c++: Add support for __real__/__imag__ modifications in constant expressions [PR88174]linaro-local/ci/tcwg_kernel/gnu-master-arm-norov-allnoconfiglinaro-local/ci/tcwg_kernel/gnu-master-arm-norov-allmodconfiglinaro-local/ci/tcwg_kernel/gnu-master-arm-mainline-defconfiglinaro-local/ci/tcwg_kernel/gnu-master-aarch64-norov-allnoconfiglinaro-local/ci/tcwg_kernel/gnu-master-aarch64-next-allnoconfiglinaro-local/ci/tcwg_bmk_llvm_tk1/llvm-master-arm-spec2k6-O2linaro-local/ci/tcwg_bmk_llvm_apm/llvm-master-arm-spec2k6-Ozlinaro-local/ci/tcwg_bmk_gnu_fx/gnu-master-aarch64-cpu2017-O2linaro-local/ci/tcwg_bmk_gnu_eabi_stm32/gnu_eabi-master-arm_eabi-coremark-Os_LTOlinaro-local/ci/tcwg_bmk_gnu_eabi_stm32/gnu_eabi-master-arm_eabi-coremark-O3_LTOlinaro-local/ci/tcwg_bmk_gnu_eabi_stm32/gnu_eabi-master-arm_eabi-coremark-O2linaro-local/ci/tcwg_bmk_gnu_apm/gnu-master-aarch64-spec2k6-Os_LTO

We claim we support P0415R1 (constexpr complex), but e.g.
 #include <complex>

constexpr bool
foo ()
{
  std::complex<double> a (1.0, 2.0);
  a += 3.0;
  a.real (6.0);
  return a.real () == 6.0 && a.imag () == 2.0;
}

static_assert (foo ());

fails with
test.C:12:20: error: non-constant condition for static assertion
   12 | static_assert (foo ());
      |                ~~~~^~
test.C:12:20:   in ‘constexpr’ expansion of ‘foo()’
test.C:8:10:   in ‘constexpr’ expansion of ‘a.std::complex<double>::real(6.0e+0)’
test.C:12:20: error: modification of ‘__real__ a.std::complex<double>::_M_value’ is not a constant expression

The problem is we don't handle REALPART_EXPR and IMAGPART_EXPR
in cxx_eval_store_expression.
The following patch attempts to support it (with a requirement
that those are the outermost expressions, ARRAY_REF/COMPONENT_REF
etc. are just not possible on the result of these, BIT_FIELD_REF
would be theoretically possible if trying to extract some bits
from one part of a complex int, but I don't see how it could appear
in the FE trees.

For these references, the code handles value being COMPLEX_CST,
COMPLEX_EXPR or CONSTRUCTOR_NO_CLEARING empty CONSTRUCTOR (what we use
to represent uninitialized values for C++20 and later) and the
code starts by rewriting it to COMPLEX_EXPR, so that we can freely
adjust the individual parts and later on possibly optimize it back
to COMPLEX_CST if both halves are constant.

2022-08-07  Jakub Jelinek  <jakub@redhat.com>

	PR c++/88174
	* constexpr.cc (cxx_eval_store_expression): Handle REALPART_EXPR
	and IMAGPART_EXPR.  Change ctors from releasing_vec to
	auto_vec<tree *>, adjust all uses.  For !preeval, update ctors
	vector.

	* g++.dg/cpp1y/constexpr-complex1.C: New test.

1 files changed, 24 insertions, 0 deletions

diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-complex1.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-complex1.C
new file mode 100644
index 00000000000..8bb24cb2775
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-complex1.C
@@ -0,0 +1,24 @@
+// PR c++/88174
+// { dg-do compile { target c++14 } }
+
+constexpr bool
+foo (double x, double y, double z, double w)
+{
+  __complex__ double a = 0;
+  __real__ a = x;
+  __imag__ a = y;
+#if __cpp_constexpr >= 201907L
+  __complex__ double b;
+  __real__ b = z;
+#else
+  __complex__ double b = z;
+#endif
+  __imag__ b = w;
+  a += b;
+  a -= b;
+  a *= b;
+  a /= b;
+  return __real__ a == x && __imag__ a == y;
+}
+
+static_assert (foo (1.0, 2.0, 3.0, 4.0), "");