Index: clang/lib/Sema/SemaExpr.cpp
===================================================================
--- clang/lib/Sema/SemaExpr.cpp
+++ clang/lib/Sema/SemaExpr.cpp
@@ -1115,6 +1115,11 @@
   bool LHSFloat = LHSType->isRealFloatingType();
   bool RHSFloat = RHSType->isRealFloatingType();
 
+  // FIXME: Implement Floating-fixedpoint conversion. (Bug 46268)
+  // Reference N1169 4.1.4 (Type conversion, usual arithmetic conversions).
+  if ((LHSType->isFixedPointType() && RHSType->isRealFloatingType()) ||
+      (LHSType->isRealFloatingType() && RHSType->isFixedPointType()))
+    return QualType();
   // If we have two real floating types, convert the smaller operand
   // to the bigger result.
   if (LHSFloat && RHSFloat) {
Index: clang/test/Frontend/fixed_point_errors.c
===================================================================
--- clang/test/Frontend/fixed_point_errors.c
+++ clang/test/Frontend/fixed_point_errors.c
@@ -250,3 +250,8 @@
 char c_const = 256.0uk;                   // expected-warning{{implicit conversion from 256.0 cannot fit within the range of values for 'char'}}
 short _Accum sa_const5 = 256;             // expected-warning{{implicit conversion from 256 cannot fit within the range of values for 'short _Accum'}}
 unsigned short _Accum usa_const2 = -2;    // expected-warning{{implicit conversion from -2 cannot fit within the range of values for 'unsigned short _Accum'}}
+
+void foo(void) {
+  _Accum x = 0.5k;
+  if (x == 0.5) {} // expected-error{{invalid operands to binary expression ('_Accum' and 'double')}}
+}