Remove warnings from -Wchar-subscripts for known positive constants (#69061)

Fixes #18763

Remove warnings when using a signed char as an array bound if the char is a known positive constant.

This goes one step farther than gcc does.

For example given the following code
```c++
char upper[300];

int main() {
  upper['a'] = 'A';
  char b = 'a';
  upper[b] = 'A';
  const char c = 'a';
  upper[c] = 'A';
  constexpr char d = 'a';
  upper[d] = 'A';
  char e = -1;
  upper[e] = 'A';
  const char f = -1;
  upper[f] = 'A';
  constexpr char g = -1;
  upper[g] = 'A';
  return 1;
}
```

clang currently gives warnings for all cases, while gcc gives warnings
for all cases except for 'a' (https://godbolt.org/z/5ahjETTv3)

With the change there is no longer any warning for 'a', 'c', or 'd'.

Author: wheatman

clang/docs/ReleaseNotes.rst

@@ -651,6 +651,8 @@ Bug Fixes in This Version
 - Fixed false positive error emitted by clang when performing qualified name
   lookup and the current class instantiation has dependent bases.
   Fixes (`#13826 <https://github.com/llvm/llvm-project/issues/13826>`_)
+- Clang's ``-Wchar-subscripts`` no longer warns on chars whose values are known non-negative constants.
+  Fixes (`#18763 <https://github.com/llvm/llvm-project/issues/18763>`_)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaExpr.cpp

@@ -6053,9 +6053,14 @@ Sema::CreateBuiltinArraySubscriptExpr(Expr *Base, SourceLocation LLoc,
                      << IndexExpr->getSourceRange());
 
   if ((IndexExpr->getType()->isSpecificBuiltinType(BuiltinType::Char_S) ||
-       IndexExpr->getType()->isSpecificBuiltinType(BuiltinType::Char_U))
-         && !IndexExpr->isTypeDependent())
-    Diag(LLoc, diag::warn_subscript_is_char) << IndexExpr->getSourceRange();
+       IndexExpr->getType()->isSpecificBuiltinType(BuiltinType::Char_U)) &&
+      !IndexExpr->isTypeDependent()) {
+    std::optional<llvm::APSInt> IntegerContantExpr =
+        IndexExpr->getIntegerConstantExpr(getASTContext());
+    if (!IntegerContantExpr.has_value() ||
+        IntegerContantExpr.value().isNegative())
+      Diag(LLoc, diag::warn_subscript_is_char) << IndexExpr->getSourceRange();
+  }
 
   // C99 6.5.2.1p1: "shall have type "pointer to *object* type". Similarly,
   // C++ [expr.sub]p1: The type "T" shall be a completely-defined object

clang/test/Sema/warn-char-subscripts.c

@@ -62,3 +62,28 @@ void t10(void) {
   UnsignedCharTy subscript = 0;
   int val = array[subscript]; // no warning for unsigned char
 }
+
+void t11(void) {
+  int array[256] = { 0 };
+  int val = array['a']; // no warning for char with known positive value
+}
+
+void t12(void) {
+  int array[256] = { 0 };
+  char b = 'a';
+  int val = array[b]; // expected-warning{{array subscript is of type 'char'}}
+}
+
+void t13(void) {
+  int array[256] = { 0 };
+  const char b = 'a';
+  int val = array[b]; // expected-warning{{array subscript is of type 'char'}}
+}
+
+void t14(void) {
+  int array[256] = { 0 }; // expected-note {{array 'array' declared here}}
+  const char b = -1;
+  // expected-warning@+2 {{array subscript is of type 'char'}}
+  // expected-warning@+1 {{array index -1 is before the beginning of the array}}
+  int val = array[b];
+}

clang/test/Sema/warn-char-subscripts.cpp

@@ -0,0 +1,103 @@
+// RUN: %clang_cc1 -Wchar-subscripts -fsyntax-only -verify %s
+
+void t1(void) {
+  int array[1] = { 0 };
+  char subscript = 0;
+  int val = array[subscript]; // expected-warning{{array subscript is of type 'char'}}
+}
+
+void t2(void) {
+  int array[1] = { 0 };
+  char subscript = 0;
+  int val = subscript[array]; // expected-warning{{array subscript is of type 'char'}}
+}
+
+void t3(void) {
+  int *array = 0;
+  char subscript = 0;
+  int val = array[subscript]; // expected-warning{{array subscript is of type 'char'}}
+}
+
+void t4(void) {
+  int *array = 0;
+  char subscript = 0;
+  int val = subscript[array]; // expected-warning{{array subscript is of type 'char'}}
+}
+
+char returnsChar(void);
+void t5(void) {
+  int *array = 0;
+  int val = array[returnsChar()]; // expected-warning{{array subscript is of type 'char'}}
+}
+
+void t6(void) {
+  int array[1] = { 0 };
+  signed char subscript = 0;
+  int val = array[subscript]; // no warning for explicit signed char
+}
+
+void t7(void) {
+  int array[1] = { 0 };
+  unsigned char subscript = 0;
+  int val = array[subscript]; // no warning for unsigned char
+}
+
+typedef char CharTy;
+void t8(void) {
+  int array[1] = { 0 };
+  CharTy subscript = 0;
+  int val = array[subscript]; // expected-warning{{array subscript is of type 'char'}}
+}
+
+typedef signed char SignedCharTy;
+void t9(void) {
+  int array[1] = { 0 };
+  SignedCharTy subscript = 0;
+  int val = array[subscript]; // no warning for explicit signed char
+}
+
+typedef unsigned char UnsignedCharTy;
+void t10(void) {
+  int array[1] = { 0 };
+  UnsignedCharTy subscript = 0;
+  int val = array[subscript]; // no warning for unsigned char
+}
+
+void t11(void) {
+  int array[256] = { 0 };
+  int val = array['a']; // no warning for char with known positive value
+}
+
+void t12(void) {
+  int array[256] = { 0 };
+  char b = 'a';
+  int val = array[b]; // expected-warning{{array subscript is of type 'char'}}
+}
+
+void t13(void) {
+  int array[256] = { 0 };
+  const char b = 'a';
+  int val = array[b]; // no warning for char with known positive value
+}
+
+void t14(void) {
+  int array[256] = { 0 };
+  constexpr char b = 'a';
+  int val = array[b]; // no warning for char with known positive value
+}
+
+void t15(void) {
+  int array[256] = { 0 }; // expected-note {{array 'array' declared here}}
+  const char b = -1;
+  // expected-warning@+2 {{array subscript is of type 'char'}}
+  // expected-warning@+1 {{array index -1 is before the beginning of the array}}
+  int val = array[b];
+}
+
+void t16(void) {
+  int array[256] = { 0 }; // expected-note {{array 'array' declared here}}
+  constexpr char b = -1;
+  // expected-warning@+2 {{array subscript is of type 'char'}}
+  // expected-warning@+1 {{array index -1 is before the beginning of the array}}
+  int val = array[b];
+}