[Clang][Sema] Fix Wswitch-default bad warning in template (#76007)

llvm/llvm-project#73077 added -Wswitch-default
diagnostic but it produced false positives in templates. This PR will
address that. llvm/llvm-project#75943

clang/lib/Sema/SemaStmt.cpp

@@ -1271,6 +1271,9 @@ Sema::ActOnFinishSwitchStmt(SourceLocation SwitchLoc, Stmt *Switch,
 
   bool CaseListIsErroneous = false;
 
+  // FIXME: We'd better diagnose missing or duplicate default labels even
+  // in the dependent case. Because default labels themselves are never
+  // dependent.
   for (SwitchCase *SC = SS->getSwitchCaseList(); SC && !HasDependentValue;
        SC = SC->getNextSwitchCase()) {
 
@@ -1327,9 +1330,6 @@ 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.
@@ -1344,6 +1344,7 @@ Sema::ActOnFinishSwitchStmt(SourceLocation SwitchLoc, Stmt *Switch,
       assert(!HasConstantCond ||
              (ConstantCondValue.getBitWidth() == CondWidth &&
               ConstantCondValue.isSigned() == CondIsSigned));
+      Diag(SwitchLoc, diag::warn_switch_default);
     }
     bool ShouldCheckConstantCond = HasConstantCond;
 

clang/test/Sema/switch-default.cpp

@@ -0,0 +1,53 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 -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;
+}
+
+// Warn even completely covered Enum cases(GCC compatibility).
+enum E { A, B };
+enum E check_enum(enum E e) {
+  switch (e) {                // expected-warning {{'switch' missing 'default' label}}
+    case A: break;
+    case B: break;
+  }
+  return e;
+}
+
+template<typename Index>
+int t1(Index i)
+{
+  switch (i) {              // expected-warning {{'switch' missing 'default' label}}
+    case 0: return 0;
+    case 1: return 1;
+  }
+  return 0;
+}
+
+template<typename Index>
+int t2(Index i)
+{
+  switch (i) {            // no-warning
+    case 0: return 0;
+    case 1: return 1;
+    default: return 2;
+  }
+  return 0;
+}
+
+int main() {
+  return t1(1);       // expected-note {{in instantiation of function template specialization 't1<int>' requested here}}
+}
+