Expect parentheses after an alternative only if they were present in its declaration (#2605)

The design of choice types expects the declaration of an alternative to match the usage: if an alternative is declared as `None`, then it should be used as `None` not `None()`, and if it is declared as `None()` then it should be used as `None()` not `None`. Update explorer to match.

Also clean up the handling of choice types and alternative values a little in general, by moving away from identifying choice types and alternatives as strings and towards identifying them symbolically.

Closes #2422

explorer/testdata/basic_syntax/choice.carbon

@@ -5,7 +5,7 @@
 // AUTOUPDATE
 // RUN: %{explorer-run}
 // RUN: %{explorer-run-trace}
-// CHECK:STDOUT: result: 0
+// CHECK:STDOUT: result: 1
 
 package ExplorerTest api;
 
@@ -15,26 +15,30 @@ choice Ints {
   Two(i32, i32)
 }
 
-fn Main() -> i32 {
-  var x: Ints = Ints.None();
-  var y: Ints = Ints.One(42);
-  var n: i32 = 0;
-  match (y) {
-    case Ints.None() => { n = n + 2; }
-    case Ints.One(x: auto) => { n = x + 1 - 42; }
-    case Ints.Two(a: auto, b: auto) => { n = 2; }
-  }
-  match (x) {
-    case Ints.One(x: auto) => { n = x + 2; }
-    case Ints.None() => { n = n - 1; }
-    case Ints.Two(x: auto, y: auto) => { n = 5; }
-  }
-  return n;
-}
-
 // Test some alternate syntaxes
 choice MoreInts {
   None(),
   One(i32),
   Two(i32, i32),
 }
+
+fn Main() -> i32 {
+  var x: Ints = Ints.None;
+  var y: Ints = Ints.One(42);
+  var z: MoreInts = MoreInts.None();
+  var n: i32 = 0;
+  match (y) {
+    case Ints.None => { n = n + 2; }
+    case Ints.One(x: auto) => { n = x + 1 - 42; }
+    case Ints.Two(a: auto, b: auto) => { n = 2; }
+  }
+  match (x) {
+    case Ints.One(x: auto) => { n = x + 2; }
+    case Ints.None => { n = n - 1; }
+    case Ints.Two(x: auto, y: auto) => { n = 5; }
+  }
+  match (z) {
+    case MoreInts.None() => { ++n; }
+  }
+  return n;
+}

explorer/testdata/basic_syntax/fail_choice_extra_parens.carbon

@@ -0,0 +1,23 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+// RUN: %{not} %{explorer-run}
+// RUN: %{not} %{explorer-run-trace}
+
+package ExplorerTest api;
+
+choice Ints {
+  None,
+  One(i32),
+  Two(i32, i32)
+}
+
+fn Main() -> i32 {
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_choice_extra_parens.carbon:[[@LINE+1]]: alternative `choice Ints.None` does not expect an argument list
+  match (Ints.None()) {
+    case Ints.None => { return 0; }
+    default => { return 1; }
+  }
+}

explorer/testdata/basic_syntax/fail_choice_no_parens.carbon

@@ -9,17 +9,17 @@
 package ExplorerTest api;
 
 choice Ints {
-  None,
+  None(),
   One(i32),
   Two(i32, i32)
 }
 
 fn Main() -> i32 {
-  match (Ints.None()) {
-    case Ints.None => { return 0; }
+  match (Ints.None) {
+    case Ints.None() => { return 0; }
     // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_choice_no_parens.carbon:[[@LINE+3]]: type error in `match` pattern type
-    // CHECK:STDERR: expected: fn () -> choice Ints
-    // CHECK:STDERR: actual: choice Ints
+    // CHECK:STDERR: expected: choice Ints
+    // CHECK:STDERR: actual: fn () -> choice Ints
     default => { return 1; }
   }
 }

explorer/testdata/basic_syntax/fail_choice_pattern_extra_parens.carbon

@@ -0,0 +1,23 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+// RUN: %{not} %{explorer-run}
+// RUN: %{not} %{explorer-run-trace}
+
+package ExplorerTest api;
+
+choice Ints {
+  None,
+  One(i32),
+  Two(i32, i32)
+}
+
+fn Main() -> i32 {
+  match (Ints.None) {
+    // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_choice_pattern_extra_parens.carbon:[[@LINE+1]]: alternative `choice Ints.None` does not expect an argument list
+    case Ints.None() => { return 0; }
+    default => { return 1; }
+  }
+}

explorer/testdata/basic_syntax/fail_choice_pattern_no_parens.carbon

@@ -0,0 +1,25 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+// RUN: %{not} %{explorer-run}
+// RUN: %{not} %{explorer-run-trace}
+
+package ExplorerTest api;
+
+choice Ints {
+  None(),
+  One(i32),
+  Two(i32, i32)
+}
+
+fn Main() -> i32 {
+  match (Ints.None()) {
+    case Ints.None => { return 0; }
+    // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/basic_syntax/fail_choice_pattern_no_parens.carbon:[[@LINE+3]]: type error in `match` pattern type
+    // CHECK:STDERR: expected: fn () -> choice Ints
+    // CHECK:STDERR: actual: choice Ints
+    default => { return 1; }
+  }
+}