x/tools/gopls: impossible type assertion failure in ChangeSignature (inliner)

[adonovan]

#!stacks
"panicdottypeI" && "inline.(*state).inlineCall:+159"

Issue created by stacks.

I audited the type checker for places that construct a variadic signature, and in all cases it derives from an Ellipsis as the final element of FuncType.Params.List, so the code below is sound. I think is yet another case of random memory corruption.

	if sig.Variadic() {
			lastParam := last(params)
...				// ordinary/ellipsis call to variadic

				// simplify decl: func(T...) -> func([]T)
				lastParamField := last(calleeDecl.Type.Params.List)
				lastParamField.Type = &ast.ArrayType{
					Elt: lastParamField.Type.(*ast.Ellipsis).Elt, // panic: type assertion failed

This stack BVYdIg was reported by telemetry:

• crash/crash
• runtime.gopanic:+69,+0x167
• runtime.panicdottypeE:=275,+0x70
• runtime.panicdottypeI:+5,+0x29
• golang.org/x/tools/internal/refactor/inline.(*state).inlineCall:+159,+0x428d
• golang.org/x/tools/internal/refactor/inline.(*state).inline:+17,+0x27e
• golang.org/x/tools/internal/refactor/inline.Inline:+13,+0x95
• golang.org/x/tools/gopls/internal/golang.inlineAllCalls:+187,+0x167e
• golang.org/x/tools/gopls/internal/golang.rewriteCalls:+118,+0xb04
• golang.org/x/tools/gopls/internal/golang.ChangeSignature:+142,+0x1027
• golang.org/x/tools/gopls/internal/server.(*commandHandler).ChangeSignature.func1:+15,+0x195
• golang.org/x/tools/gopls/internal/server.(*commandHandler).run.func2:+3,+0x86
• golang.org/x/tools/gopls/internal/server.(*commandHandler).run:+81,+0x582
• golang.org/x/tools/gopls/internal/server.(*commandHandler).ChangeSignature:+3,+0x1aa
• golang.org/x/tools/gopls/internal/protocol/command.Dispatch:+45,+0x226e
• golang.org/x/tools/gopls/internal/server.(*server).ResolveCodeAction:+21,+0x1b8
• golang.org/x/tools/gopls/internal/protocol.serverDispatch:+46,+0xcd9
• golang.org/x/tools/gopls/internal/lsprpc.(*StreamServer).ServeStream.ServerHandler.func4:+5,+0x84

golang.org/x/tools/gopls@v0.20.0 go1.25.0 windows/amd64 vscode (6)

[gabyhelp]

Related Issues

• x/tools/gopls: ChangeSignature: crash in in inliner (likely due to ill-formed variadic call) #73187
• x/tools/gopls: ChangeSignature: OOB panic in inlineCall (FuncType.Params inconsistent with Signature) #73912 (closed)
• x/tools/gopls: ChangeSignature: OOB panic in inline.updateCalleeParams (mismatched syntactic/semantic params) #73913 (closed)
• x/tools/gopls: ChangeSignature: "cannot inline: corrupted reference" bug #73779
• x/tools/gopls/internal/golang: inline variable: nil panic in inlineVariableOne #74347 (closed)
• x/tools/gopls: Inline: panic in falcon analysis (unexpected core type) #74393 (closed)
• x/tools/gopls: crash due to missing type in inline.escape #75236
• x/tools/gopls: objectpath: panic due to nil *types.Signature #75575
• x/tools/gopls: Implementation (by signatures): nil panic in dynamicFuncCallType #74305 (closed)
• x/tools/gopls: panic in internal/golang.activeParameter #69552 (closed)

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