[clang] [clang] CTAD: implement the missing IsDeducible constraint for alias templates (PR #89358)

[Haojian Wu via cfe-commits]

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@@ -3207,6 +3241,59 @@ Sema::DeduceTemplateArguments(VarTemplatePartialSpecializationDecl *Partial,
   return ::DeduceTemplateArguments(*this, Partial, TemplateArgs, Info);
 }
 
+TemplateDeductionResult
+Sema::DeduceTemplateArgumentsFromType(TemplateDecl *TD, QualType FromType,
+                                      sema::TemplateDeductionInfo &Info) {
+  if (TD->isInvalidDecl())
+    return TemplateDeductionResult::Invalid;
+
+  QualType PType;
+  if (const auto *CTD = dyn_cast<ClassTemplateDecl>(TD)) {
+    // Use the InjectedClassNameType.
+    PType = Context.getTypeDeclType(CTD->getTemplatedDecl());
+  } else if (const auto *AliasTemplate = dyn_cast<TypeAliasTemplateDecl>(TD)) {
+    PType = AliasTemplate->getTemplatedDecl()
+                ->getUnderlyingType()
+                .getCanonicalType();
+  } else {
+    // FIXME: emit a diagnostic, we only only support alias and class templates.
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hokein wrote:

It turns out we will never hit other cases in this code path (we will reject the var/function templates when parsing the type trait expr). I changed it to `assert`.

https://github.com/llvm/llvm-project/pull/89358