Panic while type checking objects with non-json keys

[patrick-east]

With some policy snippet like:

package test

x = {{{}: 1}: 1}

OPA will panic like so:

panic: unreachable [recovered]
	panic: unreachable

goroutine 1 [running]:
github.com/open-policy-agent/opa/ast.(*Compiler).compile.func1()
	/Users/patrick/projects/go/src/github.com/open-policy-agent/opa/ast/compile.go:846 +0x73
panic(0x4772080, 0x49a1730)
	/usr/local/Cellar/go/1.13.7/libexec/src/runtime/panic.go:679 +0x1b2
github.com/open-policy-agent/opa/ast.(*TypeEnv).Get.func1(0xc0003001c0, 0xc0003001e0)
	/Users/patrick/projects/go/src/github.com/open-policy-agent/opa/ast/env.go:67 +0x391
github.com/open-policy-agent/opa/ast.(*object).Foreach.func1(0xc0003001c0, 0xc0003001e0, 0xc000300200, 0x6941460)
	/Users/patrick/projects/go/src/github.com/open-policy-agent/opa/ast/term.go:1769 +0x39
github.com/open-policy-agent/opa/ast.(*object).Iter(0xc0002fb140, 0xc000264480, 0x20, 0x481b1e0)
	/Users/patrick/projects/go/src/github.com/open-policy-agent/opa/ast/term.go:1747 +0xa4
github.com/open-policy-agent/opa/ast.(*object).Foreach(0xc0002fb140, 0xc000300700)
	/Users/patrick/projects/go/src/github.com/open-policy-agent/opa/ast/term.go:1768 +0x53
github.com/open-policy-agent/opa/ast.(*TypeEnv).Get(0xc0002ebd30, 0x4851a60, 0xc000300220, 0x3, 0xc0001e04a0)
	/Users/patrick/projects/go/src/github.com/open-policy-agent/opa/ast/env.go:63 +0x3c0
github.com/open-policy-agent/opa/ast.(*typeChecker).checkRule(0xc0002fb5c0, 0xc0002ebd20, 0xc0002bfe40)
	/Users/patrick/projects/go/src/github.com/open-policy-agent/opa/ast/check.go:181 +0x92a
github.com/open-policy-agent/opa/ast.(*typeChecker).CheckTypes(0xc0002fb5c0, 0xc0002ebd20, 0xc0002ebcf0, 0x1, 0x1, 0x0, 0x0, 0x0, 0x2000107)
	/Users/patrick/projects/go/src/github.com/open-policy-agent/opa/ast/check.go:101 +0x168
github.com/open-policy-agent/opa/ast.(*Compiler).checkTypes(0xc00017f110)
	/Users/patrick/projects/go/src/github.com/open-policy-agent/opa/ast/compile.go:811 +0x117
github.com/open-policy-agent/opa/ast.(*Compiler).runStage(0xc00017f110, 0x48ba9a9, 0x19, 0xc0001e01c0)
	/Users/patrick/projects/go/src/github.com/open-policy-agent/opa/ast/compile.go:832 +0x46
github.com/open-policy-agent/opa/ast.(*Compiler).compile(0xc00017f110)
	/Users/patrick/projects/go/src/github.com/open-policy-agent/opa/ast/compile.go:851 +0x125
github.com/open-policy-agent/opa/ast.(*Compiler).Compile(0xc00017f110, 0xc000264cd0)
	/Users/patrick/projects/go/src/github.com/open-policy-agent/opa/ast/compile.go:345 +0x2a3
github.com/open-policy-agent/opa/bundle.writeModules(0x49cab20, 0xc0000cc008, 0x49d3ea0, 0xc0001a40f0, 0x49b5c60, 0xc0001de5d0, 0xc00017f110, 0x49d0ae0, 0xc0001dc980, 0xc000193200, ...)
	/Users/patrick/projects/go/src/github.com/open-policy-agent/opa/bundle/store.go:397 +0x5b9
github.com/open-policy-agent/opa/bundle.activateBundles(0xc000265358, 0xc0000cc008, 0x49d3ea0)
	/Users/patrick/projects/go/src/github.com/open-policy-agent/opa/bundle/store.go:243 +0x817
github.com/open-policy-agent/opa/bundle.Activate(...)
	/Users/patrick/projects/go/src/github.com/open-policy-agent/opa/bundle/store.go:161
github.com/open-policy-agent/opa/rego.(*Rego).compileModules(0xc0001d2200, 0x49cab20, 0xc0000cc008, 0x49b5c60, 0xc0001de5d0, 0x49d0ae0, 0xc0001dc980, 0x0, 0x0)
	/Users/patrick/projects/go/src/github.com/open-policy-agent/opa/rego/rego.go:1442 +0x1b5
github.com/open-policy-agent/opa/rego.(*Rego).prepare(0xc0001d2200, 0x49cab20, 0xc0000cc008, 0x0, 0xc0002654f8, 0x1, 0x1, 0x0, 0x1)
	/Users/patrick/projects/go/src/github.com/open-policy-agent/opa/rego/rego.go:1269 +0x1f4
github.com/open-policy-agent/opa/rego.(*Rego).PrepareForEval(0xc0001d2200, 0x49cab20, 0xc0000cc008, 0x0, 0x0, 0x0, 0x1, 0x4f0a3e0, 0x0, 0x0)
	/Users/patrick/projects/go/src/github.com/open-policy-agent/opa/rego/rego.go:1184 +0x4d3
github.com/open-policy-agent/opa/cmd.eval(0xc000192fc0, 0x1, 0x3, 0x0, 0xc000194230, 0x1, 0x1, 0x4f0a3e0, 0x0, 0x0, ...)
	/Users/patrick/projects/go/src/github.com/open-policy-agent/opa/cmd/eval.go:263 +0x9b4
github.com/open-policy-agent/opa/cmd.init.4.func2(0xc0001aa780, 0xc000192fc0, 0x1, 0x3)
	/Users/patrick/projects/go/src/github.com/open-policy-agent/opa/cmd/eval.go:203 +0xde
github.com/spf13/cobra.(*Command).execute(0xc0001aa780, 0xc000192f60, 0x3, 0x3, 0xc0001aa780, 0xc000192f60)
	/Users/patrick/projects/go/src/github.com/open-policy-agent/opa/vendor/github.com/spf13/cobra/command.go:766 +0x2aa
github.com/spf13/cobra.(*Command).ExecuteC(0x4ed9560, 0xc000000180, 0xc0000dff50, 0x400753f)
	/Users/patrick/projects/go/src/github.com/open-policy-agent/opa/vendor/github.com/spf13/cobra/command.go:852 +0x2ea
github.com/spf13/cobra.(*Command).Execute(...)
	/Users/patrick/projects/go/src/github.com/open-policy-agent/opa/vendor/github.com/spf13/cobra/command.go:800
main.main()
	/Users/patrick/projects/go/src/github.com/open-policy-agent/opa/main.go:15 +0x31