[clang][Sema] Emit more specific diagnostic for auto in lambda before C++14 (#46059) (#68540)

Namely, we specify that `auto` in a lambda parameter is a C++14
extension in the error message, which now reads:

`'auto' not allowed in lambda parameter before C++14`

This does not change the behavior for `decltype(auto)` and `__auto_type`
though.

---------

Co-authored-by: cor3ntin <[email protected]>

Author: weltschildkroete

clang/docs/ReleaseNotes.rst

@@ -494,6 +494,9 @@ Improvements to Clang's diagnostics
 Improvements to Clang's time-trace
 ----------------------------------
 
+- Clang now specifies that using ``auto`` in a lambda parameter is a C++14 extension when
+  appropriate. (`#46059: <https://github.com/llvm/llvm-project/issues/46059>`_).
+
 Bug Fixes in This Version
 -------------------------
 - Clang's ``-Wundefined-func-template`` no longer warns on pure virtual

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -2433,7 +2433,9 @@ def err_auto_not_allowed : Error<
   "|in template parameter|in friend declaration|in function prototype that is "
   "not a function declaration|in requires expression parameter"
   "|in array declaration"
-  "|in declaration of conversion function template}1">;
+  "|in declaration of conversion function template"
+  "|in lambda parameter before C++14}1">;
+
 def err_dependent_deduced_tst : Error<
   "typename specifier refers to "
   "%select{class template|function template|variable template|alias template|"

clang/lib/Sema/SemaType.cpp

@@ -3257,9 +3257,13 @@ static QualType GetDeclSpecTypeForDeclarator(TypeProcessingState &state,
         Info = &SemaRef.InventedParameterInfos.back();
       } else {
         // In C++14, generic lambdas allow 'auto' in their parameters.
-        if (!SemaRef.getLangOpts().CPlusPlus14 || !Auto ||
-            Auto->getKeyword() != AutoTypeKeyword::Auto) {
-          Error = 16;
+        if (!SemaRef.getLangOpts().CPlusPlus14 && Auto &&
+            Auto->getKeyword() == AutoTypeKeyword::Auto) {
+          Error = 25; // auto not allowed in lambda parameter (before C++14)
+          break;
+        } else if (!Auto || Auto->getKeyword() != AutoTypeKeyword::Auto) {
+          Error = 16; // __auto_type or decltype(auto) not allowed in lambda
+                      // parameter
           break;
         }
         Info = SemaRef.getCurLambda();

clang/test/SemaCXX/auto-cxx0x.cpp

@@ -12,14 +12,14 @@ thread_local auto x; // expected-error {{requires an initializer}}
 void g() {
   [](auto){}(0);
 #if __cplusplus == 201103L
-  // expected-error@-2 {{'auto' not allowed in lambda parameter}}
+  // expected-error@-2 {{'auto' not allowed in lambda parameter before C++14}}
 #endif
 }
 
 void rdar47689465() {
   int x = 0;
   [](auto __attribute__((noderef)) *){}(&x);
 #if __cplusplus == 201103L
-  // expected-error@-2 {{'auto' not allowed in lambda parameter}}
+  // expected-error@-2 {{'auto' not allowed in lambda parameter before C++14}}
 #endif
 }