Sema: add error for resolving inferred error set of generic function

Closes #14991

src/Sema.zig

@@ -31523,6 +31523,16 @@ fn resolveInferredErrorSet(
     if (ies_func_info.return_type.tag() == .generic_poison) {
         assert(ies_func_info.cc == .Inline);
     } else if (ies_func_info.return_type.errorUnionSet().castTag(.error_set_inferred).?.data == ies) {
+        if (ies_func_info.is_generic) {
+            const msg = msg: {
+                const msg = try sema.errMsg(block, src, "unable to resolve inferred error set of generic function", .{});
+                errdefer msg.destroy(sema.gpa);
+
+                try sema.mod.errNoteNonLazy(ies_func_owner_decl.srcLoc(), msg, "generic function declared here", .{});
+                break :msg msg;
+            };
+            return sema.failWithOwnedErrorMsg(msg);
+        }
         // In this case we are dealing with the actual InferredErrorSet object that
         // corresponds to the function, not one created to track an inline/comptime call.
         try sema.ensureFuncBodyAnalyzed(ies.func);

test/cases/compile_errors/resolve_inferred_error_set_of_generic_fn.zig

@@ -0,0 +1,18 @@
+fn foo(a: anytype) !void {
+    if (a == 0) return error.A;
+    return error.B;
+}
+const Error = error{ A, B };
+export fn entry() void {
+    const info = @typeInfo(@TypeOf(foo));
+    const ret_type = info.Fn.return_type.?;
+    const error_set = @typeInfo(ret_type).ErrorUnion.error_set;
+    _ = Error || error_set;
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :10:15: error: unable to resolve inferred error set of generic function
+// :1:1: note: generic function declared here