diff --git a/compiler/rustc_mir/src/interpret/validity.rs b/compiler/rustc_mir/src/interpret/validity.rs
index 423d1270ac865..0b7492631c41d 100644
--- a/compiler/rustc_mir/src/interpret/validity.rs
+++ b/compiler/rustc_mir/src/interpret/validity.rs
@@ -515,7 +515,11 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValidityVisitor<'rt, 'mir, '
                 Ok(true)
             }
             ty::Float(_) | ty::Int(_) | ty::Uint(_) => {
-                let value = self.ecx.read_scalar(value)?;
+                let value = try_validation!(
+                    self.ecx.read_scalar(value),
+                    self.path,
+                    err_unsup!(ReadPointerAsBytes) => { "read of part of a pointer" },
+                );
                 // NOTE: Keep this in sync with the array optimization for int/float
                 // types below!
                 if self.ctfe_mode.is_some() {
diff --git a/src/test/ui/consts/issue-79690.rs b/src/test/ui/consts/issue-79690.rs
new file mode 100644
index 0000000000000..a2e7b97b3187d
--- /dev/null
+++ b/src/test/ui/consts/issue-79690.rs
@@ -0,0 +1,32 @@
+// ignore-32bit
+// This test gives a different error on 32-bit architectures.
+
+union Transmute<T: Copy, U: Copy> {
+    t: T,
+    u: U,
+}
+trait Bar {
+    fn bar(&self) -> u32;
+}
+struct Foo {
+    foo: u32,
+    bar: bool,
+}
+impl Bar for Foo {
+    fn bar(&self) -> u32 {
+        self.foo
+    }
+}
+#[derive(Copy, Clone)]
+struct Fat<'a>(&'a Foo, &'static VTable);
+struct VTable {
+    size: Foo,
+}
+const FOO: &dyn Bar = &Foo {
+    foo: 128,
+    bar: false,
+};
+const G: Fat = unsafe { Transmute { t: FOO }.u };
+//~^ ERROR it is undefined behavior to use this value
+
+fn main() {}
diff --git a/src/test/ui/consts/issue-79690.stderr b/src/test/ui/consts/issue-79690.stderr
new file mode 100644
index 0000000000000..918dd4c20f96c
--- /dev/null
+++ b/src/test/ui/consts/issue-79690.stderr
@@ -0,0 +1,11 @@
+error[E0080]: it is undefined behavior to use this value
+  --> $DIR/issue-79690.rs:29:1
+   |
+LL | const G: Fat = unsafe { Transmute { t: FOO }.u };
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered read of part of a pointer at .1.<deref>.size.foo
+   |
+   = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0080`.