[SemaTemplate] Fix uncorrected typos after pack expansion

[sammccall]

Summary

This case is particularly important for clangd, as it is triggered after
inserting the snippet for variadic functions.

[kadircet]

clang/lib/Sema/SemaTemplateVariadic.cpp

622

why not directly return CorrectDelayedTyposInExpr(Pattern); ?

[sammccall]

clang/lib/Sema/SemaTemplateVariadic.cpp

622

Because the error isn't that Pattern is invalid per se (in a way that CorrectDelayedTyposInExpr can see), but rather that there's no pack to expand.

So even if CorrectDelayedTyposInExpr returns something valid, we still want to return ExprError(). We call it for its side-effects of flushing typos and emitting diagnostics.

clang/lib/Sema/SemaTemplateVariadic.cpp

ExprResult Sema::CheckPackExpansion(Expr *Pattern, SourceLocation EllipsisLoc,
 
   // C++0x [temp.variadic]p5:
   //   The pattern of a pack expansion shall name one or more
   //   parameter packs that are not expanded by a nested pack
   //   expansion.
   if (!Pattern->containsUnexpandedParameterPack()) {
     Diag(EllipsisLoc, diag::err_pack_expansion_without_parameter_packs)
     << Pattern->getSourceRange();
+    CorrectDelayedTyposInExpr(Pattern);
     return ExprError();
   }
 
   // Create the pack expansion expression and source-location information.
   return new (Context)
     PackExpansionExpr(Context.DependentTy, Pattern, EllipsisLoc, NumExpansions);
 }
 

clang/test/SemaTemplate/typo-variadic.cpp

+// RUN: %clang_cc1 -fsyntax-only %s -verify
+int x = m(s...); // expected-error{{pack expansion does not}} expected-error{{undeclared identifier}}