[clang] Create PointerToBoolean casts for C casts (#155368)

Don't create CK_BitCast casts from `nullptr_t` to `bool`.

Fixes #155126

Author: tbaederr

clang/lib/Sema/SemaCast.cpp

@@ -3171,7 +3171,12 @@ void CastOperation::CheckCStyleCast() {
       SrcExpr = ExprError();
       return;
     }
-    if (!DestType->isNullPtrType()) {
+    if (DestType->isBooleanType()) {
+      SrcExpr = ImplicitCastExpr::Create(
+          Self.Context, DestType, CK_PointerToBoolean, SrcExpr.get(), nullptr,
+          VK_PRValue, Self.CurFPFeatureOverrides());
+
+    } else if (!DestType->isNullPtrType()) {
       // Implicitly cast from the null pointer type to the type of the
       // destination.
       CastKind CK = DestType->isPointerType() ? CK_NullToPointer : CK_BitCast;

clang/test/CodeGen/issue155126.c

@@ -0,0 +1,17 @@
+// NOTE: Assertions have been autogenerated by utils/update_cc_test_checks.py UTC_ARGS: --version 5
+// RUN: %clang_cc1 -std=c23 %s -emit-llvm -o - | FileCheck %s
+// RUN: %clang_cc1 -std=c23 %s -emit-llvm -o - -fexperimental-new-constant-interpreter | FileCheck %s
+
+enum e : bool { b = true };
+// CHECK-LABEL: define dso_local void @foo(
+// CHECK-SAME: ) #[[ATTR0:[0-9]+]] {
+// CHECK-NEXT:  [[ENTRY:.*:]]
+// CHECK-NEXT:    [[E1:%.*]] = alloca i8, align 1
+// CHECK-NEXT:    store i8 0, ptr [[E1]], align 1
+// CHECK-NEXT:    ret void
+//
+void foo ()
+{
+  enum e e1;
+  e1 = (bool) nullptr;
+}

clang/test/Sema/constexpr.c

@@ -398,3 +398,6 @@ typedef short v2int16_t __attribute__((ext_vector_type(2)));
 bool issue155507(v2int16_t a, v2int16_t b) {
     return __builtin_bit_cast(unsigned char, __builtin_convertvector(a == b, __vbool2)) == 0b11;
 }
+
+constexpr bool b2 = (bool)nullptr;
+_Static_assert(!b2);