[C++ PATCH] Fix unnecessary error when top-level cv-qualifiers is dropped (PR c++/101783)

[nick huang via Gcc-patches]

The root cause of this bug is that it considers reference with cv-qualifiers 
as an error by generating value for variable "bad_quals". However, this is 
not correct for case of typedef. Here I quote spec:
"Cv-qualified references are ill-formed except when the cv-qualifiers
are introduced through the use of a typedef-name ([dcl.typedef],
[temp.param]) or decltype-specifier ([dcl.type.decltype]),
in which case the cv-qualifiers are ignored."

2021-08-09  Qingzhe Huang  <nickhuan...@hotmail.com>

PR c++/101387

gcc/cp/ChangeLog:
        PR c++/101387
        * tree.c (cp_build_qualified_type_real): Excluding typedef from error

gcc/testsuite/ChangeLog:
        PR c++/101387
        * g++.dg/parse/pr101783.C: New test.

diff --git gcc/cp/tree.c gcc/cp/tree.c
index 10b818d1370..a4ae3eee6d0 100644
--- gcc/cp/tree.c
+++ gcc/cp/tree.c
@@ -1361,11 +1361,18 @@ cp_build_qualified_type_real (tree type,
   /* A reference or method type shall not be cv-qualified.
      [dcl.ref], [dcl.fct].  This used to be an error, but as of DR 295
      (in CD1) we always ignore extra cv-quals on functions.  */
+  /* Cv-qualified references are ill-formed except when the cv-qualifiers
+     are introduced through the use of a typedef-name ([dcl.typedef],
+     [temp.param]) or decltype-specifier ([dcl.type.decltype]),
+     in which case the cv-qualifiers are ignored.  */
   if (type_quals & (TYPE_QUAL_CONST | TYPE_QUAL_VOLATILE)
       && (TYPE_REF_P (type)
          || FUNC_OR_METHOD_TYPE_P (type)))
     {
-      if (TYPE_REF_P (type))
+      if (TYPE_REF_P (type)
+       //A reference from typedef is not an error, but ignored.
+       && (!typedef_variant_p (type) || FUNC_OR_METHOD_TYPE_P (type)))
        bad_quals |= type_quals & (TYPE_QUAL_CONST | TYPE_QUAL_VOLATILE);
       type_quals &= ~(TYPE_QUAL_CONST | TYPE_QUAL_VOLATILE);
     }

diff --git gcc/testsuite/g++.dg/parse/pr101783.C 
gcc/testsuite/g++.dg/parse/pr101783.C
new file mode 100644
index 00000000000..8f8f8b1faeb
--- /dev/null
+++ gcc/testsuite/g++.dg/parse/pr101783.C
@@ -0,0 +1,10 @@
+template<class T> struct A{
+        typedef T& Type;
+};
+template<class T>
+void f(const typename A<T>::Type){}
+template<>
+void f<int>(const typename A<int>::Type){}
+struct B{};
+template<>
+void f<B>(const A<B>::Type){}