Fix false positive unsequenced access and modification warning in array subscript expression.

Summary: In the [expr.sub] p1, we can read that for a given E1[E2], E1 is sequenced before E2.

Patch by Mateusz Janek.

Reviewers: rsmith, Rakete1111

Reviewed By: rsmith, Rakete1111

Subscribers: riccibruno, lebedev.ri, Rakete1111, hiraditya, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D50766

3 files changed, 37 insertions, 16 deletions

diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index cd96200b817..a47c01daf0a 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -11908,30 +11908,42 @@ public:
       notePostUse(O, E);
   }
 
-  void VisitBinComma(BinaryOperator *BO) {
-    // C++11 [expr.comma]p1:
-    //   Every value computation and side effect associated with the left
-    //   expression is sequenced before every value computation and side
-    //   effect associated with the right expression.
-    SequenceTree::Seq LHS = Tree.allocate(Region);
-    SequenceTree::Seq RHS = Tree.allocate(Region);
+  void VisitSequencedExpressions(Expr *SequencedBefore, Expr *SequencedAfter) {
+    SequenceTree::Seq BeforeRegion = Tree.allocate(Region);
+    SequenceTree::Seq AfterRegion = Tree.allocate(Region);
     SequenceTree::Seq OldRegion = Region;
 
     {
-      SequencedSubexpression SeqLHS(*this);
-      Region = LHS;
-      Visit(BO->getLHS());
+      SequencedSubexpression SeqBefore(*this);
+      Region = BeforeRegion;
+      Visit(SequencedBefore);
     }
 
-    Region = RHS;
-    Visit(BO->getRHS());
+    Region = AfterRegion;
+    Visit(SequencedAfter);
 
     Region = OldRegion;
 
-    // Forget that LHS and RHS are sequenced. They are both unsequenced
-    // with respect to other stuff.
-    Tree.merge(LHS);
-    Tree.merge(RHS);
+    Tree.merge(BeforeRegion);
+    Tree.merge(AfterRegion);
+  }
+
+  void VisitArraySubscriptExpr(ArraySubscriptExpr *ASE) {
+    // C++17 [expr.sub]p1:
+    //   The expression E1[E2] is identical (by definition) to *((E1)+(E2)). The
+    //   expression E1 is sequenced before the expression E2.
+    if (SemaRef.getLangOpts().CPlusPlus17)
+      VisitSequencedExpressions(ASE->getLHS(), ASE->getRHS());
+    else
+      Base::VisitStmt(ASE);
+  }
+
+  void VisitBinComma(BinaryOperator *BO) {
+    // C++11 [expr.comma]p1:
+    //   Every value computation and side effect associated with the left
+    //   expression is sequenced before every value computation and side
+    //   effect associated with the right expression.
+    VisitSequencedExpressions(BO->getLHS(), BO->getRHS());
   }
 
   void VisitBinAssign(BinaryOperator *BO) {
diff --git a/clang/test/SemaCXX/warn-unsequenced-cxx17.cpp b/clang/test/SemaCXX/warn-unsequenced-cxx17.cpp
new file mode 100644
index 00000000000..3c221fb8d65
--- /dev/null
+++ b/clang/test/SemaCXX/warn-unsequenced-cxx17.cpp
@@ -0,0 +1,8 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++17 -Wno-unused %s
+
+void test() {
+  int xs[10];
+  int *p = xs;
+  // expected-no-diagnostics
+  p[(long long unsigned)(p = 0)]; // ok
+}
diff --git a/clang/test/SemaCXX/warn-unsequenced.cpp b/clang/test/SemaCXX/warn-unsequenced.cpp
index 9e8a5b46c21..6d4468cabfe 100644
--- a/clang/test/SemaCXX/warn-unsequenced.cpp
+++ b/clang/test/SemaCXX/warn-unsequenced.cpp
@@ -81,6 +81,7 @@ void test() {
   int *p = xs;
   a = *(a++, p); // ok
   a = a++ && a; // ok
+  p[(long long unsigned)(p = 0)]; // expected-warning {{unsequenced modification and access to 'p'}}
 
   A *q = &agg1;
   (q = &agg2)->y = q->x; // expected-warning {{unsequenced modification and access to 'q'}}