internal/typeparams: improve the panic in OriginMethod

Improve the panic message to help debug golang/go#61196.

Updates golang/go#61196

Change-Id: Ia3ab9583886d67b971914310e82a99a1a2b15a59
Reviewed-on: https://go-review.googlesource.com/c/tools/+/508795
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>

internal/facts/imports.go

@@ -55,7 +55,7 @@ func importMap(imports []*types.Package) map[string]*types.Package {
 			// infinite expansions:
 			//     type N[T any] struct { F *N[N[T]] }
 			// importMap() is called on such types when Analyzer.RunDespiteErrors is true.
-			T = typeparams.NamedTypeOrigin(T).(*types.Named)
+			T = typeparams.NamedTypeOrigin(T)
 			if !typs[T] {
 				typs[T] = true
 				addObj(T.Obj())

internal/typeparams/common.go

@@ -23,6 +23,7 @@
 package typeparams
 
 import (
+	"fmt"
 	"go/ast"
 	"go/token"
 	"go/types"
@@ -125,6 +126,11 @@ func OriginMethod(fn *types.Func) *types.Func {
 		}
 	}
 
+	// In golang/go#61196, we observe another crash, this time inexplicable.
+	if gfn == nil {
+		panic(fmt.Sprintf("missing origin method for %s.%s; named == origin: %t, named.NumMethods(): %d, origin.NumMethods(): %d", named, fn, named == orig, named.NumMethods(), orig.NumMethods()))
+	}
+
 	return gfn.(*types.Func)
 }
 

internal/typeparams/typeparams_go117.go

@@ -129,7 +129,7 @@ func NamedTypeArgs(*types.Named) *TypeList {
 }
 
 // NamedTypeOrigin is the identity method at this Go version.
-func NamedTypeOrigin(named *types.Named) types.Type {
+func NamedTypeOrigin(named *types.Named) *types.Named {
 	return named
 }
 

internal/typeparams/typeparams_go118.go

@@ -103,7 +103,7 @@ func NamedTypeArgs(named *types.Named) *TypeList {
 }
 
 // NamedTypeOrigin returns named.Orig().
-func NamedTypeOrigin(named *types.Named) types.Type {
+func NamedTypeOrigin(named *types.Named) *types.Named {
 	return named.Origin()
 }