avoid a type assertion panic with generic code

I was wrongly assumed that, if `used` has an `Elem` method,
then `origin` must too. But it does not if it's a type parameter.

Add a test case too, which panicked before the fix.

Fixes #577.

CHANGELOG.md

@@ -5,6 +5,7 @@
 This bugfix release continues support for Go 1.18 and 1.19.
 
 * Fix an edge case resulting in bad syntax due to comments - [#573]
+* Avoid a panic involving generic code - [#577]
 
 ## [v0.7.1] - 2022-08-02
 
@@ -144,6 +145,7 @@ Known bugs:
 * `garble test` is temporarily disabled, as it is currently broken
 
 [#573]: https://github.com/burrowers/garble/issues/573
+[#577]: https://github.com/burrowers/garble/issues/577
 
 [v0.7.1]: https://github.com/burrowers/garble/releases/tag/v0.7.1
 

main.go

@@ -1385,8 +1385,13 @@ func (tf *transformer) recordType(used, origin types.Type) {
 	type Container interface{ Elem() types.Type }
 	switch used := used.(type) {
 	case Container:
-		origin := origin.(Container)
-		tf.recordType(used.Elem(), origin.Elem())
+		// origin may be a *types.TypeParam, which is not a Container.
+		// 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 {
+			tf.recordType(used.Elem(), origin.Elem())
+		}
 	case *types.Named:
 		if tf.recordTypeDone[used] {
 			return

testdata/scripts/typeparams.txt

@@ -15,8 +15,11 @@ func main() {
 	GenericFunc[int, int](1, 2)
 	var _ GenericVector[int]
 
-	g := GenericGraph[string]{Content: "Foo"}
-	g.Edges = make([]GenericGraph[string], 1)
+	g1 := GenericGraph[string]{Content: "Foo"}
+	g1.Edges = make([]GenericGraph[string], 1)
+
+	g2 := GenericGraph[*[]byte]{Content: new([]byte)}
+	g2.Edges = make([]GenericGraph[*[]byte], 1)
 }
 
 func GenericFunc[GenericParamA, B any](x GenericParamA, y B) {}
@@ -44,3 +47,9 @@ type CombineEmbeds interface {
 	interface { EmbeddedMethod() }
 	RegularMethod()
 }
+
+type Slice[T any] []T
+
+func sliceOfPointer() Slice[*any] {
+	return []*any{}
+}