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cmd/compile: compiler crash with circular (but valid) generic alias #50779

ianlancetaylor opened this issue Jan 24, 2022 · 2 comments

cmd/compile: compiler crash with circular (but valid) generic alias #50779

ianlancetaylor opened this issue Jan 24, 2022 · 2 comments


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@ianlancetaylor ianlancetaylor commented Jan 24, 2022

Compiling this program with tip crashes the compiler.

package p

type AC interface {

type ST []int

type R[S any, P any] struct {}

type SR = R[SS, ST]

type SS interface {
	NSR(any) *SR

type C interface {
	NSR(any) *SR
<autogenerated>:1: internal compiler error: panic: runtime error: invalid memory address or nil pointer dereference

goroutine 1 [running]:
	/home/iant/go/src/runtime/debug/stack.go:24 +0x65
cmd/compile/internal/base.FatalfAt({0x4?, 0x0?}, {0xd1f6c4, 0x9}, {0xc0000d6b08, 0x1, 0x1})
	/home/iant/go/src/cmd/compile/internal/base/print.go:227 +0x1ca
	/home/iant/go/src/cmd/compile/internal/gc/main.go:49 +0x85
panic({0xca9460, 0x130cdf0})
	/home/iant/go/src/runtime/panic.go:838 +0x207
	/home/iant/go/src/cmd/compile/internal/types/size.go:646 +0x18
cmd/compile/internal/escape.(*escape).exprSkipInit(0xc0003dc4c0, {0xc000636bd0?, 0x6f3008?, 0x0?, 0x0?}, {0xe972b8, 0xc0003241b0?})
	/home/iant/go/src/cmd/compile/internal/escape/expr.go:33 +0x176
cmd/compile/internal/escape.(*escape).expr(0xc000636bd0?, {0xc000636bd0?, 0x0?, 0x0?, 0x10?}, {0xe972b8, 0xc0003241b0})
	/home/iant/go/src/cmd/compile/internal/escape/expr.go:20 +0xad
cmd/compile/internal/escape.(*escape).assignList(0xc0003dc4c0, {0xc0000a6500, 0x1, 0x0?}, {0xc0000a64b0, 0x1, 0x203000?}, {0xd1b025, 0x6}, {0xe98e10, ...})
	/home/iant/go/src/cmd/compile/internal/escape/assign.go:93 +0x3d6
cmd/compile/internal/escape.(*escape).stmt(0xc0003dc4c0, {0xe98e10, 0xc00007ca00?})
	/home/iant/go/src/cmd/compile/internal/escape/stmt.go:175 +0x1577
cmd/compile/internal/escape.(*escape).block(0xc0003dc4c0?, {0xc0000a64c0?, 0x1?, 0xc000326480?})
	/home/iant/go/src/cmd/compile/internal/escape/stmt.go:197 +0x72
cmd/compile/internal/escape.(*batch).walkFunc(0xc000326480, 0xc0003d6c60)
	/home/iant/go/src/cmd/compile/internal/escape/escape.go:227 +0xe5
cmd/compile/internal/escape.Batch({0xc0000d76b0?, 0x1, 0x1}, 0xc8?)
	/home/iant/go/src/cmd/compile/internal/escape/escape.go:143 +0x228
cmd/compile/internal/reflectdata.methodWrapper(0xc0003b2d20, 0xc0003ec1e0, 0x88?)
	/home/iant/go/src/cmd/compile/internal/reflectdata/reflect.go:1974 +0x1871
	/home/iant/go/src/cmd/compile/internal/reflectdata/reflect.go:395 +0x2d9
	/home/iant/go/src/cmd/compile/internal/reflectdata/reflect.go:1054 +0x5cd
	/home/iant/go/src/cmd/compile/internal/reflectdata/reflect.go:1266 +0x3d8
	/home/iant/go/src/cmd/compile/internal/gc/obj.go:118 +0x6e
	/home/iant/go/src/cmd/compile/internal/gc/main.go:328 +0x11ab
	/home/iant/go/src/cmd/compile/main.go:55 +0xdd

CC @randall77 @danscales

@ianlancetaylor ianlancetaylor added NeedsInvestigation release-blocker labels Jan 24, 2022
@ianlancetaylor ianlancetaylor added this to the Go1.18 milestone Jan 24, 2022
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@griesemer griesemer commented Jan 24, 2022

This will report an error with forthcoming CL 379916. Leaving open for now so I can add a test. Not a release blocker.

@griesemer griesemer self-assigned this Jan 24, 2022
@griesemer griesemer added NeedsFix and removed NeedsInvestigation release-blocker labels Jan 24, 2022
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@gopherbot gopherbot commented Jan 24, 2022

Change mentions this issue: go/types, types2: report an error when using a broken alias

jproberts pushed a commit to jproberts/go that referenced this issue Jun 21, 2022
The type checker doesn't have a general mechanism to "use" the type
of a type alias whose type depends on a recursive type declaration
which is not yet completely type-checked. In some cases, the type of
a type alias is needed before it is determined; the type is incorrect
(invalid) in that case but no error is reported. The type-checker is
happy with this (incorrect type), but the compiler may crash under
some circumstances.

A correct fix will likely require some form of forwarding type which
is a fairly pervasive change and may also affect the type checker API.

This CL introduces a simple side table, a map of broken type aliases,
which is consulted before the type associated with a type alias is
used. If the type alias is broken, an error is reported.

This is a stop-gap solution that prevents the compiler from crashing.
The reported error refers to the corresponding issue which suggests
a work-around that may be applicable in some cases.

Also fix a minor error related to type cycles: If we have a cycle
that doesn't start with a type, don't use a compiler error message
that explicitly mentions "type".

Fixes golang#50259.
Fixes golang#50276.
Fixes golang#50779.

For golang#50729.

Change-Id: Ie8e38f49ef724e742e8e78625e6d4f3d4014a52c
Trust: Robert Griesemer <>
Run-TryBot: Robert Griesemer <>
Reviewed-by: Robert Findley <>
TryBot-Result: Gopher Robot <>
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