[clang] [Clang] No longer reject call expression whose type is a not-yet-deduced auto type. (PR #208007)

[via cfe-commits]

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@@ -6828,7 +6828,7 @@ static bool MayBeFunctionType(const ASTContext &Context, const Expr *E) {
       T == Context.BuiltinFnTy || T == Context.OverloadTy ||
       T->isFunctionType() || T->isFunctionReferenceType() ||
       T->isMemberFunctionPointerType() || T->isFunctionPointerType() ||
-      T->isBlockPointerType() || T->isRecordType())
+      T->isBlockPointerType() || T->isRecordType() || T->isUndeducedType())
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Waffl3x wrote:

tl;dr undeduced types aren't leaking out, it's just a quirk of how SubstNonTypeTemplateParmPackExpr is built.

I got curious and investigated this, it looks like SubstNonTypeTemplateParmPackExpr is just always built with the declared type of the constant template parameter.
https://github.com/llvm/llvm-project/blob/2078da43e25a4623cab2d0d60decddf709aaea28/clang/lib/Sema/SemaTemplateInstantiate.cpp#L2235-L2247
And while slightly different now...
https://github.com/llvm/llvm-project/blob/2a8e0e1abbdc558f3a4b43d5a9432c07b5d01387/clang/lib/Sema/SemaTemplateInstantiate.cpp#L2189-L2215
...it doesn't seem to change the outcome.
So it's relying on a quirk of SubstNonTypeTemplateParmPackExpr that won't reasonably change.
https://clang.llvm.org/doxygen/classclang_1_1SubstNonTypeTemplateParmPackExpr.html
> When a pack expansion in the source code contains multiple parameter packs and those parameter packs correspond to different levels of template parameter lists, this node is used to represent a non-type template parameter pack from an outer level,...

https://clang.llvm.org/doxygen/ExprCXX_8cpp_source.html#l01773
```
  setDependence(ExprDependence::TypeValueInstantiation |
                ExprDependence::UnexpandedPack);
```
With that said, it looks like "E->[containsUnexpandedParameterPack](https://clang.llvm.org/doxygen/classclang_1_1Expr.html#a45c0f5a000bc434250cb5338a536e187)()" conveys the intent more accurately, and effectively relies on the same quirk. On the other hand, if TemplateInstantiator::TransformTemplateParmRefExpr gets a QoI change to store the deduced type when every element is the same, using containsUnexpandedParameterPack will suppress valid diagnostics.

https://github.com/llvm/llvm-project/pull/208007