[AutoDiff] [Sema] Limit implicit @differentiable attribute creation.

During protocol witness matching for a protocol requirement with
`@differentiable` attributes, implicit `@differentiable` attributes may be
created for the witness under specific conditions (when the witness has a
`@differentiable` attribute with superset differentiability parameters, or
when the witness has less-than-public visibility).

Do not generate implicit `@differentiable` attributes for protocol witnesses
when the protocol conformance is declared from a separate file from the witness.

Otherwise, compilation of the file containing the conformance creates references
to external symbols for the implicit `@differentiable` attributes, even though
no such symbols exist.

Resolves SR-13455.

Author: dan-zheng

include/swift/AST/DiagnosticsSema.def

@@ -3112,10 +3112,10 @@ ERROR(overriding_decl_missing_differentiable_attr,none,
       "overriding declaration is missing attribute '%0'", (StringRef))
 NOTE(protocol_witness_missing_differentiable_attr,none,
      "candidate is missing attribute '%0'", (StringRef))
-NOTE(protocol_witness_missing_differentiable_attr_nonpublic_other_file,none,
-     "non-public %1 %2 must have explicit '%0' attribute to satisfy "
-     "requirement %3 %4 (in protocol %6) because it is declared in a different "
-     "file than the conformance of %5 to %6",
+NOTE(protocol_witness_missing_differentiable_attr_other_file,none,
+     "%1 %2 must have explicit '%0' attribute to satisfy requirement %3 %4 (in "
+     "protocol %6) because it is declared in a different file than the "
+     "conformance of %5 to %6",
      (StringRef, DescriptiveDeclKind, DeclName, DescriptiveDeclKind, DeclName,
       Type, Type))
 

lib/Sema/TypeCheckProtocol.cpp

@@ -384,23 +384,15 @@ matchWitnessDifferentiableAttr(DeclContext *dc, ValueDecl *req,
         supersetConfig = witnessConfig;
     }
     if (!foundExactConfig) {
-      bool success = false;
       // If no exact witness derivative configuration was found, check
       // conditions for creating an implicit witness `@differentiable` attribute
-      // with the exact derivative configuration:
-      // - If the witness has a "superset" derivative configuration.
-      // - If the witness is less than public and is declared in the same file
-      //   as the conformance.
-      //   - `@differentiable` attributes are really only significant for public
-      //     declarations: it improves usability to not require explicit
-      //     `@differentiable` attributes for less-visible declarations.
-      bool createImplicitWitnessAttribute =
-          supersetConfig || witness->getFormalAccess() < AccessLevel::Public;
-      // If the witness has less-than-public visibility and is declared in a
-      // different file than the conformance, produce an error.
-      if (!supersetConfig && witness->getFormalAccess() < AccessLevel::Public &&
-          dc->getModuleScopeContext() !=
-              witness->getDeclContext()->getModuleScopeContext()) {
+      // with the exact derivative configuration.
+
+      // If witness is declared in a different file as the conformance, we
+      // should not create an implicit `@differentiable` attribute on the
+      // witness. Produce an error.
+      if (dc->getModuleScopeContext() !=
+          witness->getDeclContext()->getModuleScopeContext()) {
         // FIXME(TF-1014): `@differentiable` attribute diagnostic does not
         // appear if associated type inference is involved.
         if (auto *vdWitness = dyn_cast<VarDecl>(witness)) {
@@ -412,6 +404,20 @@ matchWitnessDifferentiableAttr(DeclContext *dc, ValueDecl *req,
                                   reqDiffAttr);
         }
       }
+
+      // Otherwise, the witness must:
+      // - Have a "superset" derivative configuration.
+      // - Have less than public visibility.
+      //   - `@differentiable` attributes are really only significant for
+      //     public declarations: it improves usability to not require
+      //     explicit `@differentiable` attributes for less-visible
+      //     declarations.
+      //
+      // If these conditions are met, an implicit `@differentiable` attribute
+      // with the exact derivative configuration can be created.
+      bool success = false;
+      bool createImplicitWitnessAttribute =
+          supersetConfig || witness->getFormalAccess() < AccessLevel::Public;
       if (createImplicitWitnessAttribute) {
         auto derivativeGenSig = witnessAFD->getGenericSignature();
         if (supersetConfig)
@@ -2448,16 +2454,15 @@ diagnoseMatch(ModuleDecl *module, NormalProtocolConformance *conformance,
     llvm::raw_string_ostream os(reqDiffAttrString);
     reqAttr->print(os, req, omitWrtClause);
     os.flush();
-    // If the witness has less-than-public visibility and is declared in a
-    // different file than the conformance, emit a specialized diagnostic.
-    if (witness->getFormalAccess() < AccessLevel::Public &&
-        conformance->getDeclContext()->getModuleScopeContext() !=
+    // If the witness is declared in a different file than the conformance, emit
+    // a specialized diagnostic.
+    if (conformance->getDeclContext()->getModuleScopeContext() !=
             witness->getDeclContext()->getModuleScopeContext()) {
       diags
           .diagnose(
               witness,
               diag::
-                  protocol_witness_missing_differentiable_attr_nonpublic_other_file,
+                  protocol_witness_missing_differentiable_attr_other_file,
               reqDiffAttrString, witness->getDescriptiveKind(),
               witness->getName(), req->getDescriptiveKind(),
               req->getName(), conformance->getType(),

