[llvm-branch-commits] [clang] [clang-tools-extra] [clang][HeuristicResolver] Default argument heuristic for template parameters (PR #131074)

[Nathan Ridge via llvm-branch-commits]

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@@ -125,6 +126,20 @@ TagDecl *HeuristicResolverImpl::resolveTypeToTagDecl(QualType QT) {
   if (!T)
     return nullptr;
 
+  // If T is the type of a template parameter, we can't get a useful TagDecl
+  // out of it. However, if the template parameter has a default argument,
+  // as a heuristic we can replace T with the default argument type.
+  if (const auto *TTPT = dyn_cast<TemplateTypeParmType>(T)) {
+    if (const auto *TTPD = TTPT->getDecl()) {
+      if (TTPD->hasDefaultArgument()) {
+        const auto &DefaultArg = TTPD->getDefaultArgument().getArgument();
+        if (DefaultArg.getKind() == TemplateArgument::Type) {
+          T = DefaultArg.getAsType().getTypePtrOrNull();
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HighCommander4 wrote:

Thanks for the added testcases!

The recursive case made me realize that application of this heuristic is probably better done in `simplifyType`. I updated the patch and added the testcase.

For the template template parameter case, I agree that it would be nice to make this work as well, but I'd rather leave that to a follow-up patch.

https://github.com/llvm/llvm-project/pull/131074