Factor out a Pattern sum type from Expression (#685)

`Pattern` is intended to pilot some changes I would like to apply to all our sum types:
- The alternatives are expressed as derived classes rather than members of a `std::variant`.
- The alternatives are classes in the [style guide sense](https://google.github.io/styleguide/cppguide.html#Structs_vs._Classes), meaning they can have invariants, but can't have public data members.
- Creating an object is expressed using a constructor rather than a factory function.
- Accessing an alternative is expressed as a cast (using LLVM's RTTI system) rather than `std::get` or a `Get` method.

Co-authored-by: Jon Meow <46229924+jonmeow@users.noreply.github.com>

executable_semantics/ast/pattern_test.cpp

@@ -0,0 +1,130 @@
+// 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
+
+#include "executable_semantics/ast/pattern.h"
+
+#include "executable_semantics/ast/expression.h"
+#include "executable_semantics/syntax/paren_contents.h"
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "llvm/Support/Casting.h"
+
+namespace Carbon {
+namespace {
+
+using llvm::cast;
+using llvm::isa;
+using testing::ElementsAre;
+using testing::IsEmpty;
+
+// Matches a TuplePattern::Field named `name` whose `pattern` is an
+// `AutoPattern`.
+MATCHER_P(AutoFieldNamed, name, "") {
+  return arg.name == std::string(name) && isa<AutoPattern>(arg.pattern);
+}
+
+TEST(PatternTest, EmptyAsPattern) {
+  ParenContents<Pattern> contents = {.elements = {},
+                                     .has_trailing_comma = false};
+  const Pattern* pattern = PatternFromParenContents(/*line_num=*/1, contents);
+  EXPECT_EQ(pattern->LineNumber(), 1);
+  ASSERT_TRUE(isa<TuplePattern>(pattern));
+  EXPECT_THAT(cast<TuplePattern>(pattern)->Fields(), IsEmpty());
+}
+
+TEST(PatternTest, EmptyAsTuplePattern) {
+  ParenContents<Pattern> contents = {.elements = {},
+                                     .has_trailing_comma = false};
+  const TuplePattern* tuple =
+      TuplePatternFromParenContents(/*line_num=*/1, contents);
+  EXPECT_EQ(tuple->LineNumber(), 1);
+  EXPECT_THAT(tuple->Fields(), IsEmpty());
+}
+
+TEST(PatternTest, UnaryNoCommaAsPattern) {
+  // Equivalent to a code fragment like
+  // ```
+  // (
+  //   auto
+  // )
+  // ```
+  ParenContents<Pattern> contents = {
+      .elements = {{.name = std::nullopt,
+                    .term = new AutoPattern(/*line_num=*/2)}},
+      .has_trailing_comma = false};
+
+  const Pattern* pattern = PatternFromParenContents(/*line_num=*/1, contents);
+  EXPECT_EQ(pattern->LineNumber(), 2);
+  ASSERT_TRUE(isa<AutoPattern>(pattern));
+}
+
+TEST(PatternTest, UnaryNoCommaAsTuplePattern) {
+  ParenContents<Pattern> contents = {
+      .elements = {{.name = std::nullopt,
+                    .term = new AutoPattern(/*line_num=*/2)}},
+      .has_trailing_comma = false};
+
+  const TuplePattern* tuple =
+      TuplePatternFromParenContents(/*line_num=*/1, contents);
+  EXPECT_EQ(tuple->LineNumber(), 1);
+  EXPECT_THAT(tuple->Fields(), ElementsAre(AutoFieldNamed("0")));
+}
+
+TEST(PatternTest, UnaryWithCommaAsPattern) {
+  ParenContents<Pattern> contents = {
+      .elements = {{.name = std::nullopt,
+                    .term = new AutoPattern(/*line_num=*/2)}},
+      .has_trailing_comma = true};
+
+  const Pattern* pattern = PatternFromParenContents(/*line_num=*/1, contents);
+  EXPECT_EQ(pattern->LineNumber(), 1);
+  ASSERT_TRUE(isa<TuplePattern>(pattern));
+  EXPECT_THAT(cast<TuplePattern>(pattern)->Fields(),
+              ElementsAre(AutoFieldNamed("0")));
+}
+
+TEST(PatternTest, UnaryWithCommaAsTuplePattern) {
+  ParenContents<Pattern> contents = {
+      .elements = {{.name = std::nullopt,
+                    .term = new AutoPattern(/*line_num=*/2)}},
+      .has_trailing_comma = true};
+
+  const TuplePattern* tuple =
+      TuplePatternFromParenContents(/*line_num=*/1, contents);
+  EXPECT_EQ(tuple->LineNumber(), 1);
+  EXPECT_THAT(tuple->Fields(), ElementsAre(AutoFieldNamed("0")));
+}
+
+TEST(PatternTest, BinaryAsPattern) {
+  ParenContents<Pattern> contents = {
+      .elements = {{.name = std::nullopt,
+                    .term = new AutoPattern(/*line_num=*/2)},
+                   {.name = std::nullopt,
+                    .term = new AutoPattern(/*line_num=*/3)}},
+      .has_trailing_comma = true};
+
+  const Pattern* pattern = PatternFromParenContents(/*line_num=*/1, contents);
+  EXPECT_EQ(pattern->LineNumber(), 1);
+  ASSERT_TRUE(isa<TuplePattern>(pattern));
+  EXPECT_THAT(cast<TuplePattern>(pattern)->Fields(),
+              ElementsAre(AutoFieldNamed("0"), AutoFieldNamed("1")));
+}
+
+TEST(PatternTest, BinaryAsTuplePattern) {
+  ParenContents<Pattern> contents = {
+      .elements = {{.name = std::nullopt,
+                    .term = new AutoPattern(/*line_num=*/2)},
+                   {.name = std::nullopt,
+                    .term = new AutoPattern(/*line_num=*/3)}},
+      .has_trailing_comma = true};
+
+  const TuplePattern* tuple =
+      TuplePatternFromParenContents(/*line_num=*/1, contents);
+  EXPECT_EQ(tuple->LineNumber(), 1);
+  EXPECT_THAT(tuple->Fields(),
+              ElementsAre(AutoFieldNamed("0"), AutoFieldNamed("1")));
+}
+
+}  // namespace
+}  // namespace Carbon