x/tools/gopls: SEGV in types.Type.Underlying

[adonovan]

#!stacks
"sigpanic" && "go/types.under:+4"

Issue created by stacks.

This stack 3WhcMA was reported by telemetry:

• crash/crash
• runtime.gopanic:+69,+0x167
• runtime.panicmem:=262,+0x358
• runtime.sigpanic:+19,+0x328
• go/types.under:+4,+0x35
• go/types.isBytesOrRunes:+2,+0x3b
• go/types.(*operand).convertibleTo:+47,+0x3bc
• go/types.ConvertibleTo:=46,+0x1ea
• golang.org/x/tools/gopls/internal/golang/completion.convertibleTo:+6,+0x1eb
• golang.org/x/tools/gopls/internal/golang/completion.considerTypeConversion:+12,+0x7c
• golang.org/x/tools/gopls/internal/golang/completion.(*candidateInference).candTypeMatches.func1:+27,+0x231
• golang.org/x/tools/gopls/internal/golang/completion.(*candidate).anyCandType.func1:+1,+0x5e
• golang.org/x/tools/gopls/internal/golang/completion.(*candidate).anyCandType:+28,+0xd9
• golang.org/x/tools/gopls/internal/golang/completion.(*candidateInference).candTypeMatches:+14,+0x13b
• golang.org/x/tools/gopls/internal/golang/completion.(*completer).matchingCandidate:+17,+0xc4
• golang.org/x/tools/gopls/internal/golang/completion.(*completer).addCandidate:+2,+0x5b
• golang.org/x/tools/gopls/internal/golang/completion.(*completer).deepSearch:+71,+0x4de
• golang.org/x/tools/gopls/internal/golang/completion.Completion:+165,+0x13a5
• golang.org/x/tools/gopls/internal/server.(*server).Completion:+19,+0x357

golang.org/x/tools/gopls@v0.19.1 go1.24.4 linux/amd64 vscode (1)

Dups: dumPAQ NtAk-A QUq4eg

[adonovan]

This stack dumPAQ was reported by telemetry:

• crash/crash
• runtime.gopanic:+69,+0x14f
• go/types.(*Checker).handleBailout:+25,+0x9b
• go/types.(*Checker).Files.deferwrap1:+0,+0x27
• runtime.gopanic:+50,+0x11f
• runtime.panicmem:=262,+0x2e7
• runtime.sigpanic:+19,+0x2b8
• go/types.under:+4,+0x3c
• go/types.isBasic:=38,+0x23
• go/types.isString:+0,+0x4c
• go/types.(*Checker).builtin.func2:+4,+0x93
• go/types.(*_TypeSet).typeset:+2,+0x5f
• go/types.(*TypeParam).typeset:+1,+0x43
• go/types.typeset:+2,+0x97
• go/types.(*Checker).builtin:+81,+0x413
• go/types.(*Checker).callExpr:+60,+0xa9f
• go/types.(*Checker).exprInternal:+92,+0x803
• go/types.(*Checker).rawExpr:+10,+0x113
• go/types.(*Checker).multiExpr:+2,+0x5f

golang.org/x/tools/gopls@v0.20.0 go1.25.3 darwin/arm64 gopls.client=other (5)

Use this command to reproduce the executable:
(HOME=$(mktemp -d); GOOS=darwin GOARCH=arm64 GOTOOLCHAIN=go1.25.3 go install golang.org/x/tools/gopls@v0.20.0 && find $HOME/go/bin -type f)
To disassemble: go tool objdump exe | less

[adonovan]

Found new stacks for predicate:

#!stacks
"sigpanic" && "go/types.under:+4"

This stack NtAk-A was reported by telemetry:

• crash/crash
• runtime.gopanic:+69,+0x167
• go/types.(*Checker).handleBailout:+25,+0x96
• go/types.(*Checker).Files.deferwrap1:+0,+0x24
• runtime.gopanic:+50,+0x131
• runtime.panicmem:=262,+0x358
• runtime.sigpanic:+19,+0x328
• go/types.under:+4,+0x35
• go/types.isBasic:=38,+0x1c
• go/types.isString:+0,+0x3f
• go/types.(*Checker).builtin.func2:+4,+0x98
• go/types.(*_TypeSet).typeset:+2,+0x45
• go/types.(*TypeParam).typeset:+1,+0x39
• go/types.typeset:+2,+0x85
• go/types.(*Checker).builtin:+81,+0x57b
• go/types.(*Checker).callExpr:+60,+0xdc9
• go/types.(*Checker).exprInternal:+92,+0xa1b
• go/types.(*Checker).rawExpr:+10,+0x18b
• go/types.(*Checker).multiExpr:+2,+0x78

golang.org/x/tools/gopls@v0.21.0 go1.25.6 linux/amd64 gopls.client=vscode (85)

Use this command to reproduce the executable:
(HOME=$(mktemp -d); GOOS=linux GOARCH=amd64 GOTOOLCHAIN=go1.25.6 go install golang.org/x/tools/gopls@v0.21.0 && find $HOME/go/bin -type f)
To disassemble: go tool objdump exe | less