[gcc r17-2722] c++: vrp and pmf conversion [PR126310]

https://gcc.gnu.org/g:0e429acc24a2305c77da1229c456788e50edcae9

commit r17-2722-g0e429acc24a2305c77da1229c456788e50edcae9
Author: Jason Merrill <[email protected]>
Date:   Sun Jul 26 12:44:22 2026 -0400

    c++: vrp and pmf conversion [PR126310]
    
    Here when trying to convert to EncodeFnTy, aka void (sub::*)(int&&), we were
    instead converting to void (sub::*)(ValuePart&&), and the optimization added
    by r16-5887 got confused by the values in the array initializer having
    different RECORD_TYPEs (with different FIELD_DECLs) from the type of the
    array.
    
    So let's correct the conversion to actually end with the requested type.
    
            PR c++/126310
    
    gcc/cp/ChangeLog:
    
            * call.cc (standard_conversion): Don't build a mixed METHOD_TYPE if
            it isn't needed.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/opt/pmf2.C: New test.

Diff:
---
 gcc/cp/call.cc                  | 16 ++++++++++------
 gcc/testsuite/g++.dg/opt/pmf2.C | 37 +++++++++++++++++++++++++++++++++++++
 2 files changed, 47 insertions(+), 6 deletions(-)

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 143e85760b3b..62ad77a2db6f 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -1546,12 +1546,16 @@ standard_conversion (tree to, tree from, tree expr, bool c_cast_p,
 
       if (!same_type_p (fbase, tbase))
 	{
-	  from = build_memfn_type (fstat,
-				   tbase,
-				   cp_type_quals (tbase),
-				   type_memfn_rqual (tofn));
-	  from = build_ptrmemfunc_type (build_pointer_type (from));
-	  conv = build_conv (ck_pmem, from, conv);
+	  tree first = to;
+	  if (!same_type_p (tstat, fstat))
+	    {
+	      first = build_memfn_type (fstat,
+					tbase,
+					cp_type_quals (tbase),
+					type_memfn_rqual (tofn));
+	      first = build_ptrmemfunc_type (build_pointer_type (first));
+	    }
+	  conv = build_conv (ck_pmem, first, conv);
 	  conv->base_p = true;
 	}
       if (fnptr_conv_p (tstat, fstat))
diff --git a/gcc/testsuite/g++.dg/opt/pmf2.C b/gcc/testsuite/g++.dg/opt/pmf2.C
new file mode 100644
index 000000000000..88c38a8ea2a4
--- /dev/null
+++ b/gcc/testsuite/g++.dg/opt/pmf2.C
@@ -0,0 +1,37 @@
+// PR c++/126310
+// { dg-do run { target c++11 } }
+// { dg-additional-options -O }
+
+typedef int ValuePart;
+struct base0  {
+  int t;
+};
+struct base1 {
+  void gg(ValuePart&&) {symbols = 1; }
+  int symbols;
+};
+struct sub : base0, base1 {};
+
+using EncodeFnTy = void (sub::*)(int&&);
+static const EncodeFnTy encode_fns[]
+{
+    &sub::gg,
+    &sub::gg
+};
+
+[[gnu::noinline,gnu::noclone]]
+void f(sub &m, bool i)
+{
+ (&m->*encode_fns[i])(0);
+}
+int main()
+{
+    sub a;
+    a.t = 0;
+    a.symbols = 0;
+    f(a, 0);
+    if (a.t != 0)
+      __builtin_abort ();
+    if (a.symbols != 1)
+      __builtin_abort ();
+}