[Clang] Implement P2308R1 - Template Parameter Initialization. (#73103)

https://wiki.edg.com/pub/Wg21kona2023/StrawPolls/p2308r1.html

This implements P2308R1 as a DR and resolves CWG2459, CWG2450 and
CWG2049.


Fixes #73666
Fixes #58434 
Fixes #41227
Fixes #49978
Fixes #36296

1 files changed, 8 insertions, 3 deletions

diff --git a/clang/lib/Parse/ParseTemplate.cpp b/clang/lib/Parse/ParseTemplate.cpp
index f556d0e..64fe4d5 100644
--- a/clang/lib/Parse/ParseTemplate.cpp
+++ b/clang/lib/Parse/ParseTemplate.cpp
@@ -1062,8 +1062,7 @@ Parser::ParseNonTypeTemplateParameter(unsigned Depth, unsigned Position) {
       ++CurTemplateDepthTracker;
       EnterExpressionEvaluationContext ConstantEvaluated(
           Actions, Sema::ExpressionEvaluationContext::ConstantEvaluated);
-      DefaultArg =
-          Actions.CorrectDelayedTyposInExpr(ParseAssignmentExpression());
+      DefaultArg = Actions.CorrectDelayedTyposInExpr(ParseInitializer());
       if (DefaultArg.isInvalid())
         SkipUntil(tok::comma, tok::greater, StopAtSemi | StopBeforeMatch);
     }
@@ -1582,6 +1581,8 @@ ParsedTemplateArgument Parser::ParseTemplateTemplateArgument() {
 ///         constant-expression
 ///         type-id
 ///         id-expression
+///         braced-init-list  [C++26, DR]
+///
 ParsedTemplateArgument Parser::ParseTemplateArgument() {
   // C++ [temp.arg]p2:
   //   In a template-argument, an ambiguity between a type-id and an
@@ -1619,8 +1620,12 @@ ParsedTemplateArgument Parser::ParseTemplateArgument() {
   }
 
   // Parse a non-type template argument.
+  ExprResult ExprArg;
   SourceLocation Loc = Tok.getLocation();
-  ExprResult ExprArg = ParseConstantExpressionInExprEvalContext(MaybeTypeCast);
+  if (getLangOpts().CPlusPlus11 && Tok.is(tok::l_brace))
+    ExprArg = ParseBraceInitializer();
+  else
+    ExprArg = ParseConstantExpressionInExprEvalContext(MaybeTypeCast);
   if (ExprArg.isInvalid() || !ExprArg.get()) {
     return ParsedTemplateArgument();
   }