test/AutoDiff/Sema/ImplicitDifferentiableAttributeCrossFile/Inputs/other_file.swift

@@ -0,0 +1,53 @@
+import _Differentiation
+
+protocol Protocol1: Differentiable {
+  // expected-note @+2 {{protocol requires function 'internalMethod1' with type '(Float) -> Float'}}
+  @differentiable(wrt: (self, x))
+  func internalMethod1(_ x: Float) -> Float
+
+  // expected-note @+3 {{protocol requires function 'internalMethod2' with type '(Float) -> Float'}}
+  @differentiable(wrt: x)
+  @differentiable(wrt: (self, x))
+  func internalMethod2(_ x: Float) -> Float
+
+  // expected-note @+3 {{protocol requires function 'internalMethod3' with type '(Float) -> Float'}}
+  @differentiable(wrt: x)
+  @differentiable(wrt: (self, x))
+  func internalMethod3(_ x: Float) -> Float
+}
+
+protocol Protocol2: Differentiable {
+  @differentiable(wrt: (self, x))
+  func internalMethod4(_ x: Float) -> Float
+}
+
+// Note:
+// - No `ConformingStruct: Protocol1` conformance exists in this file, so this
+//   file should compile just file.
+// - A `ConformingStruct: Protocol1` conformance in a different file should be
+//   diagnosed to prevent linker errors. Without a diagnostic, compilation of
+//   the other file creates external references to symbols for implicit
+//   `@differentiable` attributes, even though no such symbols exist.
+//   Context: https://github.com/apple/swift/pull/29771#issuecomment-585059721
+
+struct ConformingStruct: Differentiable {
+  // Error for missing `@differentiable` attribute.
+  // expected-note @+1 {{instance method 'internalMethod1' must have explicit '@differentiable' attribute to satisfy requirement instance method 'internalMethod1' (in protocol 'Protocol1') because it is declared in a different file than the conformance of 'ConformingStruct' to 'Protocol1'}} {{3-3=@differentiable }}
+  func internalMethod1(_ x: Float) -> Float {
+    x
+  }
+
+  // Error for missing `@differentiable` superset attribute.
+  // expected-note @+2 {{instance method 'internalMethod2' must have explicit '@differentiable' attribute to satisfy requirement instance method 'internalMethod2' (in protocol 'Protocol1') because it is declared in a different file than the conformance of 'ConformingStruct' to 'Protocol1'}} {{3-3=@differentiable }}
+  @differentiable(wrt: x)
+  func internalMethod2(_ x: Float) -> Float {
+    x
+  }
+
+  // Error for missing `@differentiable` subset attribute.
+  // expected-note @+2 {{instance method 'internalMethod3' must have explicit '@differentiable(wrt: x)' attribute to satisfy requirement instance method 'internalMethod3' (in protocol 'Protocol1') because it is declared in a different file than the conformance of 'ConformingStruct' to 'Protocol1'}} {{3-3=@differentiable(wrt: x) }}
+  @differentiable(wrt: (self, x))
+  func internalMethod3(_ x: Float) -> Float {
+    x
+  }
+}

test/AutoDiff/Sema/ImplicitDifferentiableAttributeCrossFile/Inputs/other_file_protocol_default_implementation_witness.swift

