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

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https://github.com/weltschildkroete updated https://github.com/llvm/llvm-project/pull/68540

>From 7e62fe6179dc63e9930a77c6996ae651d4489aa2 Mon Sep 17 00:00:00 2001
From: Leonardo Duarte <weltschildkroete at gmail.com>
Date: Sun, 8 Oct 2023 12:59:15 +0200
Subject: [PATCH] [clang][Sema] Emit more specific diagnostic for auto in
 lambda before C++14 (#46059)

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 until C++14`

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

The relevant change to `SemaType.cpp` is the addition of a branch that
sets `Error = 24`, whilst the bulk of the change comes from formatting.
---
 clang/include/clang/Basic/DiagnosticSemaKinds.td | 2 +-
 clang/lib/Sema/SemaType.cpp                      | 7 +++++--
 clang/test/SemaCXX/auto-cxx0x.cpp                | 4 ++--
 3 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index b211680a0e9b6e9..804c69c222ec0d0 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -2393,7 +2393,7 @@ def err_auto_not_allowed : Error<
   "|in type allocated by 'new'|in K&R-style function parameter"
   "|in template parameter|in friend declaration|in function prototype that is "
   "not a function declaration|in requires expression parameter"
-  "|in array declaration}1">;
+  "|in array declaration|in lambda parameter until C++14}1">;
 def err_dependent_deduced_tst : Error<
   "typename specifier refers to "
   "%select{class template|function template|variable template|alias template|"
diff --git a/clang/lib/Sema/SemaType.cpp b/clang/lib/Sema/SemaType.cpp
index 068971f8130a4aa..52a8161797e15e1 100644
--- a/clang/lib/Sema/SemaType.cpp
+++ b/clang/lib/Sema/SemaType.cpp
@@ -3605,8 +3605,11 @@ 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) {
+        if (!SemaRef.getLangOpts().CPlusPlus14 && Auto &&
+            Auto->getKeyword() == AutoTypeKeyword::Auto) {
+          Error = 24;
+          break;
+        } else if (!Auto || Auto->getKeyword() != AutoTypeKeyword::Auto) {
           Error = 16;
           break;
         }
diff --git a/clang/test/SemaCXX/auto-cxx0x.cpp b/clang/test/SemaCXX/auto-cxx0x.cpp
index b4da3f9330c1045..65398de28e10cfb 100644
--- a/clang/test/SemaCXX/auto-cxx0x.cpp
+++ b/clang/test/SemaCXX/auto-cxx0x.cpp
@@ -12,7 +12,7 @@ thread_local auto x; // expected-error {{requires an initializer}}
 void g() {
   [](auto){}(0);
 #if __cplusplus == 201103L
-  // expected-error at -2 {{'auto' not allowed in lambda parameter}}
+  // expected-error at -2 {{'auto' not allowed in lambda parameter until C++14}}
 #endif
 }
 
@@ -20,6 +20,6 @@ void rdar47689465() {
   int x = 0;
   [](auto __attribute__((noderef)) *){}(&x);
 #if __cplusplus == 201103L
-  // expected-error at -2 {{'auto' not allowed in lambda parameter}}
+  // expected-error at -2 {{'auto' not allowed in lambda parameter until C++14}}
 #endif
 }