[release-branch.go1.23] cmd/compile: more informative panic when importing generic...

When GOEXPERIMENT=aliastypeparams is set, type aliases may have type
parameters. The compiler export data doesn't export that type parameter
information yet, which leads to an index-out-of-bounds panic when a
client package imports a package with a general type alias and then
refers to one of the missing type parameters.

This CL detects this specific case and panics with a more informative
panic message explaining the shortcoming. The change is only in effect
if the respective GOEXPERIMENT is enabled.

Manually tested. No test addded since this is just a temporary fix
(Go 1.24 will have a complete implementation), and because the existing
testing framework doesn't easily support testing that a compilation
panics.

Together with @taking and input from @rfindley.

For #68526.

Change-Id: I24737b153a7e2f9b705cd29a5b70b2b9e808dffc
Reviewed-on: https://go-review.googlesource.com/c/go/+/601035
Reviewed-by: Tim King <taking@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>

diff --git a/src/cmd/compile/internal/importer/ureader.go b/src/cmd/compile/internal/importer/ureader.go
index d3c7d4516f7ee81a48c26679a846e145e7e12b40..7eda375bd52196aa93c63e5a784b2657b29d89e1 100644 (file)
--- a/src/cmd/compile/internal/importer/ureader.go
+++ b/src/cmd/compile/internal/importer/ureader.go
@@ -9,6 +9,7 @@ import (
        "cmd/compile/internal/syntax"
        "cmd/compile/internal/types2"
        "cmd/internal/src"
+       "internal/buildcfg"
        "internal/pkgbits"
 )
 
@@ -411,6 +412,14 @@ func (pr *pkgReader) objIdx(idx pkgbits.Index) (*types2.Package, string) {
                        panic("weird")
 
                case pkgbits.ObjAlias:
+                       if buildcfg.Experiment.AliasTypeParams && len(r.dict.bounds) > 0 {
+                               // Temporary work-around for issue #68526: rather than panicking
+                               // with an non-descriptive index-out-of-bounds panic when trying
+                               // to access a missing type parameter, instead panic with a more
+                               // descriptive error. Only needed for Go 1.23; Go 1.24 will have
+                               // the correct implementation.
+                               panic("importing generic type aliases is not supported in Go 1.23 (see issue #68526)")
+                       }
                        pos := r.pos()
                        typ := r.typ()
                        return newAliasTypeName(pr.enableAlias, pos, objPkg, objName, typ)