gopls/internal/golang: fix crash when hovering over implicit

After months of intermittent investigation, I was finally able to
reproduce the telemetry crash of golang/go#69362: if the operant of the
type switch is undefined, the selectedType will be nil, and therefore
logic may proceed to the point where it reaches a nil entry in
types.Info.Defs.

The fix is of course straightforward, now that we understand it: we
cannot rely on types.Info.Defs not containing nil entries. A follow-up
CL will introduce an analyzer to detect such problematic uses of the
go/types API.

Fixes golang/go#69362

Change-Id: I8f75c24710dbb2e78c79d8b9d721f45d9a040cd7
Reviewed-on: https://go-review.googlesource.com/c/tools/+/652015
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: findleyr

gopls/internal/golang/hover.go

@@ -375,15 +375,10 @@ func hover(ctx context.Context, snapshot *cache.Snapshot, fh file.Handle, pp pro
 	// use the default build config for all other types, even
 	// if they embed platform-variant types.
 	//
-	var sizeOffset string // optional size/offset description
-	// debugging #69362: unexpected nil Defs[ident] value (?)
-	_ = ident.Pos()          // (can't be nil due to check after referencedObject)
-	_ = pkg.TypesInfo()      // (can't be nil due to check in call to inferredSignature)
-	_ = pkg.TypesInfo().Defs // (can't be nil due to nature of cache.Package)
-	if def, ok := pkg.TypesInfo().Defs[ident]; ok {
-		_ = def.Pos() // can't be nil due to reasoning in #69362.
-	}
-	if def, ok := pkg.TypesInfo().Defs[ident]; ok && ident.Pos() == def.Pos() {
+	var sizeOffset string
+
+	// As painfully learned in golang/go#69362, Defs can contain nil entries.
+	if def, _ := pkg.TypesInfo().Defs[ident]; def != nil && ident.Pos() == def.Pos() {
 		// This is the declaring identifier.
 		// (We can't simply use ident.Pos() == obj.Pos() because
 		// referencedObject prefers the TypeName for an embedded field).

gopls/internal/test/marker/testdata/hover/issues.txt

@@ -20,3 +20,15 @@ package issue64237
 import "golang.org/lsptests/nonexistant" //@diag("\"golang", re"could not import")
 
 var _ = nonexistant.Value //@hovererr("nonexistant", "no package data")
+
+-- issue69362/p.go --
+package issue69362
+
+// golang/go#69362: hover panics over undefined implicits.
+
+func _() {
+	switch x := y.(type) { //@diag("y", re"undefined"), hover("x", "x", "")
+	case int:
+		_ = x
+	}
+}