"ambiguous symbol" in Bazel building C++ - c++

Question

Background
I am on Windows building with Bazel using cl to compile C++.
Subset of files:
third_party/icu/source/common/unicode/schriter.h
third_party/icu/source/common/unicode/utypes.h
third_party/icu/source/common/unicode/stringpiece.h
third_party/icu/source/common/stringpiece.cpp
third_party/icu/BUILD
a/a.cc
a/a.h
a/BUILD
b/cpp/src/strings/stringpiece.h
b/cpp/src/util/uri_utils.h
b/BUILD
schriter.h has #include "unicode/utypes.h".
uri_utils.h and b/cpp/src/strings/stringpiece.h both have class StringPiece. third_party/icu/source/common/unicode/stringpiece.h has class U_COMMON_API StringPiece : public UMemory
a.cc refers to StringPiece and has these includes:
#include "b/cpp/util/uri_utils.h"
#include "strings/stringpiece.h"
#include "third_party/icu/source/common/unicode/schriter.h"
a/BUILD:
cc_library(
name = "a",
srcs = ["a.cc"],
hdrs = ["a.h"],
deps = [
"//third_party/icu:common",
"//b:sdk_strings",
],
)
b/BUILD:
cc_library(
name = "sdk_strings",
srcs = [
"cpp/util/uri_utils.cc",
"cpp/src/strings/stringpiece.cc"
],
hdrs = [
"cpp/util/uri_utils.h",
"cpp/src/strings/stringpiece.h",
],
includes = ["cpp/src"],
)
third_party/icu/BUILD:
cc_library(
name = "common",
srcs = [
"source/common/stringpiece.cpp",
"source/stubdata/stubdata.c",
],
hdrs = glob(["**/*.h"]),
)
Problem
As is, building third_party/icu:common fails with:
third_party/icu/source/stubdata/stubdata.c(20): fatal error C1083: Cannot open include file: 'unicode/utypes.h': No such file or directory
If I add copts = ["/Ithird_party/icu/source/common",], to third_party/icu/BUILD, then icu:common builds but target a fails with:
third_party/icu/source/common/unicode/schriter.h(21): fatal error C1083: Cannot open include file: 'unicode/utypes.h': No such file or directory
If instead I add includes = ["source/common",],, then icu:common builds but target a fails with:
a/a.cc(168): error C2872: 'StringPiece': ambiguous symbol
b/cpp/util/uri_utils.h(24): note: could be 'StringPiece'
third_party\icu\source\common\unicode/stringpiece.h(52): note: or 'icu_54::StringPiece'
The source compiles fine using cmake, so I shouldn't need to change the source. How do I change the BUILD files to make this build correctly? How do I let everything in icu access the headers in unicode, but not expose unicode/stringpiece.h to targets that depend on icu?

Answer 1

You should be able to add the namespace (icu::StringPiece, I'm guessing?) to resolve the C2872 error.
For limiting visibility, check out the documentation:
For cc_library rules, headers in hdrs comprise the public interface of the library and can be directly included both from the files in hdrs and srcs of the library itself as well as from files in hdrs and srcs of cc_* rules that list the library in their deps. Headers in srcs must only be directly included from the files in hdrs and srcs of the library itself.
This means that hdrs define transitively visible headers and srcs is for "private" headers.
However, as the docs points out further down, this cannot always be perfectly enforced:
Unfortunately Bazel currently cannot distinguish between direct and transitive inclusions, so it cannot detect error cases where a file illegally includes a header directly that is only allowed to be included transitively. For example, Bazel would not complain if in the example above foo.cc directly includes baz.h. This would be illegal, because foo does not directly depend on baz.
So, it should prevent all but the most determined user to put it in the srcs of a private target with a copts = ['-Ithird_party/icu/source/common'] option.