[clang] [Sema] Fast-path simple plain auto deduction in DeduceAutoType (PR #188196)

[Matheus Izvekov via cfe-commits]

================
@@ -5224,25 +5224,40 @@ Sema::DeduceAutoType(TypeLoc Type, Expr *Init, QualType &Result,
     return TemplateDeductionResult::Success;
   }
 
-  auto *InitList = dyn_cast<InitListExpr>(Init);
-  bool IsArrayType = Type.getType()->isArrayType();
-  if (!getLangOpts().CPlusPlus && (InitList || IsArrayType)) {
-    Diag(Init->getBeginLoc(), diag::err_auto_init_list_from_c)
-        << (int)AT->getKeyword() << IsArrayType;
-    return TemplateDeductionResult::AlreadyDiagnosed;
+  // Make sure that we treat 'char[]' equaly as 'char*' in C23 mode.
+  auto *String = dyn_cast<StringLiteral>(Init);
+  if (getLangOpts().C23 && String && Type.getType()->isArrayType()) {
+    Diag(Type.getBeginLoc(), diag::ext_c23_auto_non_plain_identifier);
+    TypeLoc TL = TypeLoc(Init->getType(), Type.getOpaqueData());
+    Result = SubstituteDeducedTypeTransform(*this, DependentResult).Apply(TL);
+    assert(!Result.isNull() && "substituting DependentTy can't fail");
+    return TemplateDeductionResult::Success;
   }
 
   // Emit a warning if 'auto*' is used in pedantic and in C23 mode.
   if (getLangOpts().C23 && Type.getType()->isPointerType()) {
     Diag(Type.getBeginLoc(), diag::ext_c23_auto_non_plain_identifier);
   }
 
+  auto *InitList = dyn_cast<InitListExpr>(Init);
+  if (!getLangOpts().CPlusPlus && InitList) {
+    Diag(Init->getBeginLoc(), diag::err_auto_init_list_from_c)
+        << (int)AT->getKeyword() << getLangOpts().C23;
+    return TemplateDeductionResult::AlreadyDiagnosed;
+  }
+
   // Deduce type of TemplParam in Func(Init)
   SmallVector<DeducedTemplateArgument, 1> Deduced;
   Deduced.resize(1);
 
   SmallVector<OriginalCallArg, 4> OriginalCallArgs;
 
+  bool CanTryFastPath =
+      !InitList &&
+      !Init->getType()->isSpecificBuiltinType(BuiltinType::Overload) &&
+      !getLangOpts().ObjC && !getLangOpts().OpenCL;
----------------
mizvekov wrote:

Yes, strong qualifiers is the one kind of conversion we have for type template arguments.

That's gated on the ARC feature: getLangOpts().ObjCAutoRefCount

https://github.com/llvm/llvm-project/pull/188196