[clang] [Clang][Sema] Print more static_assert exprs (PR #74852)

[via cfe-commits]

================
@@ -0,0 +1,115 @@
+// RUN: %clang_cc1 -std=c++2a -verify %s
+
+struct A {
+  int a, b[3], c;
+  bool operator==(const A&) const = default;
+};
+
+constexpr auto a0 = A{0, 0, 3, 4, 5};
+
+// expected-note at +1 {{evaluates to 'A{0, {0, 3, 4}, 5} == A{1, {2, 3, 4}, 5}'}}
+static_assert(a0 == A{1, {2, 3, 4}, 5}); // expected-error {{failed}}
+
+struct _arr {
+  const int b[3];
+  constexpr bool operator==(const int rhs[3]) const {
+    for (unsigned i = 0; i < sizeof(b) / sizeof(int); i++)
+      if (b[i] != rhs[i])
+        return false;
+    return true;
+  }
+};
+
+// expected-note at +1 {{{evaluates to '_arr{{2, 3, 4}} == (int[3]){0, 3, 4}'}}}
+static_assert(_arr{2, 3, 4} == a0.b); // expected-error {{failed}}
+
+struct B {
+  int a, c; // named the same just to keep things fresh
+  bool operator==(const B&) const = default;
+};
+
+// expected-note at +1 {{evaluates to 'B{7, 6} == B{8, 6}'}}
+static_assert(B{7, 6} == B{8, 6}); // expected-error {{failed}}
+
+typedef int v4si __attribute__((__vector_size__(16)));
+
+struct C: A, B {
+  enum { E1, E2 } e;
+  bool operator==(const C&) const = default;
+};
+
+constexpr auto cc = C{A{1, {2, 3, 4}, 5}, B{7, 6}, C::E1};
+
+// expected-note at +1 {{{evaluates to 'C{{1, {2, 3, 4}, 5}, {7, 6}, 0} == C{{0, {0, 3, 4}, 5}, {5, 0}, 1}'}}}
+static_assert(cc == C{a0, {5}, C::E2}); // expected-error {{failed}}
----------------
sethp wrote:

It is confusing, but I don't think this change makes it any worse; the output I see from clang with my change is:

```c++
clang/test/SemaCXX/static-assert-diagnostics.cpp:177:15: error: static assertion failed due to requirement 'S{1, 2, 3} == S{1, 2, 3}'
  177 | static_assert(S{1,2,3} == S{1,2,3});
      |               ^~~~~~~~~~~~~~~~~~~~
clang/test/SemaCXX/static-assert-diagnostics.cpp:177:24: note: expression evaluates to 'S{1, 2, 3} == S{1, 2, 3}'
  177 | static_assert(S{1,2,3} == S{1,2,3});
      |               ~~~~~~~~~^~~~~~~~~~~
```

The `note` this change adds is redundant, sure, but I don't think it's more confusing than omitting it: either way, I'm going to go hunt for an `operator==` between two `S`s and see if I can figure out why two things that sure feel like they _ought_ to be `==` aren't.

A different example with the same `operator==` overload there might be something like this:

```
struct alignas(int) S {
unsigned char a, b, c;
};
constexpr bool operator==(const S&, const S&) { return false; }

static_assert(S{1, 2, 3} == std::bit_cast<S>(0x030201));
```

Which now outputs:

```
clang/test/SemaCXX/static-assert-diagnostics.cpp:177:15: error: static assertion failed due to requirement 'S{1, 2, 3} == std::bit_cast(197121)'
  177 | static_assert(S{1, 2, 3} == std::bit_cast<S>(0x030201));
      |               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
clang/test/SemaCXX/static-assert-diagnostics.cpp:177:26: note: expression evaluates to 'S{1, 2, 3} == struct S{1, 2, 3}'
  177 | static_assert(S{1, 2, 3} == std::bit_cast<S>(0x030201));
      |               ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

Here the note is, I think, quite helpful: to me it very strongly suggests to me that the issue isn't the `bit_cast` or a endian-ness mismatch or anything like that, but the `operator==` implementation.



https://github.com/llvm/llvm-project/pull/74852