c++: lambda in function template signature [PR119401]

Here we instantiate the lambda three times in producing A<0>::f:
1) in tsubst_function_type, substituting the type of A<>::f
2) in tsubst_function_decl, substituting the parameters of A<>::f
3) in regenerate_decl_from_template when instantiating A<>::f

The first one gets thrown away by maybe_rebuild_function_decl_type.  Before
r15-7202, we happily built all of them and mangled the result wrongly as
lambda gcc-mirror#3.  After r15-7202, we try to mangle gcc-mirror#3 as gcc-mirror#1, which breaks because
 gcc-mirror#1 is already mangled as gcc-mirror#1.

This patch avoids building gcc-mirror#3 by suppressing regenerate_decl_from_template
if the template signature includes a lambda, fixing the ICE.

We now mangle the lambda as gcc-mirror#2, which is still wrong.  Addressing that
should involve not calling tsubst_function_type from tsubst_function_decl,
and building the type from the parms types in the first place rather than
fixing it up in maybe_rebuild_function_decl_type.

	PR c++/119401

gcc/cp/ChangeLog:

	* pt.cc (regenerate_decl_from_template): Don't regenerate if the
	signature involves a lambda.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/lambda-targ11.C: New test.

Author: jicama

gcc/cp/pt.cc

@@ -27238,6 +27238,19 @@ regenerate_decl_from_template (tree decl, tree tmpl, tree args)
       if (DECL_UNIQUE_FRIEND_P (decl))
 	goto done;
 
+      /* A template with a lambda in the signature also changes type if
+	 regenerated (PR119401).  */
+      walk_tree_fn find_lambda
+	= [](tree *tp, int *, void *)
+	{
+	  if (TREE_CODE (*tp) == LAMBDA_EXPR)
+	    return *tp;
+	  return NULL_TREE;
+	};
+      if (cp_walk_tree_without_duplicates
+	  (&TREE_TYPE (tmpl), find_lambda, nullptr))
+	goto done;
+
       /* Use the source location of the definition.  */
       DECL_SOURCE_LOCATION (decl) = DECL_SOURCE_LOCATION (tmpl);
 

gcc/testsuite/g++.dg/cpp2a/lambda-targ11.C

@@ -0,0 +1,13 @@
+// PR c++/119401
+// { dg-do compile { target c++20 } }
+
+template <auto>
+struct B {};
+template <int N>
+struct A {
+  void f(B<[]{}>) {}
+};
+auto t = &A<0>::f;
+
+// A<0>::f(B<A<0>::{lambda()#1}{}>)
+// { dg-final { scan-assembler "_ZN1AILi0EE1fE1BIXtlNS0_UlvE_EEEE" { xfail *-*-* } } }

gcc/testsuite/g++.dg/cpp2a/lambda-targ12.C

@@ -0,0 +1,13 @@
+// PR c++/119401
+// { dg-do compile { target c++20 } }
+
+template <class>
+struct B {};
+template <int N>
+struct A {
+  void f(B<decltype([]{})>) {}
+};
+auto t = &A<0>::f;
+
+// A<0>::f(B<A<0>::{lambda()#1}>)
+// { dg-final { scan-assembler "_ZN1AILi0EE1fE1BINS0_UlvE_EE" { xfail *-*-* } } }