internal/typeparams: fix crash in interface hover with empty type set

The logic of typeparams.Free.Has was not handling ErrEmptyTypeSet from
InterfaceTermSet, leading to a crash.

Also fix a hover marker in hover/generics.txt that was apparently
missing a '@'.

Fixes golang/go#68213

Change-Id: I0e264b5c5f5ecc0d09a0b8ab381eb7d7c492c18f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/595555
Auto-Submit: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

gopls/internal/test/marker/testdata/hover/generics.txt

@@ -3,8 +3,11 @@ This file contains tests for hovering over generic Go code.
 Requires go1.20+ for the new go/doc/comment package, and a change in Go 1.20
 that affected the formatting of constraint interfaces.
 
+Its size expectations assume a 64-bit machine.
+
 -- flags --
 -min_go=go1.20
+-skip_goarch=386,arm
 
 -- go.mod --
 // A go.mod is require for correct pkgsite links.
@@ -13,10 +16,28 @@ module mod.com
 
 go 1.18
 
+-- issue68213.go --
+package generics
+
+// Hovering over an interface with empty type set must not panic.
+type empty interface { //@hover("empty", "empty", empty)
+	int
+	string
+}
+
+-- @empty --
+```go
+type empty interface { // size=16 (0x10)
+	int
+	string
+}
+```
+
+Hovering over an interface with empty type set must not panic.
 -- generics.go --
 package generics
 
-type value[T any] struct { //hover("lue", "value", value),hover("T", "T", valueT)
+type value[T any] struct { //@hover("lue", "value", value),hover("T", "T", valueT)
 	val T   //@hover("T", "T", valuevalT)
 	Q   int64 //@hover("Q", "Q", valueQ)
 }
@@ -30,6 +51,52 @@ func F[P interface{ ~int | string }]() { //@hover("P", "P", Ptparam)
 	var _ P //@hover("P","P",Pvar)
 }
 
+-- @value --
+```go
+type value[T any] struct {
+	val T     //@hover("T", "T", valuevalT)
+	Q   int64 //@hover("Q", "Q", valueQ)
+}
+```
+-- @valueT --
+```go
+type parameter T any
+```
+-- @valuevalT --
+```go
+type parameter T any
+```
+-- @valueQ --
+```go
+field Q int64 // size=8
+```
+
+@hover("Q", "Q", valueQ)
+-- @ValueT --
+```go
+type parameter T any
+```
+-- @ValuevalT --
+```go
+type parameter T any
+```
+-- @ValueQ --
+```go
+field Q int64 // size=8
+```
+
+@hover("Q", "Q", ValueQ)
+
+
+[`(generics.Value).Q` on pkg.go.dev](https://pkg.go.dev/mod.com#Value.Q)
+-- @Ptparam --
+```go
+type parameter P interface{~int | string}
+```
+-- @Pvar --
+```go
+type parameter P interface{~int | string}
+```
 -- inferred.go --
 package generics
 
@@ -44,42 +111,7 @@ func _() {
 	_ = app([]int{}, 0)        //@hover("app", "app", appint)
 }
 
--- @ValueQ --
-```go
-field Q int64 // size=8
-```
-
-@hover("Q", "Q", ValueQ)
-
-
-[`(generics.Value).Q` on pkg.go.dev](https://pkg.go.dev/mod.com#Value.Q)
--- @ValueT --
-```go
-type parameter T any
-```
--- @ValuevalT --
-```go
-type parameter T any
-```
 -- @appint --
 ```go
 func app(s []int, e int) []int // func[S interface{~[]E}, E interface{}](s S, e E) S
 ```
--- @valueQ --
-```go
-field Q int64 // size=8
-```
-
-@hover("Q", "Q", valueQ)
--- @valuevalT --
-```go
-type parameter T any
-```
--- @Ptparam --
-```go
-type parameter P interface{~int | string}
-```
--- @Pvar --
-```go
-type parameter P interface{~int | string}
-```

internal/typeparams/free.go

@@ -21,7 +21,6 @@ type Free struct {
 
 // Has reports whether the specified type has a free type parameter.
 func (w *Free) Has(typ types.Type) (res bool) {
-
 	// detect cycles
 	if x, ok := w.seen[typ]; ok {
 		return x
@@ -83,7 +82,7 @@ func (w *Free) Has(typ types.Type) (res bool) {
 		}
 		terms, err := InterfaceTermSet(t)
 		if err != nil {
-			panic(err)
+			return false // ill typed
 		}
 		for _, term := range terms {
 			if w.Has(term.Type()) {