Produce warning for performing pointer arithmetic on a null pointer.

Summary:
Test and produce warning for subtracting a pointer from null or subtracting
null from a pointer.  Reuse existing warning that this is undefined
behaviour.  Also add unit test for both warnings.

Reformat to satisfy clang-format.

Respond to review comments:  add additional test.

Respond to review comments:  Do not issue warning for nullptr - nullptr
in C++.

Fix indenting to satisfy clang-format.

Respond to review comments:  Add C++ tests.

Author: Jamie Schmeiser <schmeise@ca.ibm.com>
Reviewed By: efriedma (Eli Friedman), nickdesaulniers (Nick Desaulniers)
Differential Revision: https://reviews.llvm.org/D98798

clang/lib/Sema/SemaExpr.cpp

@@ -10779,7 +10779,17 @@ QualType Sema::CheckSubtractionOperands(ExprResult &LHS, ExprResult &RHS,
                                                LHS.get(), RHS.get()))
         return QualType();
 
-      // FIXME: Add warnings for nullptr - ptr.
+      bool LHSIsNullPtr = LHS.get()->IgnoreParenCasts()->isNullPointerConstant(
+          Context, Expr::NPC_ValueDependentIsNotNull);
+      bool RHSIsNullPtr = RHS.get()->IgnoreParenCasts()->isNullPointerConstant(
+          Context, Expr::NPC_ValueDependentIsNotNull);
+
+      // Subtracting nullptr or from nullptr should produce
+      // a warning expect nullptr - nullptr is valid in C++ [expr.add]p7
+      if (LHSIsNullPtr && (!getLangOpts().CPlusPlus || !RHSIsNullPtr))
+        diagnoseArithmeticOnNullPointer(*this, Loc, LHS.get(), false);
+      if (RHSIsNullPtr && (!getLangOpts().CPlusPlus || !LHSIsNullPtr))
+        diagnoseArithmeticOnNullPointer(*this, Loc, RHS.get(), false);
 
       // The pointee type may have zero size.  As an extension, a structure or
       // union may have zero size or an array may have zero length.  In this

clang/test/Sema/pointer-addition.c

@@ -29,4 +29,7 @@ void a(S* b, void* c) {
   // Cases that don't match the GNU inttoptr idiom get a different warning.
   f = (char*)0 - i; // expected-warning {{performing pointer arithmetic on a null pointer has undefined behavior}}
   int *g = (int*)0 + i; // expected-warning {{performing pointer arithmetic on a null pointer has undefined behavior}}
+  f = (char*)((char*)0 - f); // expected-warning {{performing pointer arithmetic on a null pointer has undefined behavior}}
+  f = (char*)(f - (char*)0); // expected-warning {{performing pointer arithmetic on a null pointer has undefined behavior}}
+  f = (char*)((char*)0 - (char*)0); // expected-warning {{performing pointer arithmetic on a null pointer has undefined behavior}} expected-warning {{performing pointer arithmetic on a null pointer has undefined behavior}}
 }

clang/test/Sema/pointer-addition.cpp

@@ -0,0 +1,8 @@
+// RUN: %clang_cc1 %s -fsyntax-only -verify -pedantic -Wextra -std=c++11
+
+void a() {
+  char *f = (char*)0;
+  f = (char*)((char*)0 - f); // expected-warning {{performing pointer arithmetic on a null pointer has undefined behavior}}
+  f = (char*)(f - (char*)0); // expected-warning {{performing pointer arithmetic on a null pointer has undefined behavior}}
+  f = (char*)((char*)0 - (char*)0); // valid in C++
+}