Add regression tests

tests/cases/compiler/strictSubtypeReduction.ts

@@ -0,0 +1,34 @@
+// @strict: true
+
+// Repro from #41977
+
+class S1 {
+  static f(a: number | string): void {}
+}
+
+class S2 {
+  static f(a: number): void {}
+  static g(a: number): void {}
+}
+
+function f(a: number): void {}
+function g(a: number): void {}
+
+// Declaring the following type aliases should have no effect
+
+type T1 = typeof S2.g;
+type T2 = typeof g;
+
+// All should have type ((a: number) => void)[]
+
+const y1 = [S1.f, f];
+const y2 = [S1.f, g];
+const y3 = [S1.f, S2.f];
+const y4 = [S1.f, S2.g];
+
+// All assignments should be errors
+
+const x1: ((ctrl: number | string) => void)[] = y1;
+const x2: ((ctrl: number | string) => void)[] = y2;
+const x3: ((ctrl: number | string) => void)[] = y3;
+const x4: ((ctrl: number | string) => void)[] = y4;