@@ -0,0 +1,17 @@
+import _Differentiation
+
+protocol P1: Differentiable {
+  @differentiable(wrt: self)
+  // expected-note @+1 {{protocol requires function 'callAsFunction' with type '(Float) -> Float'}}
+  func callAsFunction(_ input: Float) -> Float
+}
+
+protocol P2: P1 {}
+
+extension P2 {
+  @differentiable(wrt: (self, input))
+  // expected-note @+1 {{instance method 'callAsFunction' must have explicit '@differentiable(wrt: self)' attribute to satisfy requirement instance method 'callAsFunction' (in protocol 'P1') because it is declared in a different file than the conformance of 'ConformingStruct' to 'P1'}}
+  public func callAsFunction(_ input: Float) -> Float {
+    return input
+  }
+}

test/AutoDiff/Sema/ImplicitDifferentiableAttributeCrossFile/main.swift

@@ -0,0 +1,35 @@
+// Test missing protocol requirement `@differentiable` attribute errors for
+// non-public protocol witnesses, when the protocol conformance is declared in a
+// separate file from witnesses.
+//
+// Implicit `@differentiable` attributes cannot be generated for protocol
+// witnesses when the conformance is declared from a separate file from the
+// witness. Otherwise, compilation of the file containing the conformance
+// creates references to external symbols for implicit `@differentiable`
+// attributes, even though no such symbols exist.
+//
+// Context: https://github.com/apple/swift/pull/29771#issuecomment-585059721
+
+// Note: `swiftc main.swift other_file.swift` runs three commands:
+// - `swiftc -frontend -primary-file main.swift other_file.swift -o ...`
+// - `swiftc -frontend main.swift -primary-file other_file.swift -o ...`
+// - `/usr/bin/ld ...`
+//
+// `%target-build-swift` performs `swiftc main.swift other_file.swift`, so it is expected to fail (hence `not`).
+// `swiftc -frontend -primary-file main.swift other_file.swift` should fail, so `-verify` is needed.
+// `swiftc -frontend main.swift -primary-file other_file.swift` should succeed, so no need for `-verify`.
+
+// RUN: %target-swift-frontend -c -verify -primary-file %s %S/Inputs/other_file.swift
+// RUN: %target-swift-frontend -c %s -primary-file %S/Inputs/other_file.swift
+// RUN: not %target-build-swift %s %S/Inputs/other_file.swift
+
+// Error: conformance is in different file than witnesses.
+// expected-error @+1 {{type 'ConformingStruct' does not conform to protocol 'Protocol1'}}
+extension ConformingStruct: Protocol1 {}
+
+// No error: conformance is in same file as witnesses.
+extension ConformingStruct: Protocol2 {
+  func internalMethod4(_ x: Float) -> Float {
+    x
+  }
+}

test/AutoDiff/Sema/ImplicitDifferentiableAttributeCrossFile/protocol_default_implementation_witness.swift

@@ -0,0 +1,12 @@
+// RUN: %target-swift-frontend -c -verify -primary-file %s %S/Inputs/other_file_protocol_default_implementation_witness.swift
+
+// SR-13455: Test missing protocol requirement `@differentiable` attribute
+// errors for protocol witnesses declared in a different file than the protocol
+// conformance.
+//
+// This test case specifically tests protocol extension method witnesses.
+
+import _Differentiation
+
+// expected-error @+1 {{type 'ConformingStruct' does not conform to protocol 'P1'}}
+struct ConformingStruct: P2 {}