[gcc r15-7834] c++: Check invalid use of constrained auto with trailing return type [PR100589]

[Patrick Palka]

https://gcc.gnu.org/g:7439febd94368f42bc46885224e22d2f135fedb2

commit r15-7834-g7439febd94368f42bc46885224e22d2f135fedb2
Author: Da Xie <xxie_xd@163.com>
Date:   Sun Mar 2 14:45:11 2025 +0800

    c++: Check invalid use of constrained auto with trailing return type [PR100589]
    
    Add check for constrained auto type specifier in function declaration or
    function type declaration with trailing return type. Issue error if such
    usage is detected.
    
    Test file renamed, and added a new test for type declaration.
    
    Successfully bootstrapped and regretested on x86_64-pc-linux-gnu:
    Added 6 passed and 4 unsupported tests.
    
            PR c++/100589
    
    gcc/cp/ChangeLog:
    
            * decl.cc (grokdeclarator): Issue an error for a declarator with
            constrained auto type specifier and trailing return types. Include
            function names if available.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp2a/concepts-pr100589.C: New test.
    
    Signed-off-by: Da Xie <xxie_xd@163.com>
    Reviewed-by: Patrick Palka <ppalka@redhat.com>
    Reviewed-by: Jason Merrill <jason@redhat.com>

Diff:
---
 gcc/cp/decl.cc                                 | 13 +++++++++++++
 gcc/testsuite/g++.dg/cpp2a/concepts-pr100589.C |  9 +++++++++
 2 files changed, 22 insertions(+)

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 9ca8c6c44816..337ee65752e4 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -14037,6 +14037,19 @@ grokdeclarator (const cp_declarator *declarator,
 				"invalid use of %<decltype(auto)%>");
 		    return error_mark_node;
 		  }
+		else if (is_constrained_auto (type))
+		  {
+		    if (funcdecl_p)
+		      error_at (typespec_loc,
+				"%qs function with trailing return type "
+				"has constrained %<auto%> type specifier "
+				"rather than plain %<auto%>",
+				name);
+		    else
+		      error_at (typespec_loc,
+				"invalid use of constrained %<auto%> type");
+		    return error_mark_node;
+		  }
 		tree tmpl = CLASS_PLACEHOLDER_TEMPLATE (auto_node);
 		if (!tmpl)
 		  if (tree late_auto = type_uses_auto (late_return_type))
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-pr100589.C b/gcc/testsuite/g++.dg/cpp2a/concepts-pr100589.C
new file mode 100644
index 000000000000..918ba50ac707
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-pr100589.C
@@ -0,0 +1,9 @@
+// PR c++/100589
+// { dg-do compile { target c++20 } }
+
+template<class T>
+concept false_concept = false;
+
+false_concept auto f() -> int; // { dg-error "'f' \[^\r\n\]* trailing return type has constrained 'auto'" }
+
+using type = false_concept auto() -> int; // { dg-error "invalid use of constrained 'auto' type" }