go/constant: hang evaluating "-6e-1886451601"

[Josh Bleecher Snyder]

go/types appears to hang while typechecking:

package p

var x = -6e-1886451601

It appears to be stuck (or just doing a lot of work?) in karatsuba:

goroutine 5 [runnable]:
math/big.karatsubaAdd(0xc42996cb00, 0xfbc6a0, 0xfbc6a4, 0xc429973e00, 0xfbb840, 0xfbb844, 0x5c0)
	/Users/josh/go/tip/src/math/big/nat.go:236 +0x129
math/big.karatsuba(0xc42996b400, 0xfbc980, 0xfbc984, 0xc429965800, 0x5c0, 0xfbd504, 0xc429968600, 0x5c0, 0xfbcf44)
	/Users/josh/go/tip/src/math/big/nat.go:347 +0x731
math/big.karatsuba(0xc42995a000, 0xfbec00, 0xfbec04, 0xc424e3d000, 0xb80, 0x7df604, 0xc424e3d000, 0xb80, 0x7df604)
	/Users/josh/go/tip/src/math/big/nat.go:329 +0x506
math/big.karatsuba(0xc42995a000, 0xfbec00, 0xfbec04, 0xc424e3d000, 0x1700, 0x7df604, 0xc424e3d000, 0x1700, 0x7df604)
	/Users/josh/go/tip/src/math/big/nat.go:308 +0x1ad
math/big.karatsuba(0xc42995a000, 0xfbec00, 0xfbec04, 0xc424e3d000, 0x2e00, 0x7df604, 0xc424e3d000, 0x2e00, 0x7df604)
	/Users/josh/go/tip/src/math/big/nat.go:308 +0x1ad
math/big.karatsuba(0xc42992c000, 0xfc4800, 0xfc4804, 0xc424e26000, 0x5c00, 0x7e2404, 0xc424e26000, 0x5c00, 0x7e2404)
	/Users/josh/go/tip/src/math/big/nat.go:309 +0x28f
math/big.karatsuba(0xc4298d0000, 0xfd0000, 0xfd0004, 0xc424df8000, 0xb800, 0x7e8004, 0xc424df8000, 0xb800, 0x7e8004)
	/Users/josh/go/tip/src/math/big/nat.go:309 +0x28f
math/big.karatsuba(0xc4298d0000, 0xfd0000, 0xfd0004, 0xc424df8000, 0x17000, 0x7e8004, 0xc424df8000, 0x17000, 0x7e8004)
	/Users/josh/go/tip/src/math/big/nat.go:308 +0x1ad
math/big.karatsuba(0xc4298d0000, 0xfd0000, 0xfd0004, 0xc424df8000, 0x2e000, 0x7e8004, 0xc424df8000, 0x2e000, 0x7e8004)
	/Users/josh/go/tip/src/math/big/nat.go:308 +0x1ad
math/big.karatsuba(0xc4298d0000, 0xfd0000, 0xfd0004, 0xc424df8000, 0x5c000, 0x7e8004, 0xc424df8000, 0x5c000, 0x7e8004)
	/Users/josh/go/tip/src/math/big/nat.go:308 +0x1ad
math/big.karatsuba(0xc4298d0000, 0xfd0000, 0xfd0004, 0xc424df8000, 0xb8000, 0x7e8004, 0xc424df8000, 0xb8000, 0x7e8004)
	/Users/josh/go/tip/src/math/big/nat.go:308 +0x1ad
math/big.karatsuba(0xc428d50000, 0x1140000, 0x1140004, 0xc424838000, 0x170000, 0x8a0004, 0xc424838000, 0x170000, 0x8a0004)
	/Users/josh/go/tip/src/math/big/nat.go:309 +0x28f
math/big.karatsuba(0xc428d50000, 0x1140000, 0x1140004, 0xc424838000, 0x2e0000, 0x8a0004, 0xc424838000, 0x2e0000, 0x8a0004)
	/Users/josh/go/tip/src/math/big/nat.go:308 +0x1ad
math/big.nat.mul(0xc4224fe000, 0x17585a, 0x450004, 0xc424838000, 0x2eb0b3, 0x8a0004, 0xc424838000, 0x2eb0b3, 0x8a0004, 0xc424838000, ...)
	/Users/josh/go/tip/src/math/big/nat.go:432 +0x1b9
math/big.nat.expNN(0x0, 0x0, 0x0, 0x1465148, 0x1, 0x1, 0xc42001a2a0, 0x1, 0x5, 0x0, ...)
	/Users/josh/go/tip/src/math/big/nat.go:939 +0x30c
math/big.(*Rat).SetString(0xc420058c40, 0xc420014430, 0xd, 0xc42000ed20, 0x1)
	/Users/josh/go/tip/src/math/big/ratconv.go:115 +0x626
go/constant.makeFloatFromLiteral(0xc420014430, 0xd, 0x0, 0x0)
