Index: clang/include/clang/AST/Decl.h
===================================================================
--- clang/include/clang/AST/Decl.h
+++ clang/include/clang/AST/Decl.h
@@ -1638,6 +1638,14 @@
   /// declaration name embedded in the DeclaratorDecl base class.
   DeclarationNameLoc DNLoc;
 
+  /// Storage for diagnostics deferred until this function is codegen'ed (if it
+  /// ever is).
+  ///
+  /// These are rarely used, so we use a pointer-to-vector to save two words
+  /// inside FunctionDecl.  This is mutable because emitting diagnostics (which
+  /// clears this list) needs to be a logically-const operation.
+  mutable std::unique_ptr<std::vector<PartialDiagnosticAt>> DeferredDiags;
+
   /// \brief Specify that this function declaration is actually a function
   /// template specialization.
   ///
@@ -2271,6 +2279,14 @@
   /// 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
@@ -3436,6 +3436,22 @@
   return 0;
 }
 
+void FunctionDecl::addDeferredDiag(PartialDiagnosticAt PD) {
+  if (!DeferredDiags)
+    DeferredDiags = llvm::make_unique<std::vector<PartialDiagnosticAt>>();
+  DeferredDiags->emplace_back(PD);
+}
+
+std::vector<PartialDiagnosticAt> FunctionDecl::takeDeferredDiags() const {
+  if (!DeferredDiags)
+    return {};
+  assert(!DeferredDiags->empty() &&
+         "DeferredDiags should be non-null only if it's also non-empty.");
+  auto Ret = std::move(*DeferredDiags);
+  DeferredDiags.reset();
+  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 +2882,19 @@
                                                  llvm::GlobalValue *GV) {
   const auto *D = cast<FunctionDecl>(GD.getDecl());
 
+  // Check if this function has diagnostics that should be emitted when we
+  // codegen it.  If so, don't eit this function definition, but don't emit the
+  // diags just yet.  Emitting an error during codegen stops 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.
+  auto Diags = D->takeDeferredDiags();
+  bool HasDiags = !Diags.empty();
+  DeferredDiags.insert(DeferredDiags.end(),
+                       std::make_move_iterator(Diags.begin()),
+                       std::make_move_iterator(Diags.end()));
+  if (HasDiags)
+    return;
+
   // Compute the function info and LLVM type.
   const CGFunctionInfo &FI = getTypes().arrangeGlobalDeclaration(GD);
   llvm::FunctionType *Ty = getTypes().GetFunctionType(FI);