cppcoreguidelines-pro-bounds-constant-array-index: ignore implicit constructor

Summary:
The code

  struct A {
    int x[3];
  };

gets an compiler-generated copy constructor that uses ArraySubscriptExpr (see below).
Previously, the check would generate a warning on that copy constructor.
This commit disables the warning on implicitly generated code.
AST:

  |-CXXConstructorDecl 0x337b3c8 <col:8> col:8 implicit used constexpr A 'void (const struct A &) noexcept' inline
  | |-ParmVarDecl 0x337b510 <col:8> col:8 used 'const struct A &'
  | |-CXXCtorInitializer Field 0x3379238 'x' 'int [3]'
  | | `-ImplicitCastExpr 0x337e158 <col:8> 'int' <LValueToRValue>
  | |   `-ArraySubscriptExpr 0x337e130 <col:8> 'const int' lvalue
  | |     |-ImplicitCastExpr 0x337e118 <col:8> 'const int *' <ArrayToPointerDecay>
  | |     | `-MemberExpr 0x337dfc8 <col:8> 'int const[3]' lvalue .x 0x3379238
  | |     |   `-DeclRefExpr 0x337dfa0 <col:8> 'const struct A' lvalue ParmVar 0x337b510 '' 'const struct A &'
  | |     `-ImplicitCastExpr 0x337e098 <col:8> 'unsigned long' <LValueToRValue>
  | |       `-DeclRefExpr 0x337e070 <col:8> 'unsigned long' lvalue Var 0x337e010 '__i0' 'unsigned long'

Reviewers: alexfh, aaron.ballman

Subscribers: aemerson, nemanjai, cfe-commits

Differential Revision: https://reviews.llvm.org/D22381

llvm-svn: 275993

clang-tools-extra/clang-tidy/cppcoreguidelines/ProBoundsConstantArrayIndexCheck.cpp

@@ -45,9 +45,12 @@ void ProBoundsConstantArrayIndexCheck::registerMatchers(MatchFinder *Finder) {
   if (!getLangOpts().CPlusPlus)
     return;
 
+  // Note: if a struct contains an array member, the compiler-generated
+  // constructor has an arraySubscriptExpr.
   Finder->addMatcher(arraySubscriptExpr(hasBase(ignoringImpCasts(hasType(
                                             constantArrayType().bind("type")))),
-                                        hasIndex(expr().bind("index")))
+                                        hasIndex(expr().bind("index")),
+                                        unless(hasAncestor(isImplicit())))
                          .bind("expr"),
                      this);
 

clang-tools-extra/test/clang-tidy/cppcoreguidelines-pro-bounds-constant-array-index.cpp

@@ -74,3 +74,14 @@ void customOperator() {
   int i = 0;
   s[i] = 3; // OK, custom operator
 }
+
+struct A {
+  // The compiler-generated copy constructor uses an ArraySubscriptExpr. Don't warn.
+  int x[3];
+};
+
+void use_A() {
+  // Force the compiler to generate a copy constructor.
+  A a;
+  A a2(a);
+}