[lldb] Always include template arguments that have their default value in the internal type name

[teemperor]

Summary

Our type formatters/summaries match on the internal type name we generate in LLDB for Clang types.

These names were generated using Clang's default printing policy. However Clang's
default printing policy got tweaked over the last month to make the generated type
names more readable (by for example excluding inline/anonymous namespaces and
removing template arguments that have their default value). This broke the formatter
system where LLDB's matching logic now no longer can format certain types as
the new type names generated by Clang's default printing policy no longer match
the type names that LLDB/the user specified.

I already introduced LLDB's own type printing policy and fixed the inline/anonymous
namespaces in da121fff1184267a405f81a87f7314df2d474e1c (just to get the
test suite passing again).

This patch is restoring the old type printing behaviour where always include the template
arguments in the internal type name (even if they match the default args). This should get
template type formatters/summaries working again in the rare situation where we do
know template default arguments within LLDB. This can only happen when either having
a template that was parsed in the expression parser or when we get type information from a C++ module.

The Clang change that removed defaulted template arguments from Clang's printing policy was
e7f3e2103cdb567dda4fd52f81bf4bc07179f5a8

[labath]

looks good.

lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp

 
 PrintingPolicy TypeSystemClang::GetTypePrintingPolicy() {
   clang::PrintingPolicy printing_policy(getASTContext().getPrintingPolicy());
   printing_policy.SuppressTagKeyword = true;
   // Inline namespaces are important for some type formatters (e.g., libc++
   // and libstdc++ are differentiated by their inline namespaces).
   printing_policy.SuppressInlineNamespace = false;
   printing_policy.SuppressUnwrittenScope = false;
+  // Default arguments are also always important for type formatters. Otherwise
+  // we would need to always specify two type names for the setups where we do
+  // know the default arguments and where we don't know default arguments.
+  //
+  // For example, without this we would need to have formatters for both:
+  //   std::basic_string<char>
+  // and
+  //   std::basic_string<char, std::char_traits<char>, std::allocator<char> >
+  // to support setups where LLDB was able to reconstruct default arguments
+  // (and we then would have suppressed them from the type name) and also setups
+  // where LLDB wasn't able to reconstruct the default arguments.
+  printing_policy.SuppressDefaultTemplateArgs = false;
   return printing_policy;
 }
 
 std::string TypeSystemClang::GetTypeNameForDecl(const NamedDecl *named_decl) {
   clang::PrintingPolicy printing_policy = GetTypePrintingPolicy();
   std::string result;
   llvm::raw_string_ostream os(result);
   named_decl->printQualifiedName(os, printing_policy);

lldb/test/API/lang/cpp/default-template-args/Makefile

+CXX_SOURCES := main.cpp
+
+include Makefile.rules

lldb/test/API/lang/cpp/default-template-args/TestDefaultTemplateArgs.py

+"""
+Test default template arguments.
+"""
+
+import lldb
+from lldbsuite.test.decorators import *
+from lldbsuite.test.lldbtest import *
+from lldbsuite.test import lldbutil
+
+class TestCase(TestBase):
+
+    mydir = TestBase.compute_mydir(__file__)
+
+    @no_debug_info_test
+    def test(self):
+        self.build()
+        lldbutil.run_to_source_breakpoint(self, "// break here", lldb.SBFileSpec("main.cpp"))
+
+        # Declare a template with a template argument that has a default argument.
+        self.expect("expr --top-level -- template<typename T = int> struct $X { int v; };")
+
+        # The type we display to the user should omit the argument with the default
+        # value.
+        result = self.expect_expr("$X<> x; x",  result_type="$X<>")
+        # The internal name should also always show all arguments (even if they
+        # have their default value).
+        self.assertEqual(result.GetTypeName(), "$X<int>")
+
+        # Test the template but this time specify a non-default value for the
+        # template argument.
+        # Both internal type name and the one we display to the user should
+        # show the non-default value in the type name.
+        result = self.expect_expr("$X<long> x; x", result_type="$X<long>")
+        self.assertEqual(result.GetTypeName(), "$X<long>")
+
+        # Test that the formatters are using the internal type names that
+        # always include all template arguments.
+        self.expect("type summary add '$X<int>' --summary-string 'summary1'")
+        self.expect_expr("$X<> x; x", result_summary="summary1")
+        self.expect("type summary add '$X<long>' --summary-string 'summary2'")
+        self.expect_expr("$X<long> x; x", result_summary="summary2")

lldb/test/API/lang/cpp/default-template-args/main.cpp

+int main() {
+  return 0; // break here
+}