Sema: fix illegal multi level pointer coercions

Removes an incorrect check that allowed pointers to mutable pointers to
coerce to any other pointer to a mutable pointer.

Author: Justus2308

src/Sema.zig

@@ -30179,7 +30179,7 @@ fn coerceInMemoryAllowedPtrs(
         src_src,
         null,
     );
-    if (child != .ok and !dest_is_mut) allow: {
+    if (child != .ok) allow: {
         // As a special case, we also allow coercing `*[n:s]T` to `*[n]T`, akin to dropping the sentinel from a slice.
         // `*[n:s]T` cannot coerce in memory to `*[n]T` since they have different sizes.
         if (src_child.zigTypeTag(zcu) == .array and dest_child.zigTypeTag(zcu) == .array and

test/cases/compile_errors/coerce_pointers_with_uncoercable_child_pointers.zig

@@ -0,0 +1,42 @@
+export fn entry1() void {
+    const p: **u32 = undefined;
+    const q: **i32 = p;
+    _ = q;
+}
+
+export fn entry2() void {
+    const p: [*]*u32 = undefined;
+    const q: [*]*i32 = p;
+    _ = q;
+}
+
+export fn entry3() void {
+    const p: []*u32 = undefined;
+    const q: []*i32 = p;
+    _ = q;
+}
+
+export fn entry4() void {
+    const p: [*c]*u32 = undefined;
+    const q: [*c]*i32 = p;
+    _ = q;
+}
+
+// error
+//
+// :3:22: error: expected type '**i32', found '**u32'
+// :3:22: note: pointer type child '*u32' cannot cast into pointer type child '*i32'
+// :3:22: note: pointer type child 'u32' cannot cast into pointer type child 'i32'
+// :3:22: note: signed 32-bit int cannot represent all possible unsigned 32-bit values
+// :9:24: error: expected type '[*]*i32', found '[*]*u32'
+// :9:24: note: pointer type child '*u32' cannot cast into pointer type child '*i32'
+// :9:24: note: pointer type child 'u32' cannot cast into pointer type child 'i32'
+// :9:24: note: signed 32-bit int cannot represent all possible unsigned 32-bit values
+// :15:23: error: expected type '[]*i32', found '[]*u32'
+// :15:23: note: pointer type child '*u32' cannot cast into pointer type child '*i32'
+// :15:23: note: pointer type child 'u32' cannot cast into pointer type child 'i32'
+// :15:23: note: signed 32-bit int cannot represent all possible unsigned 32-bit values
+// :21:25: error: expected type '[*c]*i32', found '[*c]*u32'
+// :21:25: note: pointer type child '*u32' cannot cast into pointer type child '*i32'
+// :21:25: note: pointer type child 'u32' cannot cast into pointer type child 'i32'
+// :21:25: note: signed 32-bit int cannot represent all possible unsigned 32-bit values