skip all type parameters in recordType

We only did this for Container in the type switch, but not for Struct.
The added test case panics otherwise.
Just like in the previous case, we still don't need to recurse
into type parameters for fieldToStruct to be filled correctly.

Fixes #899

main.go

@@ -1683,21 +1683,23 @@ func computeFieldToStruct(info *types.Info) map[*types.Var]*types.Struct {
 // Since types can be recursive, we need a map to avoid cycles.
 // We only need to track named types as done, as all cycles must use them.
 func recordType(used, origin types.Type, done map[*types.Named]bool, fieldToStruct map[*types.Var]*types.Struct) {
+	used = types.Unalias(used)
 	if origin == nil {
 		origin = used
-	}
+	} else {
 		origin = types.Unalias(origin)
-	used = types.Unalias(used)
-	type Container interface{ Elem() types.Type }
-	switch used := used.(type) {
-	case Container:
-		// origin may be a *types.TypeParam, which is not a Container.
+		// origin may be a [*types.TypeParam].
 		// For now, we haven't found a need to recurse in that case.
 		// We can edit this code in the future if we find an example,
 		// because we panic if a field is not in fieldToStruct.
-		if origin, ok := origin.(Container); ok {
-			recordType(used.Elem(), origin.Elem(), done, fieldToStruct)
+		if _, ok := origin.(*types.TypeParam); ok {
+			return
+		}
 	}
+	type Container interface{ Elem() types.Type }
+	switch used := used.(type) {
+	case Container:
+		recordType(used.Elem(), origin.(Container).Elem(), done, fieldToStruct)
 	case *types.Named:
 		if done[used] {
 			return

testdata/script/typeparams.txtar

@@ -40,7 +40,7 @@ type StringableSignedInteger interface {
 type CombineEmbeds interface {
 	string | int
 
-	interface { EmbeddedMethod() }
+	interface{ EmbeddedMethod() }
 	RegularMethod()
 }
 
@@ -49,3 +49,7 @@ type Slice[T any] []T
 func sliceOfPointer() Slice[*any] {
 	return []*any{}
 }
+
+type Map[K, V comparable] map[K]V
+
+var _ = Map[string, struct{}]{}