On Mon, 16 Feb 2026, Patrick Palka wrote:
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look
> OK for trunk/15/14?
>
> -- >8 --
>
> This patch makes the default case of unify accept LAMBDA_EXPR P/A pairs,
> which we can legitimately hit during alias CTAD guide overload resolution.
> We can also less legimitately hit this during partial specialization
> matching (likely IFNDR).
>
> PR c++/122318
> PR c++/101670
>
> gcc/cp/ChangeLog:
>
> * pt.cc (unify) <default>: Handle LAMBDA_EXPR.
FWIW I was torn between handling them like any other non-deducible
expression vs handling them in a separate case (returning success iff
they are ==). I couldn't come up with a testcase for which it mattered
so I went with the simpler approach.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/cpp2a/class-deduction-alias27.C: New test.
> ---
> gcc/cp/pt.cc | 1 +
> gcc/testsuite/g++.dg/cpp2a/lambda-targ20.C | 14 ++++++++++++++
> gcc/testsuite/g++.dg/cpp2a/lambda-targ21.C | 10 ++++++++++
> 3 files changed, 25 insertions(+)
> create mode 100644 gcc/testsuite/g++.dg/cpp2a/lambda-targ20.C
> create mode 100644 gcc/testsuite/g++.dg/cpp2a/lambda-targ21.C
>
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 1d11b07d567d..5db46cd707f1 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -26650,6 +26650,7 @@ unify (tree tparms, tree targs, tree parm, tree arg,
> int strict,
> return unify_success (explain_p);
> gcc_assert (EXPR_P (parm)
> || TREE_CODE (parm) == CONSTRUCTOR
> + || TREE_CODE (parm) == LAMBDA_EXPR
> || TREE_CODE (parm) == TRAIT_EXPR);
> expr:
> /* We must be looking at an expression. This can happen with
> diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-targ20.C
> b/gcc/testsuite/g++.dg/cpp2a/lambda-targ20.C
> new file mode 100644
> index 000000000000..4d444bbe1121
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/lambda-targ20.C
> @@ -0,0 +1,14 @@
> +// PR c++/122318
> +// { dg-do compile { target c++20 } }
> +
> +template<class T, auto V>
> +struct A {
> + A() { }
> + A(T) { }
> +};
> +
> +template<class T>
> +using AT = A<T, []{}>;
> +
> +AT<int> a;
> +AT b = a;
> diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-targ21.C
> b/gcc/testsuite/g++.dg/cpp2a/lambda-targ21.C
> new file mode 100644
> index 000000000000..73656422333f
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/lambda-targ21.C
> @@ -0,0 +1,10 @@
> +// PR c++/101670
> +// { dg-do compile { target c++20 } }
> +
> +template<int, auto = []{}>
> +struct A;
> +
> +template<int N>
> +struct A<N> { }; // Partial specialization unusable due to lambda uniqueness
> +
> +A<0> a; // { dg-error "incomplete" }
> --
> 2.53.0.118.g852829b3dd
>
>
