Rollup merge of #67543 - JohnTitor:regression-tests, r=Centril

Add regression tests for fixed ICEs

Closes #61747 (fixed from 1.41.0-nightly (4007d4ef2 2019-12-01))
Closes #66205 (fixed from 1.41.0-nightly (4007d4ef2 2019-12-01))
Closes #66270 (fixed by #66246)
Closes #67424 (fixed by #67160)

Also picking a minor nit up from #67071 with 101dd7bad9

r? @Centril

src/test/ui/const-generics/issues/issue-61747.rs

@@ -0,0 +1,16 @@
+// check-pass
+
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete and may cause the compiler to crash
+
+struct Const<const N: usize>;
+
+impl<const C: usize> Const<{C}> {
+    fn successor() -> Const<{C + 1}> {
+        Const
+    }
+}
+
+fn main() {
+    let _x: Const::<2> = Const::<1>::successor();
+}

src/test/ui/const-generics/issues/issue-61747.stderr

@@ -0,0 +1,8 @@
+warning: the feature `const_generics` is incomplete and may cause the compiler to crash
+  --> $DIR/issue-61747.rs:3:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+

src/test/ui/const-generics/issues/issue-66205.rs

@@ -0,0 +1,10 @@
+// check-pass
+
+#![allow(incomplete_features, dead_code, unconditional_recursion)]
+#![feature(const_generics)]
+
+fn fact<const N: usize>() {
+    fact::<{ N - 1 }>();
+}
+
+fn main() {}