Fix crash when lambda uses non-interpolated string or empty array

src/QsCompiler/SyntaxProcessor/ContextVerification.fs

@@ -365,7 +365,7 @@ let rec internal freeVariables e =
     | CONDITIONAL (e1, e2, e3) -> freeVariables e1 |> merge (freeVariables e2) |> merge (freeVariables e3)
     | ValueTuple es
     | StringLiteral (_, es)
-    | ValueArray es -> es |> Seq.map freeVariables |> Seq.reduce merge
+    | ValueArray es -> es |> Seq.map freeVariables |> Seq.fold merge Map.empty
     | Lambda lambda ->
         let bindings = unqualifiedSymbols lambda.Param
         freeVariables lambda.Body |> Map.filter (fun name _ -> Set.contains name bindings |> not)

src/QsCompiler/Tests.Compiler/TestCases/TypeChecking.qs

@@ -843,6 +843,14 @@ namespace Microsoft.Quantum.Testing.TypeChecking {
         return () -> 0;
     }
 
+    function Lambda15<'a>() : 'a -> String {
+        return x -> "foo";
+    }
+
+    function Lambda16<'a>() : 'a -> 'a[] {
+        return x -> [];
+    }
+
     function LambdaInvalid1() : Qubit => Unit is Adj {
         return q => Operation(q);
     }

src/QsCompiler/Tests.Compiler/TypeCheckingTests.fs

@@ -79,7 +79,7 @@ module TypeCheckingTests =
 
     [<Fact>]
     let ``Supports lambda expressions`` () =
-        allValid "Lambda" 14
+        allValid "Lambda" 16
         expect "LambdaInvalid1" [ Error ErrorCode.TypeMismatchInReturn ]
         expect "LambdaInvalid2" [ Error ErrorCode.TypeMismatchInReturn ]
         expect "LambdaInvalid3" (Error ErrorCode.InfiniteType |> List.replicate 2)