Index: clang/include/clang/Sema/Sema.h
===================================================================
--- clang/include/clang/Sema/Sema.h
+++ clang/include/clang/Sema/Sema.h
@@ -1452,8 +1452,9 @@
   static QualType GetTypeFromParser(ParsedType Ty,
                                     TypeSourceInfo **TInfo = nullptr);
   CanThrowResult canThrow(const Expr *E);
-  const FunctionProtoType *ResolveExceptionSpec(SourceLocation Loc,
-                                                const FunctionProtoType *FPT);
+  const FunctionProtoType *
+  ResolveExceptionSpec(SourceLocation Loc, const FunctionProtoType *FPT,
+                       FunctionDecl *OldDecl = nullptr);
   void UpdateExceptionSpec(FunctionDecl *FD,
                            const FunctionProtoType::ExceptionSpecInfo &ESI);
   bool CheckSpecifiedExceptionType(QualType &T, SourceRange Range);
@@ -7124,7 +7125,8 @@
   getTemplateInstantiationArgs(NamedDecl *D,
                                const TemplateArgumentList *Innermost = nullptr,
                                bool RelativeToPrimary = false,
-                               const FunctionDecl *Pattern = nullptr);
+                               const FunctionDecl *Pattern = nullptr,
+                               FunctionTemplateDecl *PrevFTD = nullptr);
 
   /// A context in which code is being synthesized (where a source location
   /// alone is not sufficient to identify the context). This covers template
@@ -7861,7 +7863,8 @@
              const MultiLevelTemplateArgumentList &TemplateArgs);
 
   void InstantiateExceptionSpec(SourceLocation PointOfInstantiation,
-                                FunctionDecl *Function);
+                                FunctionDecl *Function,
+                                FunctionDecl *OldFunction = nullptr);
   FunctionDecl *InstantiateFunctionDeclaration(FunctionTemplateDecl *FTD,
                                                const TemplateArgumentList *Args,
                                                SourceLocation Loc);
Index: clang/lib/Sema/SemaDecl.cpp
===================================================================
--- clang/lib/Sema/SemaDecl.cpp
+++ clang/lib/Sema/SemaDecl.cpp
@@ -10009,6 +10009,7 @@
   }
 
   if (Redeclaration) {
+
     // NewFD and OldDecl represent declarations that need to be
     // merged.
     if (MergeFunctionDecl(NewFD, OldDecl, S, MergeTypeWithPrevious)) {
Index: clang/lib/Sema/SemaExceptionSpec.cpp
===================================================================
--- clang/lib/Sema/SemaExceptionSpec.cpp
+++ clang/lib/Sema/SemaExceptionSpec.cpp
@@ -186,7 +186,8 @@
 }
 
 const FunctionProtoType *
-Sema::ResolveExceptionSpec(SourceLocation Loc, const FunctionProtoType *FPT) {
+Sema::ResolveExceptionSpec(SourceLocation Loc, const FunctionProtoType *FPT,
+                           FunctionDecl *OldDecl) {
   if (FPT->getExceptionSpecType() == EST_Unparsed) {
     Diag(Loc, diag::err_exception_spec_not_parsed);
     return nullptr;
@@ -207,7 +208,7 @@
   if (SourceFPT->getExceptionSpecType() == EST_Unevaluated)
     EvaluateImplicitExceptionSpec(Loc, cast<CXXMethodDecl>(SourceDecl));
   else
-    InstantiateExceptionSpec(Loc, SourceDecl);
+    InstantiateExceptionSpec(Loc, SourceDecl, OldDecl);
 
   const FunctionProtoType *Proto =
     SourceDecl->getType()->castAs<FunctionProtoType>();
@@ -246,7 +247,8 @@
     const FunctionProtoType *New, SourceLocation NewLoc,
     bool *MissingExceptionSpecification = nullptr,
     bool *MissingEmptyExceptionSpecification = nullptr,
-    bool AllowNoexceptAllMatchWithNoSpec = false, bool IsOperatorNew = false);
+    bool AllowNoexceptAllMatchWithNoSpec = false, bool IsOperatorNew = false,
+    FunctionDecl *OldDecl = nullptr);
 
