Fix crash in alias template CTAD with member template call in constructor noexcept [GH #532]

src/expr.c

@@ -54859,8 +54859,10 @@ is TRUE if the expression is the immediate operand of an "&" operator.
             }  /* if */
           } else if (is_nontype_template_param_symbol(fund_sym)) {
             /* A dependent construct.  Represent it generically with a
-               tpck_template_ref constant. */
-            expl_templ_arg_list =
+               tpck_template_ref constant.  Leave expl_templ_arg_list
+               unsubstituted, since scan_identifier will also substitute
+               it. */
+            a_template_arg_ptr  subst_templ_arg_list =
               copy_template_arg_list_with_substitution_rebuilding_arg_operands(
                                              fund_sym, expl_templ_arg_list,
                                              (a_template_param_ptr)NULL,
@@ -54875,7 +54877,7 @@ is TRUE if the expression is the immediate operand of an "&" operator.
               con->variant.template_param.variant.template_ref.con =
                                                          sym->variant.constant;
               con->variant.template_param.variant.template_ref.arg_list =
-                                                           expl_templ_arg_list;
+                                                          subst_templ_arg_list;
               con->type = type_of_unknown_templ_param_nontype;
               sym = alloc_symbol(sk_constant, sym->header,
                                  &sym->decl_position);

src/pending_changes/gh-532-alias-ctad-noexcept-template-call.txt

@@ -0,0 +1,18 @@
+[GH #532]
+Alias template CTAD with a template-id in a constructor noexcept specification
+
+Class template argument deduction through an alias template could crash
+when the noexcept specification of a constructor template of the underlying
+class template called a member function template with the constructor's
+template parameter pack as explicit template arguments, e.g. aliases of
+std::tuple with libstdc++.  The explicit template arguments were substituted
+twice.  For example, with --c++20:
+
+  template<typename... E> struct S {
+    template<typename... U> static constexpr bool n() { return true; }
+    S(const E&...) {}
+    template<typename... U> S(U&&...) noexcept(n<U...>()) {}
+  };
+  template<typename X, typename T> struct F {};
+  template<typename X, typename... T> using A = S<F<X, T>...>;
+  A a{F<int, int>{}};

tests/tests/regressions/.gh-532.rto/default.1.1.txt

@@ -0,0 +1 @@
+fe_only -DTEST_NUMBER=1 --c++20 Test_name.c

tests/tests/regressions/.gh-532.rto/default.2.1.txt

@@ -0,0 +1 @@
+fe_only -DTEST_NUMBER=2 --c++20 --g++ --gnu_version=150200 Test_name.c

tests/tests/regressions/.gh-532.rto/default.3.1.txt

@@ -0,0 +1 @@
+fe_only -DTEST_NUMBER=3 --c++20 --clang --clang_version=220100 Test_name.c

tests/tests/regressions/.gh-532.rto/default.4.1.txt

@@ -0,0 +1 @@
+fe_only -DTEST_NUMBER=4 --ms_c++20 --microsoft_version 1950 Test_name.c

tests/tests/regressions/gh-532.sft.cpp

@@ -0,0 +1,18 @@
+//type:fp
+//options:--c++20:--c++20 --g++ --gnu_version=150200:--c++20 --clang --clang_version=220100:--ms_c++20 --microsoft_version 1950
+
+// Class template argument deduction through an alias template crashed when
+// the noexcept specification of a constructor template called a member
+// function template with the constructor's parameter pack as explicit
+// template arguments, e.g. aliases of std::tuple with libstdc++.  The
+// explicit template arguments of the call were substituted twice.
+
+template<typename... E> struct S
+{
+  template<typename... U> static constexpr bool n() { return true; }
+  S(const E&...) {}
+  template<typename... U> S(U&&...) noexcept(n<U...>()) {}
+};
+template<typename X, typename T> struct F {};
+template<typename X, typename... T> using A = S<F<X, T>...>;
+A a{F<int, int>{}};