[clang] [clang] implement current direction of CWG2765 for string literal comparisons in constant evaluation (PR #109208)

[Richard Smith via cfe-commits]

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@@ -108,22 +108,16 @@ constexpr auto p2 = "test2";
 constexpr bool b1 = foo(p1) == foo(p1);
 static_assert(b1);
 
-constexpr bool b2 = foo(p1) == foo(p2); // ref-error {{must be initialized by a constant expression}} \
-                                        // ref-note {{comparison of addresses of literals}} \
-                                        // ref-note {{declared here}}
-static_assert(!b2); // ref-error {{not an integral constant expression}} \
-                    // ref-note {{not a constant expression}}
+constexpr bool b2 = foo(p1) == foo(p2);
+static_assert(!b2);
 
 constexpr auto name1() { return "name1"; }
 constexpr auto name2() { return "name2"; }
 
-constexpr auto b3 = name1() == name1();
-static_assert(b3);
-constexpr auto b4 = name1() == name2(); // ref-error {{must be initialized by a constant expression}} \
-                                        // ref-note {{has unspecified value}} \
-                                        // ref-note {{declared here}}
-static_assert(!b4); // ref-error {{not an integral constant expression}} \
-                    // ref-note {{not a constant expression}}
+constexpr auto b3 = name1() == name1(); // ref-error {{must be initialized by a constant expression}} \
+                                        // ref-note {{comparison of addresses of literals}}
+constexpr auto b4 = name1() == name2();
+static_assert(!b4);
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zygoloid wrote:

The C++ model in general is that each evaluation of a string literal expression can produce a distinct object, and that these objects can fully or partially overlap each other in memory when they have suitable values; that's the model that this PR is implementing.

`b3` is non-constant because each call to `name1()` can produce a distinct value, so the comparison result is unspecified. `b4` is constant and `false` under this PR because the values returned by `name1()` and `name2()` cannot possibly be the same -- those two string literal evaluations can't produce the same value because the strings have different contents.

https://github.com/llvm/llvm-project/pull/109208