compiler.go: createBuiltin: accept alias for slice. Helps 1.18 tests pass.

Author: dkegel-fastly

compiler/compiler.go

@@ -1238,6 +1238,12 @@ func (b *builder) createBuiltin(argTypes []types.Type, argValues []llvm.Value, c
 			llvmCap = b.createRuntimeCall("chanCap", []llvm.Value{value}, "cap")
 		case *types.Slice:
 			llvmCap = b.CreateExtractValue(value, 2, "cap")
+		case *types.Named:
+			if _, ok := argTypes[0].Underlying().(*types.Slice); ok {
+				llvmCap = b.CreateExtractValue(value, 2, "cap")
+			} else {
+				return llvm.Value{}, b.makeError(pos, "todo: cap: unknown type")
+			}
 		default:
 			return llvm.Value{}, b.makeError(pos, "todo: cap: unknown type")
 		}