[C2X] N3007 Type inference for object definitions

This patches implements the auto keyword from the N3007 standard
specification.
This allows deducing the type of the variable like in C++:
```
auto nb = 1;
auto chr = 'A';
auto str = "String";
```
The list of statements which allows the usage of auto:

    * Basic variables declarations (int, float, double, char, char*...)
    * Macros declaring a variable with the auto type

The list of statements which will not work with the auto keyword:

    * auto arrays
    * sizeof(), alignas()
    * auto parameters, auto return type
    * auto as a struct/typedef member
    * uninitialized auto variables
    * auto in an union
    * auto as a enum type specifier
    * auto casts
    * auto in an compound literals

Differential Revision: https://reviews.llvm.org/D133289

1 files changed, 3 insertions, 2 deletions

diff --git a/clang/lib/Sema/DeclSpec.cpp b/clang/lib/Sema/DeclSpec.cpp
index 2d0d575..781f24c 100644
--- a/clang/lib/Sema/DeclSpec.cpp
+++ b/clang/lib/Sema/DeclSpec.cpp
@@ -1375,8 +1375,9 @@ void DeclSpec::Finish(Sema &S, const PrintingPolicy &Policy) {
     StorageClassSpecLoc = SourceLocation();
   }
   // Diagnose if we've recovered from an ill-formed 'auto' storage class
-  // specifier in a pre-C++11 dialect of C++.
-  if (!S.getLangOpts().CPlusPlus11 && TypeSpecType == TST_auto)
+  // specifier in a pre-C++11 dialect of C++ or in a pre-C23 dialect of C.
+  if (!S.getLangOpts().CPlusPlus11 && !S.getLangOpts().C23 &&
+      TypeSpecType == TST_auto)
     S.Diag(TSTLoc, diag::ext_auto_type_specifier);
   if (S.getLangOpts().CPlusPlus && !S.getLangOpts().CPlusPlus11 &&
       StorageClassSpec == SCS_auto)