Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -3093,6 +3093,9 @@
 def warn_impcast_float_precision : Warning<
   "implicit conversion loses floating-point precision: %0 to %1">,
   InGroup<Conversion>, DefaultIgnore;
+def warn_impcast_float_result_precision : Warning<
+  "implicit conversion when assigning computation result loses floating-point precision: %0 to %1">,
+  InGroup<Conversion>, DefaultIgnore;
 def warn_impcast_double_promotion : Warning<
   "implicit conversion increases floating-point precision: %0 to %1">,
   InGroup<DoublePromotion>, DefaultIgnore;
Index: lib/Sema/SemaChecking.cpp
===================================================================
--- lib/Sema/SemaChecking.cpp
+++ lib/Sema/SemaChecking.cpp
@@ -8625,6 +8625,9 @@
 }
 
 static void AnalyzeImplicitConversions(Sema &S, Expr *E, SourceLocation CC);
+static void CheckImplicitConversion(Sema &S, Expr *E, QualType T,
+                                    SourceLocation CC,
+                                    bool *ICContext = nullptr);
 
 static bool IsEnumConstOrFromMacro(Sema &S, Expr *E) {
   // Suppress cases where we are comparing against an enum constant.
@@ -9188,6 +9191,28 @@
   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");
+
+  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,
@@ -9501,9 +9526,8 @@
   return true;
 }
 
-static void
-CheckImplicitConversion(Sema &S, Expr *E, QualType T, SourceLocation CC,
-                        bool *ICContext = nullptr) {
+static void CheckImplicitConversion(Sema &S, Expr *E, QualType T,
+                                    SourceLocation CC, bool *ICContext) {
   if (E->isTypeDependent() || E->isValueDependent()) return;
 
   const Type *Source = S.Context.getCanonicalType(E->getType()).getTypePtr();
@@ -9555,7 +9579,7 @@
         return;
       return DiagnoseImpCast(S, E, T, CC, diag::warn_impcast_vector_scalar);
     }
-    
+
     // If the vector cast is cast between two vectors of the same size, it is
     // a bitcast, not a conversion.
     if (S.Context.getTypeSize(Source) == S.Context.getTypeSize(Target))
@@ -9832,7 +9856,7 @@
 
   if (E->isTypeDependent() || E->isValueDependent())
     return;
-  
+
   // For conditional operators, we analyze the arguments as if they
   // were being fed directly into the output.
   if (isa<ConditionalOperator>(E)) {
@@ -9876,6 +9900,9 @@
     // And with simple assignments.
     if (BO->getOpcode() == BO_Assign)
       return AnalyzeAssignment(S, BO);
+    // And with compound assignments.
+    if (BO->isAssignmentOp())
+      AnalyzeCompoundAssignment(S, BO);
   }
 
   // These break the otherwise-useful invariant below.  Fortunately,
Index: test/Sema/conversion.c
===================================================================
--- test/Sema/conversion.c
+++ test/Sema/conversion.c
@@ -436,10 +436,15 @@
 }
 
 void double2float_test2(double a, float *b) {
-    *b += a;
+  *b += a; // expected-warning {{implicit conversion when assigning computation result loses floating-point precision: 'double' to 'float'}}
 }
 
 float sinf (float x);
 double double2float_test3(double a) {
     return sinf(a); // expected-warning {{implicit conversion loses floating-point precision: 'double' to 'float'}}
 }
+
+float double2float_test4(double a, float b) {
+  b -= a; // expected-warning {{implicit conversion when assigning computation result loses floating-point precision: 'double' to 'float'}}
+  return b;
+}