[clang] [C99] Claim conformance for _Complex support (PR #88161)

[Joshua Cranmer via cfe-commits]

================
@@ -0,0 +1,97 @@
+// RUN: %clang_cc1 -verify -std=c99 %s
+
+/* WG14 N620, N638, N657, N694, N809: Yes*
+ * Complex and imaginary support in <complex.h>
+ *
+ * NB: Clang supports _Complex but not _Imaginary. In C99, _Complex support is
+ * required outside of freestanding, but _Imaginary support is fully optional.
+ * In C11, both are made fully optional. We claim full conformance because we
+ * are actually conforming, but this gets an asterisk because it's also only
+ * partially implemented in a way and users should know about that.
+ *
+ * Because the functionality is so intertwined between the various papers,
+ * we're testing all of the functionality in one file.
+ */
+
+// Demonstrate that we support spelling complex floating-point objects.
+float _Complex f1;
+_Complex float f2;
+
+double _Complex d1;
+_Complex double d2;
+
+long double _Complex ld1;
+_Complex long double ld2;
+
+// Show that we don't support spelling imaginary types.
+float _Imaginary fi1; // expected-error {{imaginary types are not supported}}
+_Imaginary float fi2; // expected-error {{imaginary types are not supported}}
+
+double _Imaginary di1; // expected-error {{imaginary types are not supported}}
+_Imaginary double di2; // expected-error {{imaginary types are not supported}}
+
+long double _Imaginary ldi1; // expected-error {{imaginary types are not supported}}
+_Imaginary long double ldi2; // expected-error {{imaginary types are not supported}}
+
+// Each complex type has the same representation and alignment as an array
+// containing two elements of the corresponding real type.
+_Static_assert(sizeof(float _Complex) == sizeof(struct { float mem[2]; }), "");
+_Static_assert(_Alignof(float _Complex) == _Alignof(struct { float mem[2]; }), "");
----------------
jcranmer-intel wrote:

I'm not entirely certain that there's a mandatory rule in C99 (or any other version of C) that requires that
`struct { float mem[2]; }` and `float mem[2];` are the same in layout (particularly alignment, see C99 6.7.2.1p12 and read it somewhat maliciously). Though I will concede that all non-malicious compilers will treat them as the same.

https://github.com/llvm/llvm-project/pull/88161