Do not call fn_sig on non-functions.

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -1918,6 +1918,14 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         receiver: Option<&'tcx hir::Expr<'tcx>>,
         args: &'tcx [hir::Expr<'tcx>],
     ) -> bool {
+        // Do not call `fn_sig` on non-functions.
+        if !matches!(
+            self.tcx.def_kind(def_id),
+            DefKind::Fn | DefKind::AssocFn | DefKind::Variant | DefKind::Ctor(..)
+        ) {
+            return false;
+        }
+
         let sig = self.tcx.fn_sig(def_id).skip_binder();
         let args_referencing_param: Vec<_> = sig
             .inputs()

src/test/ui/suggestions/assoc-const-as-fn.rs

@@ -0,0 +1,18 @@
+unsafe fn pointer(v: usize, w: u32) {}
+
+pub trait UniformScalar {}
+impl UniformScalar for u32 {}
+
+pub trait GlUniformScalar: UniformScalar {
+    const FACTORY: unsafe fn(usize, Self) -> ();
+}
+impl GlUniformScalar for u32 {
+    const FACTORY: unsafe fn(usize, Self) -> () = pointer;
+}
+
+pub fn foo<T: UniformScalar>(value: T) {
+    <T as GlUniformScalar>::FACTORY(1, value);
+    //~^ ERROR the trait bound `T: GlUniformScalar` is not satisfied
+}
+
+fn main() {}

src/test/ui/suggestions/assoc-const-as-fn.stderr

@@ -0,0 +1,14 @@
+error[E0277]: the trait bound `T: GlUniformScalar` is not satisfied
+  --> $DIR/assoc-const-as-fn.rs:14:5
+   |
+LL |     <T as GlUniformScalar>::FACTORY(1, value);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `GlUniformScalar` is not implemented for `T`
+   |
+help: consider further restricting this bound
+   |
+LL | pub fn foo<T: UniformScalar + GlUniformScalar>(value: T) {
+   |                             +++++++++++++++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.