFrameworkVersion.Version461)) <> null then Net461
         elif box (ToolLocationHelper.GetPathToDotNetFramework(TargetDotNetFrameworkVersion.Version46)) <> null then Net46
@@ -121,7 +122,8 @@ module internal Microsoft.FSharp.Compiler.MSBuildReferenceResolver
         if box (ToolLocationHelper.GetPathToDotNetFramework(TargetDotNetFrameworkVersion.Version451)) <> null then Net451 
 #endif
         elif box (ToolLocationHelper.GetPathToDotNetFramework(TargetDotNetFrameworkVersion.Version45)) <> null then Net45 
-        else Net40 // version is 4.0 assumed since this code is running. 
+        else Net45 // version is 4.0 assumed since this code is running. 
+      with _ -> Net45
 
     /// Derive the target framework directories.        
     let DeriveTargetFrameworkDirectories (targetFrameworkVersion:string, logMessage) =