Add regression test for #141738

Using a struct constructor (DefKind::Ctor(Struct, Const)) as an array
repeat count with `#![feature(min_generic_const_args)]` used to trigger
an ICE in const alias normalization. This was fixed by PR #150704, which
added const constructor support for MGCA. Add a regression test to
ensure the compiler produces a proper type error instead of panicking.

Closes #141738

Author: AprilNEA

tests/ui/const-generics/mgca/struct-ctor-in-array-len.rs

@@ -0,0 +1,18 @@
+// Regression test for https://github.com/rust-lang/rust/issues/141738
+//
+// Using a struct constructor as an array repeat count with
+// `min_generic_const_args` used to ICE with "unexpected `DefKind`
+// for const alias to resolve to: Ctor(Struct, Const)".
+// It should now produce a proper type error.
+
+#![feature(min_generic_const_args)]
+//~^ WARN the feature `min_generic_const_args` is incomplete
+
+struct S
+where
+    (): ;
+
+fn main() {
+    let _b = [0; S];
+    //~^ ERROR the constant `S` is not of type `usize`
+}

tests/ui/const-generics/mgca/struct-ctor-in-array-len.stderr

@@ -0,0 +1,19 @@
+warning: the feature `min_generic_const_args` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/struct-ctor-in-array-len.rs:8:12
+   |
+LL | #![feature(min_generic_const_args)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #132980 <https://github.com/rust-lang/rust/issues/132980> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error: the constant `S` is not of type `usize`
+  --> $DIR/struct-ctor-in-array-len.rs:16:14
+   |
+LL |     let _b = [0; S];
+   |              ^^^^^^ expected `usize`, found `S`
+   |
+   = note: the length of array `[{integer}; S]` must be type `usize`
+
+error: aborting due to 1 previous error; 1 warning emitted
+