[clang] Expose unreachable fallthrough annotation warning

The Linux kernel has a macro called IS_ENABLED(), which evaluates to a
constant 1 or 0 based on Kconfig selections, allowing C code to be
unconditionally enabled or disabled at build time. For example:

int foo(struct *a, int b) {
    switch (b) {
    case 1:
        if (a->flag || !IS_ENABLED(CONFIG_64BIT))
            return 1;
        __attribute__((fallthrough));
    case 2:
        return 2;
    default:
        return 3;
    }
}

There is an unreachable warning about the fallthrough annotation in the
first case because !IS_ENABLED(CONFIG_64BIT) can be evaluated to 1,
which looks like

        return 1;
        __attribute__((fallthrough));

to clang.

This type of warning is pointless for the Linux kernel because it does
this trick all over the place due to the sheer number of configuration
options that it has.

Add -Wunreachable-code-fallthrough, enabled under -Wunreachable-code, so
that projects that want to warn on unreachable code get this warning but
projects that do not care about unreachable code can still use
-Wimplicit-fallthrough without having to make changes to their code
base.

Fixes PR51094.

Reviewed By: aaron.ballman, nickdesaulniers

Differential Revision: https://reviews.llvm.org/D107933

clang/include/clang/Basic/DiagnosticGroups.td

@@ -821,8 +821,10 @@ def ReservedIdentifier : DiagGroup<"reserved-identifier",
 //  under separate flags.
 //
 def UnreachableCodeLoopIncrement : DiagGroup<"unreachable-code-loop-increment">;
+def UnreachableCodeFallthrough : DiagGroup<"unreachable-code-fallthrough">;
 def UnreachableCode : DiagGroup<"unreachable-code",
-                                [UnreachableCodeLoopIncrement]>;
+                                [UnreachableCodeLoopIncrement,
+                                 UnreachableCodeFallthrough]>;
 def UnreachableCodeBreak : DiagGroup<"unreachable-code-break">;
 def UnreachableCodeReturn : DiagGroup<"unreachable-code-return">;
 def UnreachableCodeAggressive : DiagGroup<"unreachable-code-aggressive",

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -682,6 +682,9 @@ def warn_unreachable_return : Warning<
 def warn_unreachable_loop_increment : Warning<
   "loop will run at most once (loop increment never executed)">,
   InGroup<UnreachableCodeLoopIncrement>, DefaultIgnore;
+def warn_unreachable_fallthrough_attr : Warning<
+  "fallthrough annotation in unreachable code">,
+  InGroup<UnreachableCodeFallthrough>, DefaultIgnore;
 def note_unreachable_silence : Note<
   "silence by adding parentheses to mark code as explicitly dead">;
 
@@ -9578,9 +9581,6 @@ def err_fallthrough_attr_outside_switch : Error<
   "fallthrough annotation is outside switch statement">;
 def err_fallthrough_attr_invalid_placement : Error<
   "fallthrough annotation does not directly precede switch label">;
-def warn_fallthrough_attr_unreachable : Warning<
-  "fallthrough annotation in unreachable code">,
-  InGroup<ImplicitFallthrough>, DefaultIgnore;
 
 def warn_unreachable_default : Warning<
   "default label in switch which covers all enumeration values">,

clang/lib/Sema/AnalysisBasedWarnings.cpp

@@ -1125,7 +1125,7 @@ namespace {
                 // unreachable in all instantiations of the template.
                 if (!IsTemplateInstantiation)
                   S.Diag(AS->getBeginLoc(),
-                         diag::warn_fallthrough_attr_unreachable);
+                         diag::warn_unreachable_fallthrough_attr);
                 markFallthroughVisited(AS);
                 ++AnnotatedCnt;
                 break;

clang/test/SemaCXX/P30636.cpp

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 -Wimplicit-fallthrough %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 -Wimplicit-fallthrough -Wunreachable-code-fallthrough %s
 // expected-no-diagnostics
 
 template<bool param>

clang/test/SemaCXX/switch-implicit-fallthrough.cpp

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 -Wimplicit-fallthrough %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 -Wimplicit-fallthrough -Wunreachable-code-fallthrough %s
 
 
 int fallthrough(int n) {
@@ -193,6 +193,26 @@ int fallthrough_position(int n) {
         ;
   }
 
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wunreachable-code-fallthrough"
+  switch (n) {
+      n += 300;
+      [[clang::fallthrough]];  // no warning here
+    case 221:
+      return 1;
+      [[clang::fallthrough]];  // no warning here
+    case 222:
+      return 2;
+      __attribute__((fallthrough)); // no warning here
+    case 223:
+      if (1)
+        return 3;
+      __attribute__((fallthrough)); // no warning here
+    case 224:
+      n += 400;
+  }
+#pragma clang diagnostic pop
+
   long p = static_cast<long>(n) * n;
   switch (sizeof(p)) {
     case 9: