What version of Go are you using (go version)?

gccgo tip, linux/amd64

$ go version
go version go1.12.2 gccgo (GCC) 10.0.0 20190822 (experimental) linux/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

$ go env

What did you do?

This actually cropped up while building a package in module mode, via

  git clone https://go.googlesource.com/protobuf
  cd protobuf
  go test ./proto 

due to a module path of the form "protobuf/internal/testprotos/legacy/proto2.v0.0.0-20160225-2fc053c5". There is a simpler way to reproduce, however, via the following.

In $GOPATH/src, create a subdir "a.0.0", then in that dir place this file:

package a

type T struct {
	q interface{}
	x uint64
}

func (t T) X() uint64 {
	return t.x
}

Then do:

cd $GOPATH/src/a.0.0
go build .

What did you expect to see?

Clean build.

What did you see instead?

This error:

# a.0.0
go1: internal compiler error: in mangled_name, at go/gofrontend/names.cc:590
0x66d873 Type::mangled_name[abi:cxx11](Gogo*) const
	../../gcc-trunk/gcc/go/gofrontend/names.cc:590
0x8c12ea Gogo::specific_type_function_names(Type const*, Named_type const*, std::__cxx11::basic_string, std::allocator >*, std::__cxx11::basic_string, std::allocator >*)
	../../gcc-trunk/gcc/go/gofrontend/names.cc:302
0x9045c7 Type::specific_type_functions(Gogo*, Named_type*, long, Function_type*, Function_type*, Named_object**, Named_object**)
	../../gcc-trunk/gcc/go/gofrontend/types.cc:2023
0x903075 Type::type_functions(Gogo*, Named_type*, Function_type*, Function_type*, Named_object**, Named_object**)
	../../gcc-trunk/gcc/go/gofrontend/types.cc:1955
0x890342 Specific_type_functions::type(Type*)
	../../gcc-trunk/gcc/go/gofrontend/gogo.cc:2755
0x890342 Specific_type_functions::type(Type*)
	../../gcc-trunk/gcc/go/gofrontend/gogo.cc:2743
0x8e9f32 Type::traverse(Type*, Traverse*)
	../../gcc-trunk/gcc/go/gofrontend/types.cc:318
0x89371c Bindings::traverse(Traverse*, bool)
	../../gcc-trunk/gcc/go/gofrontend/gogo.cc:9158
0x893941 Gogo::traverse(Traverse*)
	../../gcc-trunk/gcc/go/gofrontend/gogo.cc:2829
0x8939d7 Gogo::write_specific_type_functions()
	../../gcc-trunk/gcc/go/gofrontend/gogo.cc:2805
0x88ed33 go_parse_input_files(char const**, unsigned int, bool, bool)
	../../gcc-trunk/gcc/go/gofrontend/go.cc:173
0x88a89f go_langhook_parse_file
	../../gcc-trunk/gcc/go/go-lang.c:330
Please submit a full bug report,