Index: lib/Sema/SemaChecking.cpp
===================================================================
--- lib/Sema/SemaChecking.cpp
+++ lib/Sema/SemaChecking.cpp
@@ -10292,33 +10292,6 @@
   DiagnoseImpCast(S, E, E->getType(), T, CContext, diag, pruneControlFlow);
 }
 
-/// Analyze the given compound assignment for the possible losing of
-/// floating-point precision.
-static void AnalyzeCompoundAssignment(Sema &S, BinaryOperator *E) {
-  assert(isa<CompoundAssignOperator>(E) &&
-         "Must be compound assignment operation");
-  // Recurse on the LHS and RHS in here
-  AnalyzeImplicitConversions(S, E->getLHS(), E->getOperatorLoc());
-  AnalyzeImplicitConversions(S, E->getRHS(), E->getOperatorLoc());
-
-  // Now check the outermost expression
-  const auto *ResultBT = E->getLHS()->getType()->getAs<BuiltinType>();
-  const auto *RBT = cast<CompoundAssignOperator>(E)
-                        ->getComputationResultType()
-                        ->getAs<BuiltinType>();
-
-  // If both source and target are floating points.
-  if (ResultBT && ResultBT->isFloatingPoint() && RBT && RBT->isFloatingPoint())
-    // Builtin FP kinds are ordered by increasing FP rank.
-    if (ResultBT->getKind() < RBT->getKind())
-      // We don't want to warn for system macro.
-      if (!S.SourceMgr.isInSystemMacro(E->getOperatorLoc()))
-        // warn about dropping FP rank.
-        DiagnoseImpCast(S, E->getRHS(), E->getLHS()->getType(),
-                        E->getOperatorLoc(),
-                        diag::warn_impcast_float_result_precision);
-}
-
 /// Diagnose an implicit cast from a floating point value to an integer value.
 static void DiagnoseFloatingImpCast(Sema &S, Expr *E, QualType T,
                                     SourceLocation CContext) {
@@ -10421,6 +10394,39 @@
   }
 }
 
+/// Analyze the given compound assignment for the possible losing of
+/// floating-point precision.
+static void AnalyzeCompoundAssignment(Sema &S, BinaryOperator *E) {
+  assert(isa<CompoundAssignOperator>(E) &&
+         "Must be compound assignment operation");
+  // Recurse on the LHS and RHS in here
+  AnalyzeImplicitConversions(S, E->getLHS(), E->getOperatorLoc());
+  AnalyzeImplicitConversions(S, E->getRHS(), E->getOperatorLoc());
+
+  // Now check the outermost expression
+  const auto *ResultBT = E->getLHS()->getType()->getAs<BuiltinType>();
+  const auto *RBT = cast<CompoundAssignOperator>(E)
+                        ->getComputationResultType()
+                        ->getAs<BuiltinType>();
+
+  // The below checks assume source is floating point.
+  if (!ResultBT || !RBT || !RBT->isFloatingPoint()) return;
+
+  // If source is floating point but target is not.
+  if (!ResultBT->isFloatingPoint())
+    return DiagnoseFloatingImpCast(S, E, E->getRHS()->getType(),
+                                   E->getExprLoc());
+
+  // If both source and target are floating points.
+  // Builtin FP kinds are ordered by increasing FP rank.
+  if (ResultBT->getKind() < RBT->getKind() &&
+      // We don't want to warn for system macro.
+      !S.SourceMgr.isInSystemMacro(E->getOperatorLoc()))
+    // warn about dropping FP rank.
+    DiagnoseImpCast(S, E->getRHS(), E->getLHS()->getType(), E->getOperatorLoc(),
+                    diag::warn_impcast_float_result_precision);
+}
+
 static std::string PrettyPrintInRange(const llvm::APSInt &Value,
                                       IntRange Range) {
   if (!Range.Width) return "0";
Index: test/SemaCXX/warn-float-conversion.cpp
===================================================================
--- test/SemaCXX/warn-float-conversion.cpp
+++ test/SemaCXX/warn-float-conversion.cpp
@@ -41,6 +41,32 @@
   l = ld;  //expected-warning{{conversion}}
 }
 
+void CompoundAssignment() {
+  int x = 3;
+
+  x += 1.234;  //expected-warning{{conversion}}
+  x -= -0.0;  //expected-warning{{conversion}}
+  x *= 1.1f;  //expected-warning{{conversion}}
+  x /= -2.2f;  //expected-warning{{conversion}}
+
+  int y = x += 1.4f;  //expected-warning{{conversion}}
+
+  float z = 1.1f;
+  double w = -2.2;
+
+  y += z + w;  //expected-warning{{conversion}}
+}
+
+# 1 "foo.h" 3
+//          ^ the following text comes from a system header file.
+#define SYSTEM_MACRO_FLOAT(x) do { (x) += 1.1; } while(0)
+# 1 "warn-float-conversion.cpp" 1
+//                              ^ start of a new file.
+void SystemMacro() {
+  float x = 0.0f;
+  SYSTEM_MACRO_FLOAT(x);
+}
+
 void Test() {
   int a1 = 10.0/2.0;  //expected-warning{{conversion}}
   int a2 = 1.0/2.0;  //expected-warning{{conversion}}