Basic support for .Self within :! bindings and where expressions. (#1311)

`.Self` is modeled as a new kind of expression, `DotSelfExpression`. Name resolution associates each occurrence of a `DotSelfExpression` with a generic binding, much like for an `IdentifierExpression`.

Both `:!` bindings and `where` expressions bring `.Self` into scope. The tentative intent is that if there are multiple `.Self`s in scope and they refer to different bindings, the result of using a `.Self` expression is an ambiguity error, but that is not implemented in this patch. Instead, like for `IdentifierExpression`s, we find the innermost enclosing definition of `.Self`.

`.Foo` expressions are rewritten to `.Self.Foo`, but this isn't enough to make them do anything useful, because associated constants aren't supported in general yet.

explorer/testdata/constraint/binding_self.carbon

@@ -0,0 +1,30 @@
+// 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
+//
+// RUN: %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+// CHECK: result: 12
+
+package ExplorerTest api;
+
+interface SwizzleWith(T:! Type) {
+  fn Op[me: Self](x: T) -> i32;
+}
+
+impl i32 as SwizzleWith(i32) {
+  fn Op[me: Self](x: Self) -> Self { return me * 10 + x; }
+}
+
+fn F[T:! SwizzleWith(.Self)](v: T, w: T) -> i32 {
+  return v.Op(w);
+}
+
+fn Main() -> i32 {
+  var one: i32 = 1;
+  var two: i32 = 2;
+  return F(one, two);
+}

explorer/testdata/constraint/combine_equality.carbon

@@ -15,7 +15,7 @@ impl i32 as I {}
 
 fn F(A:! i32, B:! i32, C:! i32, D:! i32, E:! i32,
      T:! I where A == B and C == D and C == E and B == D) {
-  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/combine_equality.carbon:[[@LINE+1]]: member access, F not in constraint interface I where .Self:! Type is interface I, A:! i32 == B:! i32 == E:! i32 == C:! i32 == D:! i32
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/combine_equality.carbon:[[@LINE+1]]: member access, F not in constraint interface I where .Self:! I is interface I, A:! i32 == B:! i32 == E:! i32 == C:! i32 == D:! i32
   T.F();
 }
 

explorer/testdata/constraint/dot_self_is_other.carbon

@@ -0,0 +1,31 @@
+// 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
+//
+// RUN: %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+// CHECK: result: 12
+
+package ExplorerTest api;
+
+interface A { fn F() -> i32; }
+interface B { fn F() -> i32; }
+
+impl i32 as A { fn F() -> i32 { return 1; } }
+impl i32 as B { fn F() -> i32 { return 2; } }
+
+fn GetA[T:! A](x: T) -> i32 { return x.F(); }
+fn GetAB[T:! B where .Self is A](x: T) -> i32 {
+  // Note that there's no ambiguity in the use of the name F here.
+  // `x.F` is `x.(B.F)` because the constraint only treats `B`, not
+  // `A`, as a lookup context.
+  return 10 * GetA(x) + x.F();
+}
+
+fn Main() -> i32 {
+  var v: i32 = 0;
+  return GetAB(v);
+}

explorer/testdata/constraint/fail_dot_self_after_scope.carbon

@@ -0,0 +1,22 @@
+// 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
+//
+// RUN: %{not} %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+
+package ExplorerTest api;
+
+interface FrobWith(T:! Type) {}
+
+fn F[T:! FrobWith(.Self)]
+  // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/fail_dot_self_after_scope.carbon:[[@LINE+1]]: could not resolve '.Self'
+  (U: .Self) {
+}
+
+fn Main() -> i32 {
+  return 0;
+}

explorer/testdata/constraint/fail_dot_self_after_scope_2.carbon

@@ -0,0 +1,22 @@
+// 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
+//
+// RUN: %{not} %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+
+package ExplorerTest api;
+
+interface FrobWith(T:! Type) {}
+
+fn F[T:! FrobWith(.Self),
+     // CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/fail_dot_self_after_scope_2.carbon:[[@LINE+1]]: `.Self` used in type of non-type binding `U`
+     U:! .Self]() {
+}
+
+fn Main() -> i32 {
+  return 0;
+}

explorer/testdata/constraint/fail_dot_self_not_in_scope.carbon

@@ -0,0 +1,16 @@
+// 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
+//
+// RUN: %{not} %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+
+package ExplorerTest api;
+
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/fail_dot_self_not_in_scope.carbon:[[@LINE+1]]: could not resolve '.Self'
+fn Main() -> .Self {
+  return 0;
+}

explorer/testdata/constraint/fail_infinite_self.carbon

@@ -0,0 +1,16 @@
+// 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
+//
+// RUN: %{not} %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+
+package ExplorerTest api;
+
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/fail_infinite_self.carbon:[[@LINE+1]]: `.Self` used in type of non-type binding `T`
+fn F[T:! .Self]() {}
+
+fn Main() -> i32 { return 0; }

explorer/testdata/constraint/fail_infinite_self_ptr.carbon

@@ -0,0 +1,16 @@
+// 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
+//
+// RUN: %{not} %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+
+package ExplorerTest api;
+
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/fail_infinite_self_ptr.carbon:[[@LINE+1]]: `.Self` used in type of non-type binding `T`
+fn F[T:! .Self*]() {}
+
+fn Main() -> i32 { return 0; }

explorer/testdata/constraint/fail_nontype_self.carbon

@@ -0,0 +1,18 @@
+// 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
+//
+// RUN: %{not} %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{not} %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+
+package ExplorerTest api;
+
+class X(T:! Type) {}
+
+// CHECK: COMPILATION ERROR: {{.*}}/explorer/testdata/constraint/fail_nontype_self.carbon:[[@LINE+1]]: `.Self` used in type of non-type binding `T`
+fn F[T:! X(.Self)](x: T) {}
+
+fn Main() -> i32 { return 0; }

explorer/testdata/constraint/qualified_lookup_in_where.carbon

@@ -0,0 +1,26 @@
+// 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
+//
+// RUN: %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+// CHECK: result: 122
+
+package ExplorerTest api;
+
+interface A { fn F() -> i32; }
+interface B { fn F() -> i32; }
+
+impl i32 as A { fn F() -> i32 { return 1; } }
+impl i32 as B { fn F() -> i32 { return 2; } }
+
+fn GetAB(T:! B where .Self is A) -> i32 {
+  return 100 * T.(A.F)() + 10 * T.(B.F)() + T.F();
+}
+
+fn Main() -> i32 {
+  return GetAB(i32);
+}

explorer/testdata/constraint/where_self.carbon

@@ -0,0 +1,34 @@
+// 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
+//
+// RUN: %{explorer} %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes=false %s
+// RUN: %{explorer} --parser_debug --trace_file=- %s 2>&1 | \
+// RUN:   %{FileCheck} --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: %{explorer} %s
+// CHECK: result: 12
+
+package ExplorerTest api;
+
+interface SwizzleWith(T:! Type) {
+  fn Op[me: Self](x: T) -> i32;
+}
+
+// TODO: There should be some way to write this that includes
+// `SwizzleWith(.Self)` in the list of lookup contexts.
+alias Swizzle = Type where .Self is SwizzleWith(.Self);
+
+impl i32 as SwizzleWith(i32) {
+  fn Op[me: Self](x: Self) -> Self { return me * 10 + x; }
+}
+
+fn F[T:! Swizzle](v: T, w: T) -> i32 {
+  return v.(SwizzleWith(T).Op)(w);
+}
+
+fn Main() -> i32 {
+  var one: i32 = 1;
+  var two: i32 = 2;
+  return F(one, two);
+}