c++: Only reject reinterpret casts from pointers to integers for manifestly_const_eval evaluation [PR99456]

My PR82304/PR95307 fix moved reinterpret cast from pointer to integer
diagnostics from cxx_eval_outermost_constant_expr where it caught
invalid code only at the outermost level down into
cxx_eval_constant_expression.
Unfortunately, it regressed following testcase, we emit worse code
including dynamic initialization of some vars.
While the initializers are not constant expressions due to the
reinterpret_cast in there, there is no reason not to fold them as an
optimization.

I've tried to make this dependent on !ctx->quiet, but that regressed
two further tests, and on ctx->strict, which regressed other tests,
so this patch bases that on manifestly_const_eval.

The new testcase is now optimized as much as it used to be in GCC 10
and the only regression it causes is an extra -Wnarrowing warning
on vla22.C test on invalid code (which the patch adjusts).

2021-03-19  Jakub Jelinek  <jakub@redhat.com>

	PR c++/99456
	* constexpr.c (cxx_eval_constant_expression): For CONVERT_EXPR from
	INDIRECT_TYPE_P to ARITHMETIC_TYPE_P, when !ctx->manifestly_const_eval
	don't diagnose it, set *non_constant_p nor return t.

	* g++.dg/opt/pr99456.C: New test.
	* g++.dg/ext/vla22.C: Expect a -Wnarrowing warning for c++11 and
	later.

3 files changed, 36 insertions, 2 deletions

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index c946744..42d00ec 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -6656,7 +6656,8 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 
 	if (TREE_CODE (t) == CONVERT_EXPR
 	    && ARITHMETIC_TYPE_P (type)
-	    && INDIRECT_TYPE_P (TREE_TYPE (op)))
+	    && INDIRECT_TYPE_P (TREE_TYPE (op))
+	    && ctx->manifestly_const_eval)
 	  {
 	    if (!ctx->quiet)
 	      error_at (loc,
diff --git a/gcc/testsuite/g++.dg/ext/vla22.C b/gcc/testsuite/g++.dg/ext/vla22.C
index 2308ee7..967adb9 100644
--- a/gcc/testsuite/g++.dg/ext/vla22.C
+++ b/gcc/testsuite/g++.dg/ext/vla22.C
@@ -6,4 +6,4 @@ void
 f ()
 {
   const int tbl[(long) "h"] = { 12 }; // { dg-error "size of array .tbl. is not an integral constant-expression" }
-}
+}				      // { dg-warning "narrowing conversion" "" { target c++11 } .-1 }
diff --git a/gcc/testsuite/g++.dg/opt/pr99456.C b/gcc/testsuite/g++.dg/opt/pr99456.C
new file mode 100644
index 0000000..66f19c8
--- /dev/null
+++ b/gcc/testsuite/g++.dg/opt/pr99456.C
@@ -0,0 +1,33 @@
+// PR c++/99456
+// { dg-do compile { target c++17 } }
+// { dg-options "-g0" }
+// { dg-final { scan-assembler-not "PR99456Var0\[1234]" } }
+// { dg-final { scan-assembler-not "__static_initialization_and_destruction" } }
+// { dg-final { scan-assembler-not "_GLOBAL__sub_I" } }
+// { dg-final { scan-assembler-not "_ZGV12PR99456Var1\[1234]" } }
+
+typedef __UINTPTR_TYPE__ uintptr_t;
+
+class Container
+{
+public:
+  uintptr_t m;
+};
+
+extern unsigned desc;
+static constexpr unsigned &descRef = desc;
+
+inline Container PR99456Var01 {reinterpret_cast<uintptr_t> (&descRef)};
+inline Container PR99456Var02 {reinterpret_cast<uintptr_t> (&desc)};
+inline uintptr_t PR99456Var03 {reinterpret_cast<uintptr_t> (&descRef)};
+inline uintptr_t PR99456Var04 {reinterpret_cast<uintptr_t> (&desc)};
+
+inline Container PR99456Var11 {reinterpret_cast<uintptr_t> (&descRef)};
+inline Container PR99456Var12 {reinterpret_cast<uintptr_t> (&desc)};
+inline uintptr_t PR99456Var13 {reinterpret_cast<uintptr_t> (&descRef)};
+inline uintptr_t PR99456Var14 {reinterpret_cast<uintptr_t> (&desc)};
+
+auto *PR99456Ref11 = &PR99456Var11;
+auto *PR99456Ref12 = &PR99456Var12;
+auto *PR99456Ref13 = &PR99456Var13;
+auto *PR99456Ref14 = &PR99456Var14;