x/tools/gopls: "expected exported symbol" assert in getOneTransitiveRefLocked

[adonovan]

#!stacks
"cache.assert" && "evaluatePackageHandle.func1:+2"

Issue created by stacks.

// getOneTransitiveRefLocked computes the full set packages transitively
// reachable through the given sym reference.
//
// It may return nil if the reference is invalid (i.e. the referenced name does
// not exist).
func (b *packageHandleBuilder) getOneTransitiveRefLocked(sym typerefs.Symbol) *typerefs.PackageSet {
	assert(token.IsExported(sym.Name), "expected exported symbol")  <--- here

	trefs := b.transitiveRefs[sym.Package]
	if trefs == nil {
		trefs = &partialRefs{
			refs:     make(map[string]*typerefs.PackageSet),
			complete: false,
		}
		b.transitiveRefs[sym.Package] = trefs
	}

This stack N2fS6w was reported by telemetry:

• crash/crash
• runtime.gopanic:+69
• golang.org/x/tools/gopls/internal/cache.assert:=10
• golang.org/x/tools/gopls/internal/cache.(*packageHandleBuilder).evaluatePackageHandle.func1:+2
• runtime.gopanic:+50
• golang.org/x/tools/gopls/internal/cache.assert:=10
• golang.org/x/tools/gopls/internal/cache.(*packageHandleBuilder).getOneTransitiveRefLocked:+1
• golang.org/x/tools/gopls/internal/cache.(*packageHandleBuilder).getTransitiveRefs:+22
• golang.org/x/tools/gopls/internal/cache.(*packageHandleBuilder).evaluatePackageHandle:+129
• golang.org/x/tools/gopls/internal/cache.(*Snapshot).getPackageHandles.func2.1:+8
• golang.org/x/sync/errgroup.(*Group).Go.func1:+3
• runtime.goexit:+0

golang.org/x/tools/gopls@v0.17.0 go1.23.4 linux/amd64 vscode (1)

[adonovan]

What we know:

• sym.Name is non-exported.
• sym comes from a value in the node.ph.refs map.
• the refs map is set by evaluatePackageHandle to the result of Snapshot.typerefs()
• its values from the Refs field of the result of typerefs.Decode.
• assuming the encode/decode pair is lossless, this value must have come from typesrefs.state.class, which is inserted by state.getClassIndex.
• this value comes from typestate.declNode.extRefs.
• extRefs is inserted in three places, two of which are unions (inductive case); the third (base case) is the importEdge lambda in typerefs.visitFile, which is dominated a token.IsExported check.

So, this can't happen (modulo: bugs in encode/decode pair, compiler, runtime, hardware). What am I missing?