Don't apply lvalue-to-rvalue conversion in DefaultLValueConversion to

the expression that is passed to it if it has a function type or array
type

lvalue-to-rvalue conversion should only be applied to non-function,
non-array types, but clang was applying the conversion to discarded
value expressions of array types.

rdar://problem/61203170

Differential Revision: https://reviews.llvm.org/D78134

(cherry picked from commit 50a81ea)

Author: ahatanaka

clang/include/clang/Sema/Sema.h

@@ -10412,9 +10412,8 @@ class Sema final {
                                                   bool Diagnose = true);
 
   // DefaultLvalueConversion - performs lvalue-to-rvalue conversion on
-  // the operand.  This is DefaultFunctionArrayLvalueConversion,
-  // except that it assumes the operand isn't of function or array
-  // type.
+  // the operand. This function is a no-op if the operand has a function type
+  // or an array type.
   ExprResult DefaultLvalueConversion(Expr *E);
 
   // DefaultArgumentPromotion (C99 6.5.2.2p6). Used for function calls that

clang/lib/Sema/SemaExpr.cpp

@@ -569,6 +569,10 @@ ExprResult Sema::DefaultLvalueConversion(Expr *E) {
   QualType T = E->getType();
   assert(!T.isNull() && "r-value conversion on typeless expression?");
 
+  // lvalue-to-rvalue conversion cannot be applied to function or array types.
+  if (T->isFunctionType() || T->isArrayType())
+    return E;
+
   // We don't want to throw lvalue-to-rvalue casts on top of
   // expressions of certain types in C++.
   if (getLangOpts().CPlusPlus &&

clang/lib/Sema/SemaStmt.cpp

@@ -367,7 +367,10 @@ void Sema::DiagnoseUnusedExprResult(const Stmt *S) {
     }
   }
 
-  if (E->isGLValue() && E->getType().isVolatileQualified()) {
+  // Tell the user to assign it into a variable to force a volatile load if this
+  // isn't an array.
+  if (E->isGLValue() && E->getType().isVolatileQualified() &&
+      !E->getType()->isArrayType()) {
     Diag(Loc, diag::warn_unused_volatile) << R1 << R2;
     return;
   }

clang/test/CXX/expr/p10-0x.cpp

@@ -44,3 +44,12 @@ void f2(volatile int *x) {
   refcall();
   1 ? refcall() : *x;
 }
+
+// CHECK: define void @_Z2f3v()
+// CHECK-NOT: load
+// CHECK-NOT: memcpy
+
+void f3(void) {
+  volatile char a[10];
+  a;
+}

clang/test/SemaCXX/warn-unused-value-cxx11.cpp

@@ -41,4 +41,13 @@ void j() {
   (void)noexcept(s.g() = 5); // Ok
 }
 
-}
+}
+
+namespace volatile_array {
+void test() {
+  char a[10];
+  volatile char b[10];
+  a; // expected-warning-re {{expression result unused{{$}}}}
+  b; // expected-warning-re {{expression result unused{{$}}}}
+}
+}