[PATCH] c++: explicit() ignored on deduction guide [PR100065]

[Marek Polacek via Gcc-patches]

When we have explicit() with a value-dependent argument, we can't
evaluate it at parsing time, so cp_parser_function_specifier_opt stashes
the argument into the decl-specifiers and grokdeclarator then stores it
into explicit_specifier_map, which is then used when substituting the
function decl.  grokdeclarator stores it for constructors and conversion
functions, but we also need to do it for deduction guides, otherwise
we'll forget that we've seen an explicit-specifier as in the attached
test.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/branches?

        PR c++/100065

gcc/cp/ChangeLog:

        * decl.c (grokdeclarator): Store a value-dependent
        explicit-specifier even for deduction guides.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp2a/explicit18.C: New test.
---
 gcc/cp/decl.c                           |  2 ++
 gcc/testsuite/g++.dg/cpp2a/explicit18.C | 23 +++++++++++++++++++++++
 2 files changed, 25 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/explicit18.C

diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index a3687dbb0dd..cbf647dd569 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -14043,6 +14043,8 @@ grokdeclarator (const cp_declarator *declarator,
                storage_class = sc_none;
              }
          }
+       if (declspecs->explicit_specifier)
+         store_explicit_specifier (decl, declspecs->explicit_specifier);
       }
     else
       {
diff --git a/gcc/testsuite/g++.dg/cpp2a/explicit18.C 
b/gcc/testsuite/g++.dg/cpp2a/explicit18.C
new file mode 100644
index 00000000000..c8916fa4743
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/explicit18.C
@@ -0,0 +1,23 @@
+// PR c++/100065
+// { dg-do compile { target c++20 } }
+
+template<bool B>
+struct bool_constant {
+  static constexpr bool value = B;
+  constexpr operator bool() const { return value; }
+};
+
+using true_type = bool_constant<true>;
+using false_type = bool_constant<false>;
+
+template<bool>
+struct X {
+    template<typename T>
+    X(T);
+};
+
+template<bool b>
+explicit(b) X(bool_constant<b>) -> X<b>;
+
+X false_ = false_type{}; // OK
+X true_  = true_type{};  // { dg-error "explicit deduction guide" }

base-commit: e89759fdfc80db223bd852aba937acb2d7c2cd80
-- 
2.31.1