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
@@ -679,21 +679,14 @@
   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);
@@ -1408,6 +1401,27 @@
   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
diff --git a/llvm/include/llvm/IR/ModuleSummaryIndex.h b/llvm/include/llvm/IR/ModuleSummaryIndex.h
--- a/llvm/include/llvm/IR/ModuleSummaryIndex.h
+++ b/llvm/include/llvm/IR/ModuleSummaryIndex.h
@@ -147,7 +147,7 @@
     StringRef Name;
   } U;
 
-  GlobalValueSummaryInfo(bool HaveGVs) : U(HaveGVs) {}
+  inline GlobalValueSummaryInfo(bool HaveGVs);
 
   /// List of global value summary structures for a particular value held
   /// in the GlobalValueMap. Requires a vector in the case of multiple
@@ -575,6 +575,8 @@
   friend class ModuleSummaryIndex;
 };
 
+GlobalValueSummaryInfo::GlobalValueSummaryInfo(bool HaveGVs) : U(HaveGVs) {}
+
 /// Alias summary information.
 class AliasSummary : public GlobalValueSummary {
   ValueInfo AliaseeValueInfo;