Replace flex vars with lambda set vars during type_to_var

crates/compiler/solve/src/solve.rs

@@ -2452,12 +2452,18 @@ impl AmbientFunctionPolicy {
                 ambient_function,
             }),
             Content::FlexVar(_) => {
+                Content::LambdaSet(LambdaSet {
+                    solved: UnionLabels::default(),
+                    recursion_var: OptVariable::NONE,
+                    unspecialized: SubsSlice::default(),
+                    ambient_function,
+                })
                 // Something like
                 //   Encoder fmt <a> : List U8, fmt -a-> List U8 | fmt has EncoderFormatting
                 // THEORY: Just allow this, it's fine, because the lambda set is unbound,
                 // but it's not part of an ability signature, so it doesn't have an unspecialized
                 // lambda set, and hence doesn't need a link to the ambient function.
-                *content
+                // *content
             }
             content => internal_error!("{:?}({:?}) not a lambda set", content, var),
         };