[clang][Sema] Add -Wswitch-default warning option (llvm#73077)

Adds a warning, issued by the clang semantic analysis. The patch warns
on switch which don't have the default branch.

This is a counterpart of gcc's Wswitch-default.

clang/docs/ReleaseNotes.rst

@@ -353,6 +353,8 @@ Improvements to Clang's diagnostics
   of a base class is not called in the constructor of its derived class.
 - Clang no longer emits ``-Wmissing-variable-declarations`` for variables declared
   with the ``register`` storage class.
+- Clang's ``-Wswitch-default`` flag now diagnoses whenever a ``switch`` statement
+  does not have a ``default`` label.
 - Clang's ``-Wtautological-negation-compare`` flag now diagnoses logical
   tautologies like ``x && !x`` and ``!x || x`` in expressions. This also
   makes ``-Winfinite-recursion`` diagnose more cases.

clang/include/clang/Basic/DiagnosticGroups.td

@@ -632,7 +632,7 @@ def ShadowAll : DiagGroup<"shadow-all", [Shadow, ShadowFieldInConstructor,
 def Shorten64To32 : DiagGroup<"shorten-64-to-32">;
 def : DiagGroup<"sign-promo">;
 def SignCompare : DiagGroup<"sign-compare">;
-def : DiagGroup<"switch-default">;
+def SwitchDefault  : DiagGroup<"switch-default">;
 def : DiagGroup<"synth">;
 def SizeofArrayArgument : DiagGroup<"sizeof-array-argument">;
 def SizeofArrayDecay : DiagGroup<"sizeof-array-decay">;

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -10064,6 +10064,8 @@ def warn_missing_case : Warning<"%plural{"
   "3:enumeration values %1, %2, and %3 not handled in switch|"
   ":%0 enumeration values not handled in switch: %1, %2, %3...}0">,
   InGroup<Switch>;
+def warn_switch_default : Warning<"'switch' missing 'default' label">,
+  InGroup<SwitchDefault>, DefaultIgnore;
 
 def warn_unannotated_fallthrough : Warning<
   "unannotated fall-through between switch labels">,

clang/lib/Sema/SemaStmt.cpp

@@ -1327,6 +1327,9 @@ Sema::ActOnFinishSwitchStmt(SourceLocation SwitchLoc, Stmt *Switch,
     }
   }
 
+  if (!TheDefaultStmt)
+    Diag(SwitchLoc, diag::warn_switch_default);
+
   if (!HasDependentValue) {
     // If we don't have a default statement, check whether the
     // condition is constant.

clang/test/Sema/switch-default.c

@@ -0,0 +1,17 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -Wswitch-default %s
+
+int f1(int a) {
+  switch (a) {                // expected-warning {{'switch' missing 'default' label}}
+    case 1: a++; break;
+    case 2: a += 2; break;
+  }
+  return a;
+}
+
+int f2(int a) {
+  switch (a) {                // no-warning
+    default:
+      ;
+  }
+  return a;
+}