Further tweak -Wurneachable-code and templates by allowing the warning to run on

explicit template specializations (which represent actual functions somebody wrote).

Along the way, refactor some other code which similarly cares about whether or
not they are looking at a template instantiation.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@145547 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/AST/Decl.h

@@ -1909,6 +1909,10 @@ public:
   /// be implicitly instantiated.
   bool isImplicitlyInstantiable() const;
 
+  /// \brief Determines if the given function was instantiated from a
+  /// function template.
+  bool isTemplateInstantiation() const;
+
   /// \brief Retrieve the function declaration from which this function could
   /// be instantiated, if it is an instantiation (rather than a non-template
   /// or a specialization, for example).

lib/AST/Decl.cpp

@@ -1951,7 +1951,20 @@ bool FunctionDecl::isImplicitlyInstantiable() const {
     return true;
 
   return PatternDecl->isInlined();
-}                      
+}
+
+bool FunctionDecl::isTemplateInstantiation() const {
+  switch (getTemplateSpecializationKind()) {
+    case TSK_Undeclared:
+    case TSK_ExplicitSpecialization:
+      return false;      
+    case TSK_ImplicitInstantiation:
+    case TSK_ExplicitInstantiationDeclaration:
+    case TSK_ExplicitInstantiationDefinition:
+      return true;
+  }
+  llvm_unreachable("All TSK values handled.");
+}
    
 FunctionDecl *FunctionDecl::getTemplateInstantiationPattern() const {
   // Handle class scope explicit specialization special case.

lib/Sema/AnalysisBasedWarnings.cpp

@@ -241,19 +241,8 @@ struct CheckFallThroughDiagnostics {
     
     // Don't suggest that template instantiations be marked "noreturn"
     bool isTemplateInstantiation = false;
-    if (const FunctionDecl *Function = dyn_cast<FunctionDecl>(Func)) {
-      switch (Function->getTemplateSpecializationKind()) {
-      case TSK_Undeclared:
-      case TSK_ExplicitSpecialization:
-        break;
-        
-      case TSK_ImplicitInstantiation:
-      case TSK_ExplicitInstantiationDeclaration:
-      case TSK_ExplicitInstantiationDefinition:
-        isTemplateInstantiation = true;
-        break;
-      }
-    }
+    if (const FunctionDecl *Function = dyn_cast<FunctionDecl>(Func))
+      isTemplateInstantiation = Function->isTemplateInstantiation();
         
     if (!isVirtualMethod && !isTemplateInstantiation)
       D.diag_NeverFallThroughOrReturn =
@@ -919,7 +908,10 @@ AnalysisBasedWarnings::IssueWarnings(sema::AnalysisBasedWarnings::Policy P,
     // Different template instantiations can effectively change the control-flow
     // and it is very difficult to prove that a snippet of code in a template
     // is unreachable for all instantiations.
-    if (S.ActiveTemplateInstantiations.empty())
+    bool isTemplateInstantiation = false;
+    if (const FunctionDecl *Function = dyn_cast<FunctionDecl>(D))
+      isTemplateInstantiation = Function->isTemplateInstantiation();
+    if (!isTemplateInstantiation)
       CheckUnreachable(S, AC);
   }
 

lib/StaticAnalyzer/Checkers/UnreachableCodeChecker.cpp

@@ -54,10 +54,11 @@ void UnreachableCodeChecker::checkEndAnalysis(ExplodedGraph &G,
                                               BugReporter &B,
                                               ExprEngine &Eng) const {
   CFGBlocksSet reachable, visited;
-
+  
   if (Eng.hasWorkRemaining())
     return;
 
+  const Decl *D = 0;
   CFG *C = 0;
   ParentMap *PM = 0;
   const LocationContext *LC = 0;
@@ -67,6 +68,8 @@ void UnreachableCodeChecker::checkEndAnalysis(ExplodedGraph &G,
     const ProgramPoint &P = I->getLocation();
     LC = P.getLocationContext();
 
+    if (!D)
+      D = LC->getAnalysisDeclContext()->getDecl();
     // Save the CFG if we don't have it already
     if (!C)
       C = LC->getAnalysisDeclContext()->getUnoptimizedCFG();
@@ -80,8 +83,15 @@ void UnreachableCodeChecker::checkEndAnalysis(ExplodedGraph &G,
   }
 
   // Bail out if we didn't get the CFG or the ParentMap.
-  if (!C || !PM)
+  if (!D || !C || !PM)
     return;
+  
+  // Don't do anything for template instantiations.  Proving that code
+  // in a template instantiation is unreachable means proving that it is
+  // unreachable in all instantiations.
+  if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(D))
+    if (FD->isTemplateInstantiation())
+      return;
 
   // Find CFGBlocks that were not covered by any node
   for (CFG::const_iterator I = C->begin(), E = C->end(); I != E; ++I) {

test/SemaCXX/warn-unreachable.cpp

@@ -98,3 +98,12 @@ void test_unreachable_templates_harness() {
   test_unreachable_templates<TestUnreachableB>(); 
 }
 
+// Do warn about explict template specializations, as they represent
+// actual concrete functions that somebody wrote.
+
+template <typename T> void funcToSpecialize() {}
+template <> void funcToSpecialize<int>() {
+  halt();
+  dead(); // expected-warning {{will never be executed}}
+}
+