[Support] Make support types more easily printable.

Summary:
Error's new operator<< is the first way to print an error without consuming it.

formatv() can now print objects with an operator<< that works with raw_ostream.

Reviewers: bkramer

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D48966

llvm-svn: 336412

1 files changed, 9 insertions, 0 deletions

diff --git a/llvm/unittests/Support/FormatVariadicTest.cpp b/llvm/unittests/Support/FormatVariadicTest.cpp
index 54373da51c4..6d621464c0e 100644
--- a/llvm/unittests/Support/FormatVariadicTest.cpp
+++ b/llvm/unittests/Support/FormatVariadicTest.cpp
@@ -671,3 +671,12 @@ TEST(FormatVariadicTest, CopiesAndMoves) {
   EXPECT_EQ(0, R.Copied);
   EXPECT_EQ(0, R.Moved);
 }
+
+namespace adl {
+struct X {};
+raw_ostream &operator<<(raw_ostream &OS, const X &) { return OS << "X"; }
+} // namespace adl
+TEST(FormatVariadicTest, FormatStreamable) {
+  adl::X X;
+  EXPECT_EQ("X", formatv("{0}", X).str());
+}