Fix assertion failure when constant evaluation of a switch jumps over an

uninitialized variable in an init-statement of a 'for' or 'if'.

llvm-svn: 372437

Author: zygoloid

clang/lib/AST/ExprConstant.cpp

@@ -4131,6 +4131,21 @@ static EvalStmtResult EvaluateStmt(StmtResult &Result, EvalInfo &Info,
       // preceded by our switch label.
       BlockScopeRAII Scope(Info);
 
+      // Step into the init statement in case it brings an (uninitialized)
+      // variable into scope.
+      if (const Stmt *Init = IS->getInit()) {
+        EvalStmtResult ESR = EvaluateStmt(Result, Info, Init, Case);
+        if (ESR != ESR_CaseNotFound) {
+          assert(ESR != ESR_Succeeded);
+          return ESR;
+        }
+      }
+
+      // Condition variable must be initialized if it exists.
+      // FIXME: We can skip evaluating the body if there's a condition
+      // variable, as there can't be any case labels within it.
+      // (The same is true for 'for' statements.)
+
       EvalStmtResult ESR = EvaluateStmt(Result, Info, IS->getThen(), Case);
       if (ESR != ESR_CaseNotFound || !IS->getElse())
         return ESR;
@@ -4147,6 +4162,18 @@ static EvalStmtResult EvaluateStmt(StmtResult &Result, EvalInfo &Info,
 
     case Stmt::ForStmtClass: {
       const ForStmt *FS = cast<ForStmt>(S);
+      BlockScopeRAII Scope(Info);
+
+      // Step into the init statement in case it brings an (uninitialized)
+      // variable into scope.
+      if (const Stmt *Init = FS->getInit()) {
+        EvalStmtResult ESR = EvaluateStmt(Result, Info, Init, Case);
+        if (ESR != ESR_CaseNotFound) {
+          assert(ESR != ESR_Succeeded);
+          return ESR;
+        }
+      }
+
       EvalStmtResult ESR =
           EvaluateLoopBody(Result, Info, FS->getBody(), Case);
       if (ESR != ESR_Continue)

clang/test/SemaCXX/constant-expression-cxx2a.cpp

@@ -620,4 +620,17 @@ namespace Uninit {
   constexpr int s1 = switch_var(1);
   constexpr int s2 = switch_var(2);
   static_assert(s1 == 1 && s2 == 2);
+
+  constexpr bool switch_into_init_stmt() {
+    switch (1) {
+      if (int n; false) {
+        for (int m; false;) {
+        case 1:
+          n = m = 1;
+          return n == 1 && m == 1;
+        }
+      }
+    }
+  }
+  static_assert(switch_into_init_stmt());
 }