PR c++/79548 - missing -Wunused-variable on a typedef'd variable in a function template

gcc/c-family/ChangeLog:

	PR c++/79548
	* c-common.c (set_underlying_type): Mark type used only when
	original del is declared unused.

gcc/testsuite/ChangeLog:

	PR c++/79548
	* g++.dg/warn/Wunused-var-26.C: New test.

diff --git a/gcc/c-family/c-common.c b/gcc/c-family/c-common.c
index 885ea6d..65ffd8c 100644
--- a/gcc/c-family/c-common.c
+++ b/gcc/c-family/c-common.c
@@ -7476,7 +7476,12 @@ set_underlying_type (tree x)
       tt = build_variant_type_copy (tt);
       TYPE_STUB_DECL (tt) = TYPE_STUB_DECL (DECL_ORIGINAL_TYPE (x));
       TYPE_NAME (tt) = x;
-      TREE_USED (tt) = TREE_USED (x);
+
+      /* Mark the type as used only when its type decl is decorated
+	 with attribute unused.  */
+      if (lookup_attribute ("unused", DECL_ATTRIBUTES (x)))
+	TREE_USED (tt) = 1;
+
       TREE_TYPE (x) = tt;
     }
 }
diff --git a/gcc/testsuite/g++.dg/warn/Wunused-var-26.C b/gcc/testsuite/g++.dg/warn/Wunused-var-26.C
new file mode 100644
index 0000000..b3e020b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wunused-var-26.C
@@ -0,0 +1,147 @@
+// PR c++/79548 - missing -Wunused-variable on a typedef'd variable
+// in a function template
+// { dg-do compile }
+// { dg-options "-Wunused" }
+
+
+#define UNUSED __attribute__ ((unused))
+
+template <class T>
+void f_int ()
+{
+  T t;                        // { dg-warning "unused variable" }
+
+  typedef T U;
+  U u;                        // { dg-warning "unused variable" }
+}
+
+template void f_int<int>();
+
+
+template <class T>
+void f_intptr ()
+{
+  T *t = 0;                   // { dg-warning "unused variable" }
+
+  typedef T U;
+  U *u = 0;                   // { dg-warning "unused variable" }
+}
+
+template void f_intptr<int>();
+
+
+template <class T>
+void f_var_unused ()
+{
+  // The variable is marked unused.
+  T t UNUSED;
+
+  typedef T U;
+  U u UNUSED;
+}
+
+template void f_var_unused<int>();
+
+
+template <class T>
+void f_var_type_unused ()
+{
+  // The variable's type is marked unused.
+  T* UNUSED t = new T;   // { dg-bogus "unused variable" "bug 79585" { xfail *-*-* } }
+
+  typedef T U;
+  U* UNUSED u = new U;   // { dg-bogus "unused variable" "bug 79585" { xfail *-*-* } }
+
+  typedef T UNUSED U;
+  U v = U ();   // { dg-bogus "unused variable" "bug 79585" }
+}
+
+template void f_var_type_unused<int>();
+
+
+struct A { int i; };
+
+template <class T>
+void f_A ()
+{
+  T t;                        // { dg-warning "unused variable" }
+
+  typedef T U;
+  U u;                        // { dg-warning "unused variable" }
+}
+
+template void f_A<A>();
+
+
+template <class T>
+void f_A_unused ()
+{
+  T t UNUSED;
+
+  typedef T U;
+  U u UNUSED;
+}
+
+template void f_A_unused<A>();
+
+
+struct B { B (); };
+
+template <class T>
+void f_B ()
+{
+  T t;
+
+  typedef T U;
+  U u;
+}
+
+template void f_B<B>();
+
+
+struct NonTrivialDtor { ~NonTrivialDtor (); };
+
+template <class T>
+void f_with_NonTrivialDtor ()
+{
+  // Expect no warnings when instantiated on a type with a non-trivial
+  // destructor.
+  T t;
+
+  typedef T U;
+  U u;
+}
+
+template void f_with_NonTrivialDtor<NonTrivialDtor>();
+
+
+struct D { NonTrivialDtor b; };
+
+template <class T>
+void f_D ()
+{
+  // Same as f_with_NonTrivialDtor but with a class that has a member
+  // with a non-trivial dtor.
+  T t;
+
+  typedef T U;
+  U u;
+}
+
+template void f_D<D>();
+
+
+struct UNUSED DeclaredUnused { };
+
+template <class T>
+void f_with_unused ()
+{
+  // Expect no warnings when instantiatiated on a type declared
+  // with attribute unused.
+  T t;
+
+  typedef T U;
+  U u;
+}
+
+template void f_with_unused<DeclaredUnused>();