diff --git a/llvm/include/llvm/ADT/APFloat.h b/llvm/include/llvm/ADT/APFloat.h
--- a/llvm/include/llvm/ADT/APFloat.h
+++ b/llvm/include/llvm/ADT/APFloat.h
@@ -635,21 +635,14 @@
   DoubleAPFloat(DoubleAPFloat &&RHS);
 
   DoubleAPFloat &operator=(const DoubleAPFloat &RHS);
-
-  DoubleAPFloat &operator=(DoubleAPFloat &&RHS) {
-    if (this != &RHS) {
-      this->~DoubleAPFloat();
-      new (this) DoubleAPFloat(std::move(RHS));
-    }
-    return *this;
-  }
+  inline DoubleAPFloat &operator=(DoubleAPFloat &&RHS);
 
   bool needsCleanup() const { return Floats != nullptr; }
 
-  APFloat &getFirst() { return Floats[0]; }
-  const APFloat &getFirst() const { return Floats[0]; }
-  APFloat &getSecond() { return Floats[1]; }
-  const APFloat &getSecond() const { return Floats[1]; }
+  inline APFloat &getFirst();
+  inline const APFloat &getFirst() const;
+  inline APFloat &getSecond();
+  inline const APFloat &getSecond() const;
 
   opStatus add(const DoubleAPFloat &RHS, roundingMode RM);
   opStatus subtract(const DoubleAPFloat &RHS, roundingMode RM);
@@ -1353,6 +1346,27 @@
   return A < B ? B : A;
 }
 
+// We want the following functions to be available in the header for inlining.
+// We cannot define them inline in the class definition of `DoubleAPFloat`
+// because doing so would instantiate `std::unique_ptr<APFloat[]>` before
+// `APFloat` is defined, and that would be undefined behavior.
+namespace detail {
+
+DoubleAPFloat & DoubleAPFloat::operator=(DoubleAPFloat &&RHS) {
+  if (this != &RHS) {
+    this->~DoubleAPFloat();
+    new (this) DoubleAPFloat(std::move(RHS));
+  }
+  return *this;
+}
+
+APFloat & DoubleAPFloat::getFirst() { return Floats[0]; }
+const APFloat & DoubleAPFloat::getFirst() const { return Floats[0]; }
+APFloat & DoubleAPFloat::getSecond() { return Floats[1]; }
+const APFloat & DoubleAPFloat::getSecond() const { return Floats[1]; }
+
+} // namespace detail
+
 } // namespace llvm
 
 #undef APFLOAT_DISPATCH_ON_SEMANTICS