cmd/compile/internal/noder: offset type parameter indices

When a writer goes to encode use of a type parameter, it only records
an index because the writer knows all of the type parameters in scope
for that object.

The writer orders the type parameteres as implicits first, then
receivers, then explicits actually on the object. Note that receivers
*are* explicit type arguments, they're just inherited from the type.

For instance, given:

  func (T[P]) m[Q, R any]()

This has:
  - 0 implicits
  - 1 receiver
  - 2 explicits

With this ordering, P is at 0, Q is at 1, and R is at 2. In contrast,
inspecting TypeParam.index for Q yields 0, and for R yields 1; hence
the offset is needed.

Change-Id: If12f342e109fbc5935ba278f574ac2809c889335
Reviewed-on: https://go-review.googlesource.com/c/go/+/762021
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Mark Freeman <markfreeman@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

1 files changed, 8 insertions, 3 deletions

diff --git a/src/cmd/compile/internal/noder/writer.go b/src/cmd/compile/internal/noder/writer.go
index 85686a481a..4944b7bd54 100644
--- a/src/cmd/compile/internal/noder/writer.go
+++ b/src/cmd/compile/internal/noder/writer.go
@@ -230,8 +230,7 @@ type itabInfo struct {
 
 // typeParamIndex returns the index of the given type parameter within
 // the dictionary. This may differ from typ.Index() when there are
-// implicit type parameters due to defined types declared within a
-// generic function or method.
+// implicit or receiver type parameters.
 func (dict *writerDict) typeParamIndex(typ *types2.TypeParam) int {
 	for idx, implicit := range dict.implicits {
 		if implicit == typ {
@@ -239,7 +238,13 @@ func (dict *writerDict) typeParamIndex(typ *types2.TypeParam) int {
 		}
 	}
 
-	return len(dict.implicits) + typ.Index()
+	for idx, receiver := range dict.receivers {
+		if receiver == typ {
+			return len(dict.implicits) + idx
+		}
+	}
+
+	return len(dict.implicits) + len(dict.receivers) + typ.Index()
 }
 
 // A derivedInfo represents a reference to an encoded generic Go type.