Index: clang/include/clang/AST/ASTContext.h
===================================================================
--- clang/include/clang/AST/ASTContext.h
+++ clang/include/clang/AST/ASTContext.h
@@ -324,6 +324,12 @@
   };
   llvm::DenseMap<Module*, PerModuleInitializers*> ModuleInitializers;
 
+  /// Diagnostics that are emitted if and only if the given function is
+  /// codegen'ed.  Access these through FunctionDecl::addDeferredDiag() and
+  /// FunctionDecl::takeDeferredDiags().
+  llvm::DenseMap<const FunctionDecl *, std::vector<PartialDiagnosticAt>>
+      DeferredDiags;
+
 public:
   /// \brief A type synonym for the TemplateOrInstantiation mapping.
   typedef llvm::PointerUnion<VarTemplateDecl *, MemberSpecializationInfo *>
@@ -597,6 +603,11 @@
     return DiagAllocator;
   }
 
+  decltype(DeferredDiags) &getDeferredDiags() { return DeferredDiags; }
+  const decltype(DeferredDiags) &getDeferredDiags() const {
+    return DeferredDiags;
+  }
+
   const TargetInfo &getTargetInfo() const { return *Target; }
   const TargetInfo *getAuxTargetInfo() const { return AuxTarget; }
 
Index: clang/include/clang/AST/Decl.h
===================================================================
--- clang/include/clang/AST/Decl.h
+++ clang/include/clang/AST/Decl.h
@@ -2271,6 +2271,14 @@
   /// returns 0.
   unsigned getMemoryFunctionKind() const;
 
+  /// Add a diagnostic to be emitted if and when this function is codegen'ed.
+  void addDeferredDiag(PartialDiagnosticAt PD);
+
+  /// Gets this object's list of deferred diagnostics, if there are any.
+  ///
+  /// Although this is logically const, it clears our list of deferred diags.
+  std::vector<PartialDiagnosticAt> takeDeferredDiags() const;
+
   // Implement isa/cast/dyncast/etc.
   static bool classof(const Decl *D) { return classofKind(D->getKind()); }
   static bool classofKind(Kind K) {
Index: clang/lib/AST/Decl.cpp
===================================================================
--- clang/lib/AST/Decl.cpp
+++ clang/lib/AST/Decl.cpp
@@ -3446,6 +3446,20 @@
   return 0;
 }
 
+void FunctionDecl::addDeferredDiag(PartialDiagnosticAt PD) {
+  getASTContext().getDeferredDiags()[this].push_back(std::move(PD));
+}
+
+std::vector<PartialDiagnosticAt> FunctionDecl::takeDeferredDiags() const {
+  auto &DD = getASTContext().getDeferredDiags();
+  auto It = DD.find(this);
+  if (It == DD.end())
+    return {};
+  auto Ret = std::move(It->second);
+  DD.erase(It);
+  return Ret;
+}
+
 //===----------------------------------------------------------------------===//
 // FieldDecl Implementation
 //===----------------------------------------------------------------------===//
Index: clang/lib/CodeGen/CodeGenModule.h
===================================================================
--- clang/lib/CodeGen/CodeGenModule.h
+++ clang/lib/CodeGen/CodeGenModule.h
@@ -490,6 +490,10 @@
   /// MDNodes.
   llvm::DenseMap<QualType, llvm::Metadata *> MetadataIdMap;
 
+  /// Diags gathered from FunctionDecl::takeDeferredDiags().  Emitted at the
+  /// very end of codegen.
+  std::vector<std::pair<SourceLocation, PartialDiagnostic>> DeferredDiags;
+
 public:
   CodeGenModule(ASTContext &C, const HeaderSearchOptions &headersearchopts,
                 const PreprocessorOptions &ppopts,
Index: clang/lib/CodeGen/CodeGenModule.cpp
===================================================================
--- clang/lib/CodeGen/CodeGenModule.cpp
+++ clang/lib/CodeGen/CodeGenModule.cpp
@@ -497,6 +497,16 @@
   EmitVersionIdentMetadata();
 
   EmitTargetMetadata();
+
+  // Emit any deferred diagnostics gathered during codegen.  We didn't emit them
+  // when we first discovered them because that would have halted codegen,
+  // preventing us from gathering other deferred diags.
+  for (const PartialDiagnosticAt &DiagAt : DeferredDiags) {
+    SourceLocation Loc = DiagAt.first;
+    const PartialDiagnostic &PD = DiagAt.second;
+    DiagnosticBuilder Builder(getDiags().Report(Loc, PD.getDiagID()));
+    PD.Emit(Builder);
+  }
 }
 
 void CodeGenModule::UpdateCompletedType(const TagDecl *TD) {
@@ -2872,6 +2882,33 @@
                                                  llvm::GlobalValue *GV) {
   const auto *D = cast<FunctionDecl>(GD.getDecl());
 
+  // Emit this function's deferred diagnostics, if none of them are errors.  If
+  // any of them are errors, don't codegen the function, but also don't emit any
+  // of the diagnostics just yet.  Emitting an error during codegen stops
+  // further codegen, and we want to display as many deferred diags as possible.
+  // We'll emit the now twice-deferred diags at the very end of codegen.
+  //
+  // (If a function has both error and non-error diags, we don't emit the
+  // non-error diags here, because order can be significant, e.g. with notes
+  // that follow errors.)
+  auto Diags = D->takeDeferredDiags();
+  bool HasError = llvm::any_of(Diags, [this](const PartialDiagnosticAt &PDAt) {
+    return getDiags().getDiagnosticLevel(PDAt.second.getDiagID(), PDAt.first) >=
+           DiagnosticsEngine::Error;
+  });
+  if (HasError) {
+    DeferredDiags.insert(DeferredDiags.end(),
+                         std::make_move_iterator(Diags.begin()),
+                         std::make_move_iterator(Diags.end()));
+    return;
+  }
+  for (PartialDiagnosticAt &PDAt : Diags) {
+    const SourceLocation &Loc = PDAt.first;
+    const PartialDiagnostic &PD = PDAt.second;
+    DiagnosticBuilder Builder(getDiags().Report(Loc, PD.getDiagID()));
+    PD.Emit(Builder);
+  }
+
   // Compute the function info and LLVM type.
   const CGFunctionInfo &FI = getTypes().arrangeGlobalDeclaration(GD);
   llvm::FunctionType *Ty = getTypes().GetFunctionType(FI);