Add regression test

tests/cases/compiler/functionCallOnConstrainedTypeVariable.ts

@@ -0,0 +1,22 @@
+// @strict: true
+
+// Repro from #20196
+
+type A = {
+  a: (x: number) => string
+};
+type B = {
+  a: (x: boolean) => string
+};
+
+function call0(p: A | B) { 
+  p.a("s"); // Error
+}
+
+function callN<T extends A | B>(p: T) {
+  p.a("s"); // Error
+  
+  var a: T["a"] = p.a;
+  a(""); // Error
+  a("", "", "", ""); // Error
+}