 /// Determine whether a function has an implicitly-generated exception
 /// specification.
@@ -303,7 +305,7 @@
         Old->getType()->getAs<FunctionProtoType>(), Old->getLocation(),
         New->getType()->getAs<FunctionProtoType>(), New->getLocation(),
         &MissingExceptionSpecification, &MissingEmptyExceptionSpecification,
-        /*AllowNoexceptAllMatchWithNoSpec=*/true, IsOperatorNew)) {
+        /*AllowNoexceptAllMatchWithNoSpec=*/true, IsOperatorNew, Old)) {
     // C++11 [except.spec]p4 [DR1492]:
     //   If a declaration of a function has an implicit
     //   exception-specification, other declarations of the function shall
@@ -487,7 +489,8 @@
     const FunctionProtoType *New, SourceLocation NewLoc,
     bool *MissingExceptionSpecification,
     bool *MissingEmptyExceptionSpecification,
-    bool AllowNoexceptAllMatchWithNoSpec, bool IsOperatorNew) {
+    bool AllowNoexceptAllMatchWithNoSpec, bool IsOperatorNew,
+    FunctionDecl *OldDecl) {
   if (MissingExceptionSpecification)
     *MissingExceptionSpecification = false;
 
@@ -497,7 +500,7 @@
   Old = S.ResolveExceptionSpec(NewLoc, Old);
   if (!Old)
     return false;
-  New = S.ResolveExceptionSpec(NewLoc, New);
+  New = S.ResolveExceptionSpec(NewLoc, New, OldDecl);
   if (!New)
     return false;
 
Index: clang/lib/Sema/SemaTemplateInstantiate.cpp
===================================================================
--- clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -51,11 +51,16 @@
 /// instantiating the definition of the given declaration, \p D. This is
 /// used to determine the proper set of template instantiation arguments for
 /// friend function template specializations.
+///
+/// \param PrevFunTmpl If non-NULL, D is a pattern declaration of a function
+/// template that hasn't yet been attached to the redeclaration chain of
+/// PrevFunTmpl's pattern.
 MultiLevelTemplateArgumentList
 Sema::getTemplateInstantiationArgs(NamedDecl *D,
                                    const TemplateArgumentList *Innermost,
                                    bool RelativeToPrimary,
-                                   const FunctionDecl *Pattern) {
+                                   const FunctionDecl *Pattern,
+                                   FunctionTemplateDecl *PrevFunTmpl) {
   // Accumulate the set of template argument lists in this structure.
   MultiLevelTemplateArgumentList Result;
 
@@ -151,6 +156,16 @@
 
       } else if (FunctionTemplateDecl *FunTmpl
                                    = Function->getDescribedFunctionTemplate()) {
+        if (PrevFunTmpl) {
+          // FunTmpl hasn't been linked into the same redecl chain as
+          // PrevFunTmpl yet, so we can't retreive it's injected template
+          // arguments just yet; use PrevFunTmpl's instead.
+          //
+          // Note that we don't need to verify that FunTmpl is the function
+          // template that is a redeclaration of PrevFunTmpl because only one
+          // function template decl can appear in this search.
+          FunTmpl = PrevFunTmpl;
+        }
         // Add the "injected" template arguments.
         Result.addOuterTemplateArguments(FunTmpl->getInjectedTemplateArgs());
       }
Index: clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
===================================================================
--- clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -3575,7 +3575,8 @@
 }
 
 void Sema::InstantiateExceptionSpec(SourceLocation PointOfInstantiation,
-                                    FunctionDecl *Decl) {
+                                    FunctionDecl *Decl,
+                                    FunctionDecl *OldDecl) {
   const FunctionProtoType *Proto = Decl->getType()->castAs<FunctionProtoType>();
   if (Proto->getExceptionSpecType() != EST_Uninstantiated)
     return;
@@ -3601,8 +3602,9 @@
   Sema::ContextRAII savedContext(*this, Decl);
   LocalInstantiationScope Scope(*this);
 
-  MultiLevelTemplateArgumentList TemplateArgs =
-    getTemplateInstantiationArgs(Decl, nullptr, /*RelativeToPrimary*/true);
+  MultiLevelTemplateArgumentList TemplateArgs = getTemplateInstantiationArgs(
+      Decl, nullptr, /*RelativeToPrimary*/ true, nullptr,
+      !OldDecl ? nullptr : OldDecl->getDescribedFunctionTemplate());
 
   FunctionDecl *Template = Proto->getExceptionSpecTemplate();
   if (addInstantiatedParametersToScope(*this, Decl, Template, Scope,
Index: clang/test/SemaCXX/friend-template-redecl.cpp
===================================================================
--- /dev/null
+++ clang/test/SemaCXX/friend-template-redecl.cpp
@@ -0,0 +1,20 @@
+// RUN: %clang_cc1 -std=c++17 -verify -emit-llvm-only %s
+
+// expected-no-diagnostics
+
+template <class T> void bar(const T &t) { foo(t); }
+
+template <class>
+struct HasFriend {
+  template <class T>
+  friend void foo(const HasFriend<T> &m) noexcept(false);
+};
+
+template <class T>
+void foo(const HasFriend<T> &m) noexcept(false) {}
+
+void f() {
+  HasFriend<int> x;
+  foo(x);
+  bar(x);
+}