gccgo: internal compiler error in do_flatten (3)

[dvyukov]

gccgo crashes on the following program:

package a
func f() {
    _, _ = <-chan int(n)
}

go1: internal compiler error: in do_get_backend, at go/gofrontend/expressions.cc:3450
0x5e2224 Unsafe_type_conversion_expression::do_get_backend(Translate_context*)
    ../../gcc/go/gofrontend/expressions.cc:3450
0x5fb6ca Call_expression::do_get_backend(Translate_context*)
    ../../gcc/go/gofrontend/expressions.cc:9299
0x64de42 Temporary_statement::do_get_backend(Translate_context*)
    ../../gcc/go/gofrontend/statements.cc:487
0x61c5b7 Block::get_backend(Translate_context*)
    ../../gcc/go/gofrontend/gogo.cc:5862
0x64b65c Block_statement::do_get_backend(Translate_context*)
    ../../gcc/go/gofrontend/statements.cc:1836
0x61c5b7 Block::get_backend(Translate_context*)
    ../../gcc/go/gofrontend/gogo.cc:5862
0x61ddac Function::build(Gogo*, Named_object*)
    ../../gcc/go/gofrontend/gogo.cc:5468
0x61f2c7 Named_object::get_backend(Gogo*, std::vector<Bexpression*, std::allocator<Bexpression*> >&, std::vector<Btype*, std::allocator<Btype*> >&, std::vector<Bfunction*, std::allocator<Bfunction*> >&)
    ../../gcc/go/gofrontend/gogo.cc:7187
0x625532 Gogo::write_globals()
    ../../gcc/go/gofrontend/gogo.cc:1345

gofrontend is on commit 81810917af7ba19e1f9f8efc8b1989f7d6419d30

[dvyukov]

@ianlancetaylor

[gopherbot]

CL https://golang.org/cl/13932 mentions this issue.