gopls/internal/golang: downgrade assertion in methodsets index

A query for exported method T.M triggers a paradoxical
methodset(T)={} assertion. But an ill-typed T may have
both a field and a method named M causing the methodset
to indeed be empty.

+ Test.

Fixes golang/go#67978

Change-Id: Iecbf98ff53afab00ebb51f83ed261a0318b42771
Reviewed-on: https://go-review.googlesource.com/c/tools/+/605015
Auto-Submit: Alan Donovan <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: adonovan

gopls/internal/golang/references.go

@@ -31,7 +31,6 @@ import (
 	"golang.org/x/tools/gopls/internal/cache/parsego"
 	"golang.org/x/tools/gopls/internal/file"
 	"golang.org/x/tools/gopls/internal/protocol"
-	"golang.org/x/tools/gopls/internal/util/bug"
 	"golang.org/x/tools/gopls/internal/util/safetoken"
 	"golang.org/x/tools/internal/event"
 )
@@ -508,7 +507,10 @@ func expandMethodSearch(ctx context.Context, snapshot *cache.Snapshot, workspace
 	// Compute the method-set fingerprint used as a key to the global search.
 	key, hasMethods := methodsets.KeyOf(recv)
 	if !hasMethods {
-		return bug.Errorf("KeyOf(%s)={} yet %s is a method", recv, method)
+		// The query object was method T.m, but methodset(T)={}:
+		// this indicates that ill-typed T has conflicting fields and methods.
+		// Rather than bug-report (#67978), treat the empty method set at face value.
+		return nil
 	}
 	// Search the methodset index of each package in the workspace.
 	indexes, err := snapshot.MethodSets(ctx, workspaceIDs...)

gopls/internal/test/marker/testdata/references/issue67978.txt

@@ -0,0 +1,18 @@
+
+This test exercises a references query on an exported method that
+conflicts with a field name. This ill-typed input violates the
+assumption that if type T has a method, then the method set of T is
+nonempty, which led to a crash.
+
+See https://github.com/golang/go/issues/67978.
+
+-- a.go --
+package p
+
+type E struct { X int } //@ diag(re"()X", re"field.*same name")
+
+func (E) X() {} //@ loc(a, "X"), refs("X", a, b), diag(re"()X", re"method.*same name")
+
+var _ = new(E).X //@ loc(b, "X")
+
+