x/tools/gopls: OOB index crash in completion.expectedReturnStmtType

[adonovan]

#!stacks
"runtime.goPanicIndex" && "go/types.(*Tuple).At" && "completion.expectedReturnStmtType"

Issue created by stacks.

// expectedReturnStmtType returns the expected type of a return statement.
// Returns nil if enclosingSig is nil.
func expectedReturnStmtType(enclosingSig *types.Signature, node *ast.ReturnStmt, pos token.Pos) types.Type {
	if enclosingSig != nil {
		if resultIdx := exprAtPos(pos, node.Results); resultIdx < len(node.Results) {
			return enclosingSig.Results().At(resultIdx).Type() // <--- At panics
		}
	}
	return nil
}

This stack 69yMYw was reported by telemetry:

• crash/crash
• runtime.gopanic:+69
• runtime.goPanicIndex:+2
• go/types.(*Tuple).At:=34
• golang.org/x/tools/gopls/internal/golang/completion.expectedReturnStmtType:+3
• golang.org/x/tools/gopls/internal/golang/completion.expectedCandidate:+37
• golang.org/x/tools/gopls/internal/golang/completion.Completion:+150
• golang.org/x/tools/gopls/internal/server.(*server).Completion:+19
• golang.org/x/tools/gopls/internal/protocol.serverDispatch:+193
• golang.org/x/tools/gopls/internal/lsprpc.(*streamServer).ServeStream.ServerHandler.func3:+5
• golang.org/x/tools/gopls/internal/lsprpc.(*streamServer).ServeStream.handshaker.func4:+52
• golang.org/x/tools/gopls/internal/protocol.Handlers.MustReplyHandler.func1:+2
• golang.org/x/tools/gopls/internal/protocol.Handlers.AsyncHandler.func2.2:+3
• runtime.goexit:+0

golang.org/x/tools/gopls@v0.17.0-pre.3 go1.23.2 linux/amd64 other,vscode (5)

[adonovan]

Closely related to #70634.

[gabyhelp]

Related Issues

• x/tools/gopls: nil Signature (?) deref in Completion #70634
• x/tools/gopls: analysisinternal.ZeroValue crash on (presumably) unsafe.Pointer #70585 (closed)
• x/tools/gopls: automated issue report (crash) - unhandled basic type #40956 (closed)
• x/tools/gopls: Hover: nil deref in parseDocLink #70635
• x/tools/gopls: Hover: panic in lookupObjectByName #69616
• x/tools/gopls: crash in Hover (telemetry) #69362
• x/tools/gopls: crash in packageReferences (reported by telemetry) #66250 (closed)
• x/tools/gopls: out-of-bounds index in "stub methods" code action #64087 (closed)
• x/tools/gopls: "slice bounds out of range" crash in ExtractToNewFile #70553
• x/tools/gopls: OOB index in stubmethods.fromValueSpec (reported by telemetry) #64545 (closed)

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[findleyr]

This is indeed a logic bug, as described in #70634 (comment).

[gopherbot]

Change https://go.dev/cl/632936 mentions this issue: gopls/internal/golang/completion: fix crash with extra results