	/Users/josh/go/tip/src/go/constant/value.go:250 +0xf0
go/constant.MakeFromLiteral(0xc420014430, 0xd, 0x6, 0x0, 0x0, 0x0)
	/Users/josh/go/tip/src/go/constant/value.go:325 +0x80
go/types.(*operand).setConst(0xc420058c00, 0x6, 0xc420014430, 0xd)
	/Users/josh/go/tip/src/go/types/operand.go:187 +0xa5
go/types.(*Checker).exprInternal(0xc4200e0000, 0xc420058c00, 0x1469520, 0xc42000a320, 0x0, 0x0, 0x0)
	/Users/josh/go/tip/src/go/types/expr.go:998 +0x2a5a
go/types.(*Checker).rawExpr(0xc4200e0000, 0xc420058c00, 0x1469520, 0xc42000a320, 0x0, 0x0, 0x0)
	/Users/josh/go/tip/src/go/types/expr.go:945 +0x81
go/types.(*Checker).multiExpr(0xc4200e0000, 0xc420058c00, 0x1469520, 0xc42000a320)
	/Users/josh/go/tip/src/go/types/expr.go:1481 +0x62
go/types.(*Checker).expr(0xc4200e0000, 0xc420058c00, 0x1469520, 0xc42000a320)
	/Users/josh/go/tip/src/go/types/expr.go:1475 +0x49
go/types.(*Checker).exprInternal(0xc4200e0000, 0xc420058c00, 0x1469fa0, 0xc42000a340, 0x0, 0x0, 0xc42004e940)
	/Users/josh/go/tip/src/go/types/expr.go:1395 +0x1a9a
go/types.(*Checker).rawExpr(0xc4200e0000, 0xc420058c00, 0x1469fa0, 0xc42000a340, 0x0, 0x0, 0x0)
	/Users/josh/go/tip/src/go/types/expr.go:945 +0x81
go/types.(*Checker).multiExpr(0xc4200e0000, 0xc420058c00, 0x1469fa0, 0xc42000a340)
	/Users/josh/go/tip/src/go/types/expr.go:1481 +0x62
go/types.(*Checker).expr(0xc4200e0000, 0xc420058c00, 0x1469fa0, 0xc42000a340)
	/Users/josh/go/tip/src/go/types/expr.go:1475 +0x49
go/types.(*Checker).varDecl(0xc4200e0000, 0xc4200857c0, 0xc42000c0a8, 0x1, 0x1, 0x0, 0x0, 0x1469fa0, 0xc42000a340)
	/Users/josh/go/tip/src/go/types/decl.go:166 +0x27c
go/types.(*Checker).objDecl(0xc4200e0000, 0x146c2c0, 0xc4200857c0, 0x0, 0xc42004ac50, 0x0, 0x8)
	/Users/josh/go/tip/src/go/types/decl.go:81 +0x223
go/types.(*Checker).packageObjects(0xc4200e0000, 0xc42004e950, 0x1, 0x1)
	/Users/josh/go/tip/src/go/types/resolver.go:457 +0x105
go/types.(*Checker).checkFiles(0xc4200e0000, 0xc42004ae30, 0x1, 0x1, 0x0, 0x0)
	/Users/josh/go/tip/src/go/types/check.go:239 +0xdb
go/types.(*Checker).Files(0xc4200e0000, 0xc42004ae30, 0x1, 0x1, 0xc42000ecc0, 0x0)
	/Users/josh/go/tip/src/go/types/check.go:230 +0x49
go/types.(*Config).Check(0xc420058bc0, 0x12f539e, 0x1, 0xc420058b40, 0xc42004ae30, 0x1, 0x1, 0xc420085680, 0x0, 0x2, ...)
	/Users/josh/go/tip/src/go/types/api.go:351 +0x1b3
go/types_test.TestIssue16902(0xc4200de0f0)
	/Users/josh/go/tip/src/go/types/sizes_test.go:102 +0x31f
testing.tRunner(0xc4200de0f0, 0x130bd00)
	/Users/josh/go/tip/src/testing/testing.go:747 +0xd0
created by testing.(*T).Run
	/Users/josh/go/tip/src/testing/testing.go:789 +0x2de

Does not affect cmd/compile.

Found using go-fuzz.

[Robert Griesemer]

Probably doing insane amounts of work... - though it may finish before the Universe ends, with some luck :-)

(and yes, should probably fix but low priority)

[Josh Bleecher Snyder]

I was curious...so I fixed it. :)

[GopherBot]

CL https://golang.org/cl/42592 mentions this issue.