From 9d46fe917af93e436b347abd44bd0879b8a87de7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Daniel=20Mart=C3=AD?= <mvdan@mvdan.cc>
Date: Tue, 6 Sep 2022 10:26:49 +0100
Subject: [PATCH] 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                    |  2 ++
 main.go                         |  9 +++++++--
 testdata/scripts/typeparams.txt | 13 +++++++++++--
 3 files changed, 20 insertions(+), 4 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index c5ec030..c2f05aa 100644
--- a/CHANGELOG.md
+++ b/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
 
diff --git a/main.go b/main.go
index 74f8b99..fce82a0 100644
--- a/main.go
+++ b/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
diff --git a/testdata/scripts/typeparams.txt b/testdata/scripts/typeparams.txt
index 061495c..f2f9898 100644
--- a/testdata/scripts/typeparams.txt
+++ b/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{}
+}