[COMMITTED] c++: [PR90782] Add testcase

This testcase was fixed by r12-1744-g3eecc1 as it make
sense it fixed a few other class deduction issues.
So I thought I would add a testcase for this PR and close
it as fixed.

Committed after a quick test of the testcase.

	PR c++/90782

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/class-deduction100.C: New test.

gcc/testsuite/g++.dg/cpp1z/class-deduction100.C

@@ -0,0 +1,13 @@
+// PR c++/90782
+// { dg-do compile { target c++17 } }
+
+template<class... A>
+struct bar {
+    template<class B>
+    bar(B& obj, void(B::*f)(A...)const=&B::operator()){}
+};
+int main() {
+    const auto f1 = [](){};
+    bar f8(f1);
+}
+