`demangle`: Parse optional single template argument for built-in types

The optional single template argument for built-in types is not documented in LLVM's comments that list the grammar, but it is [intentionally implemented](https://github.com/llvm/llvm-project/commit/4bc4d51c18d5c087dfbdad1753c84bba8dbf3be0).

PiperOrigin-RevId: 606843797
Change-Id: Ib5ef1983812e9ea68a39fe64b87b6ad6ce81b93c

1 files changed, 32 insertions, 0 deletions

diff --git a/absl/debugging/internal/demangle_test.cc b/absl/debugging/internal/demangle_test.cc
index 86e66886..555e5908 100644
--- a/absl/debugging/internal/demangle_test.cc
+++ b/absl/debugging/internal/demangle_test.cc
@@ -150,6 +150,38 @@ TEST(Demangle, FunctionTemplateTemplateParamWithConstrainedArg) {
   EXPECT_STREQ(tmp, "foo<>()");
 }
 
+TEST(Demangle, NonTemplateBuiltinType) {
+  char tmp[100];
+
+  // void foo(__my_builtin_type t);
+  //
+  // foo({});
+  ASSERT_TRUE(Demangle("_Z3foou17__my_builtin_type", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "foo()");
+}
+
+TEST(Demangle, SingleArgTemplateBuiltinType) {
+  char tmp[100];
+
+  // template <typename T>
+  // __my_builtin_type<T> foo();
+  //
+  // foo<int>();
+  ASSERT_TRUE(Demangle("_Z3fooIiEu17__my_builtin_typeIT_Ev", tmp, sizeof(tmp)));
+  EXPECT_STREQ(tmp, "foo<>()");
+}
+
+TEST(Demangle, FailsOnTwoArgTemplateBuiltinType) {
+  char tmp[100];
+
+  // template <typename T, typename U>
+  // __my_builtin_type<T, U> foo();
+  //
+  // foo<int, char>();
+  ASSERT_FALSE(
+      Demangle("_Z3fooIicEu17__my_builtin_typeIT_T0_Ev", tmp, sizeof(tmp)));
+}
+
 // Test corner cases of boundary conditions.
 TEST(Demangle, CornerCases) {
   char tmp[10];