[clang] f348d96 - [Clang] Enable -Wunused-template under -Wall (#206123)

[via cfe-commits]

Author: Aditya Medhane
Date: 2026-07-06T22:54:28+03:00
New Revision: f348d964af441ec454e5fc12ab1b599cfc1e7d14

URL: https://github.com/llvm/llvm-project/commit/f348d964af441ec454e5fc12ab1b599cfc1e7d14
DIFF: https://github.com/llvm/llvm-project/commit/f348d964af441ec454e5fc12ab1b599cfc1e7d14.diff

LOG: [Clang] Enable -Wunused-template under -Wall (#206123)

Uncomment `UnusedTemplate` in the `Unused` diagnostic group so
`-Wunused-template` becomes part of `-Wall`.

`-Wunused-template` flags unused function and variable templates with
internal linkage. In a header, such a template gives every translation
unit its own internal-linkage copy, which is a latent ODR violation
(ill-formed, no diagnostic required), so enabling this surfaces a real
class of bugs and keeps the pattern from reappearing.

### Depends on

The in-tree occurrences were cleaned up first in a series of NFC PRs
tracked by #202945. This change only builds clean once those land, so CI
will be red on any area whose cleanup is not yet merged. Remaining
prerequisites:

Approved
- #202969
- #202971
- #202973
- #202974
- #206309
- #206312

Needs to be reviewed 
- #202988
- #206308

Closes #202945

cc @AaronBallman @vgvassilev

Added: 
    

Modified: 
    clang/docs/ReleaseNotes.md
    clang/include/clang/Basic/DiagnosticGroups.td
    clang/test/Misc/warning-wall.c
    clang/test/SemaCXX/warn-func-not-needed.cpp
    clang/test/SemaCXX/warn-variable-not-needed.cpp

Removed: 
    


################################################################################
diff  --git a/clang/docs/ReleaseNotes.md b/clang/docs/ReleaseNotes.md
index 77b6f89e39f83..3ec47eb8b18c3 100644
--- a/clang/docs/ReleaseNotes.md
+++ b/clang/docs/ReleaseNotes.md
@@ -548,6 +548,11 @@ latest release, please see the [Clang Web Site](https://clang.llvm.org) or the
   allowing it to be disabled independently with `-Wno-unused-but-set-global`.
   (#GH148361)
 
+- `-Wunused-template` is now part of `-Wunused` (which is enabled by `-Wall`).
+  It diagnoses unused function and variable templates with internal linkage,
+  which in a header is a latent ODR hazard. It can be disabled with
+  `-Wno-unused-template`. (#GH202945)
+
 - Added `-Wlifetime-safety` to enable lifetime safety analysis,
   a CFG-based intra-procedural analysis that detects use-after-free and related
   temporal safety bugs. See the

diff  --git a/clang/include/clang/Basic/DiagnosticGroups.td b/clang/include/clang/Basic/DiagnosticGroups.td
index 73fb97aeeb302..79583534b9bbd 100644
--- a/clang/include/clang/Basic/DiagnosticGroups.td
+++ b/clang/include/clang/Basic/DiagnosticGroups.td
@@ -1369,7 +1369,7 @@ def Conversion
 def Unused : DiagGroup<"unused",
                        [UnusedArgument, UnusedFunction, UnusedLabel,
                         // UnusedParameter, (matches GCC's behavior)
-                        // UnusedTemplate, (clean-up libc++ before enabling)
+                        UnusedTemplate,
                         // UnusedMemberFunction, (clean-up llvm before enabling)
                         UnusedPrivateField, UnusedLambdaCapture,
                         UnusedLocalTypedef, UnusedValue, UnusedVariable,

diff  --git a/clang/test/Misc/warning-wall.c b/clang/test/Misc/warning-wall.c
index 6d6c4e562400f..ade9cbaace77e 100644
--- a/clang/test/Misc/warning-wall.c
+++ b/clang/test/Misc/warning-wall.c
@@ -73,6 +73,8 @@ CHECK-NEXT:      -Wunused-argument
 CHECK-NEXT:      -Wunused-function
 CHECK-NEXT:        -Wunneeded-internal-declaration
 CHECK-NEXT:      -Wunused-label
+CHECK-NEXT:      -Wunused-template
+CHECK-NEXT:        -Wunneeded-internal-declaration
 CHECK-NEXT:      -Wunused-private-field
 CHECK-NEXT:      -Wunused-lambda-capture
 CHECK-NEXT:      -Wunused-local-typedef

diff  --git a/clang/test/SemaCXX/warn-func-not-needed.cpp b/clang/test/SemaCXX/warn-func-not-needed.cpp
index cb3cae4cd6c76..74438543c43b7 100644
--- a/clang/test/SemaCXX/warn-func-not-needed.cpp
+++ b/clang/test/SemaCXX/warn-func-not-needed.cpp
@@ -10,7 +10,7 @@ void foo() {
 }
 
 namespace test1_template {
-template <typename T> static void f() {}
+template <typename T> static void f() {} // expected-warning {{unused function template}}
 template <> void f<int>() {} // expected-warning {{function 'f<int>' is not needed and will not be emitted}}
 template <typename T>
 void foo() {

diff  --git a/clang/test/SemaCXX/warn-variable-not-needed.cpp b/clang/test/SemaCXX/warn-variable-not-needed.cpp
index 272c8998d15c0..d234e9140e3fd 100644
--- a/clang/test/SemaCXX/warn-variable-not-needed.cpp
+++ b/clang/test/SemaCXX/warn-variable-not-needed.cpp
@@ -4,7 +4,7 @@ namespace test1 {
   static int abc = 42; // expected-warning {{variable 'abc' is not needed and will not be emitted}}
 
   namespace {
-  template <typename T> int abc_template = 0;
+  template <typename T> int abc_template = 0; // expected-warning {{unused variable template}}
   template <> int abc_template<int> = 0; // expected-warning {{variable 'abc_template<int>' is not needed and will not be emitted}}
   }                                      // namespace
   template <typename T>