[clang] [Clang] Make `-Wreturn-type` default to an error in all language modes (PR #123470)

[via cfe-commits]

https://github.com/Sirraide created https://github.com/llvm/llvm-project/pull/123470

This makes `-Wreturn-type` default to an error in all language modes as there is basically never a reason as to why one would *want* to fall off the end of a non-void function.

Most of the tedious work of migrating the tests was done in #123464 (except for one test that for some reason didn’t want to cooperate earlier, which is why that one is handled in this pr).

>From 441385e14210f23a4fe5368d44620073a4347a31 Mon Sep 17 00:00:00 2001
From: Sirraide <aeternalmail at gmail.com>
Date: Sat, 18 Jan 2025 19:35:01 +0100
Subject: [PATCH] [Clang] Make -Wreturn-type default to an error in all
 language modes

---
 clang/docs/ReleaseNotes.rst                      |  4 ++++
 clang/include/clang/Basic/DiagnosticSemaKinds.td | 12 ++++++------
 clang/test/Index/warning-flags.c                 |  2 +-
 3 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index aa1c02d04f7caa..a086c443984408 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -81,6 +81,10 @@ code bases.
   ``-fno-strict-overflow`` to opt-in to a language dialect where signed integer
   and pointer overflow are well-defined.
 
+- ``-Wreturn-type`` now defaults to an error in all language modes; like other
+  warnings, it can be reverted to just being a warning by specifying
+  ``-Wno-error=return-type``.
+
 C/C++ Language Potentially Breaking Changes
 -------------------------------------------
 
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index db54312ad965e8..b70d152781a2a8 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -709,10 +709,10 @@ def err_thread_unsupported : Error<
 // FIXME: Combine fallout warnings to just one warning.
 def warn_maybe_falloff_nonvoid_function : Warning<
   "non-void function does not return a value in all control paths">,
-  InGroup<ReturnType>;
+  InGroup<ReturnType>, DefaultError;
 def warn_falloff_nonvoid_function : Warning<
   "non-void function does not return a value">,
-  InGroup<ReturnType>;
+  InGroup<ReturnType>, DefaultError;
 def warn_const_attr_with_pure_attr : Warning<
   "'const' attribute imposes more restrictions; 'pure' attribute ignored">,
   InGroup<IgnoredAttributes>;
@@ -726,10 +726,10 @@ def err_falloff_nonvoid_block : Error<
   "non-void block does not return a value">;
 def warn_maybe_falloff_nonvoid_coroutine : Warning<
   "non-void coroutine does not return a value in all control paths">,
-  InGroup<ReturnType>;
+  InGroup<ReturnType>, DefaultError;
 def warn_falloff_nonvoid_coroutine : Warning<
   "non-void coroutine does not return a value">,
-  InGroup<ReturnType>;
+  InGroup<ReturnType>, DefaultError;
 def warn_suggest_noreturn_function : Warning<
   "%select{function|method}0 %1 could be declared with attribute 'noreturn'">,
   InGroup<MissingNoreturn>, DefaultIgnore;
@@ -8365,10 +8365,10 @@ let CategoryName = "Lambda Issue" in {
     "lambda declared 'noreturn' should not return">;
   def warn_maybe_falloff_nonvoid_lambda : Warning<
     "non-void lambda does not return a value in all control paths">,
-    InGroup<ReturnType>;
+    InGroup<ReturnType>, DefaultError;
   def warn_falloff_nonvoid_lambda : Warning<
     "non-void lambda does not return a value">,
-    InGroup<ReturnType>;
+    InGroup<ReturnType>, DefaultError;
   def err_access_lambda_capture : Error<
     // The ERRORs represent other special members that aren't constructors, in
     // hopes that someone will bother noticing and reporting if they appear
diff --git a/clang/test/Index/warning-flags.c b/clang/test/Index/warning-flags.c
index 1694c6abab5624..3229f000c4ae0c 100644
--- a/clang/test/Index/warning-flags.c
+++ b/clang/test/Index/warning-flags.c
@@ -9,7 +9,7 @@ int *bar(float *f) { return f; }
 // RUN: c-index-test -test-load-source-reparse 5 all -w %s 2>&1 | FileCheck -check-prefix=NOWARNINGS %s
 // RUN: c-index-test -test-load-source all -w -O4 %s 2>&1 | FileCheck -check-prefix=NOWARNINGS %s
 
-// CHECK-BOTH-WARNINGS: warning: non-void function does not return a value
+// CHECK-BOTH-WARNINGS: error: non-void function does not return a value
 // CHECK-BOTH-WARNINGS: warning: incompatible pointer types returning 'float *' from a function with result type 'int *'
 
 // CHECK-SECOND-WARNING-NOT:non-void function does not return a value