Don't diagnose a redeclaration of a deduction guide if the prior

declaration is not visible.

In passing, add a test for a similar case of conflicting redeclarations
of internal-linkage structured bindings. (This case already works).

clang/lib/Sema/SemaDeclCXX.cpp

@@ -678,7 +678,7 @@ bool Sema::MergeCXXFunctionDecl(FunctionDecl *New, FunctionDecl *Old,
   //   for the same class template shall not have equivalent
   //   parameter-declaration-clauses.
   if (isa<CXXDeductionGuideDecl>(New) &&
-      !New->isFunctionTemplateSpecialization()) {
+      !New->isFunctionTemplateSpecialization() && isVisible(Old)) {
     Diag(New->getLocation(), diag::err_deduction_guide_redeclared);
     Diag(Old->getLocation(), diag::note_previous_declaration);
   }

clang/test/Modules/Inputs/cxx17/module.modulemap

@@ -1 +1,4 @@
-module Decls { header "decls.h" }
+module Decls {
+  header "decls.h"
+  explicit module Unimported { header "unimported.h" }
+}

clang/test/Modules/Inputs/cxx17/unimported.h

@@ -0,0 +1,2 @@
+template<typename T> struct DeductionGuide {};
+DeductionGuide() -> DeductionGuide<int>;

clang/test/Modules/Inputs/cxx20/module.modulemap

@@ -0,0 +1,4 @@
+module Decls {
+  header "decls.h"
+  explicit module Unimported { header "unimported.h" }
+}

clang/test/Modules/Inputs/cxx20/unimported.h

@@ -0,0 +1,4 @@
+namespace StructuredBinding {
+  struct Q { int p, q; };
+  static auto [a, b] = Q();
+}

clang/test/Modules/cxx17.cpp

@@ -9,3 +9,7 @@ struct MergeExceptionSpec {
 #include "decls.h"
 
 MergeExceptionSpec mergeExceptionSpec2;
+
+template<typename T> struct DeductionGuide {};
+DeductionGuide() -> DeductionGuide<int>;
+DeductionGuide a;

clang/test/Modules/cxx20.cpp

@@ -0,0 +1,11 @@
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -x c++ -std=c++20 -fmodules-cache-path=%t -fmodules -fimplicit-module-maps -I %S/Inputs/cxx20 %s -verify -fno-modules-error-recovery
+
+// expected-no-diagnostics
+
+#include "decls.h"
+
+namespace StructuredBinding {
+  struct R { int x, y; };
+  static auto [a, b] = R();
+}