[ConstraintSystem] Handle ambiguities caused by requirement failures

Aggregate all requirement failures (regardless of kind) that belong
to the same locator and diagnose them as an ambiguity (if there is
more than one overload) or as the singular failure if all solutions
point to the same overload.

Author: xedin

lib/Sema/ConstraintSystem.cpp

@@ -4427,6 +4427,67 @@ static bool diagnoseAmbiguityWithContextualType(
   return true;
 }
 
+/// Diagnose problems with generic requirement fixes that are anchored on
+/// one callee location. The list could contain different kinds of fixes
+/// i.e. missing protocol conformances at different positions,
+/// same-type requirement mismatches, etc.
+static bool diagnoseAmbiguityWithGenericRequirements(
+    ConstraintSystem &cs,
+    ArrayRef<std::pair<const Solution *, const ConstraintFix *>> aggregate) {
+  // If all of the fixes point to the same overload choice,
+  // we can diagnose this an a single error.
+  bool hasNonDeclOverloads = false;
+
+  llvm::SmallSet<ValueDecl *, 4> overloadChoices;
+  for (const auto &entry : aggregate) {
+    const auto &solution = *entry.first;
+    auto *calleeLocator = solution.getCalleeLocator(entry.second->getLocator());
+
+    if (auto overload = solution.getOverloadChoiceIfAvailable(calleeLocator)) {
+      if (auto *D = overload->choice.getDeclOrNull()) {
+        overloadChoices.insert(D);
+      } else {
+        hasNonDeclOverloads = true;
+      }
+    }
+  }
+
+  auto &primaryFix = aggregate.front();
+  {
+    if (overloadChoices.size() > 0) {
+      // Some of the choices are non-declaration,
+      // let's delegate that to ambiguity diagnostics.
+      if (hasNonDeclOverloads)
+        return false;
+
+      if (overloadChoices.size() == 1)
+        return primaryFix.second->diagnose(*primaryFix.first);
+
+      // fall through to the tailored ambiguity diagnostic.
+    } else {
+      // If there are no overload choices it means that
+      // the issue is with types, delegate that to the primary fix.
+      return primaryFix.second->diagnoseForAmbiguity(aggregate);
+    }
+  }
+
+  // Produce "no exact matches" diagnostic.
+  auto &ctx = cs.getASTContext();
+  auto *choice = *overloadChoices.begin();
+  auto name = choice->getName();
+
+  ctx.Diags.diagnose(getLoc(primaryFix.second->getLocator()->getAnchor()),
+                     diag::no_overloads_match_exactly_in_call,
+                     /*isApplication=*/false, choice->getDescriptiveKind(),
+                     name.isSpecial(), name.getBaseName());
+
+  for (const auto &entry : aggregate) {
+    entry.second->diagnose(*entry.first, /*asNote=*/true);
+  }
+
+  return true;
+}
+
 static bool diagnoseAmbiguity(
     ConstraintSystem &cs, const SolutionDiff::OverloadDiff &ambiguity,
     ArrayRef<std::pair<const Solution *, const ConstraintFix *>> aggregateFix,
@@ -4850,14 +4911,78 @@ bool ConstraintSystem::diagnoseAmbiguityWithFixes(
   // overload choices.
   fixes.set_subtract(consideredFixes);
 
+  // Aggregate all requirement fixes that belong to the same callee
+  // and attempt to diagnose possible ambiguities.
+  {
+    auto isResultBuilderRef = [&](ASTNode node) {
+      auto *UDE = getAsExpr<UnresolvedDotExpr>(node);
+      if (!UDE)
+        return false;
+
+      auto &ctx = getASTContext();
+      return UDE->isImplicit() &&
+        UDE->getName().compare(DeclNameRef(ctx.Id_buildBlock)) == 0;
+    };
+
+    llvm::MapVector<SourceLoc,
+                    SmallVector<FixInContext, 4>>
+        builderFixes;
+
+    llvm::MapVector<ConstraintLocator *, SmallVector<FixInContext, 4>>
+        requirementFixes;
+
+    // TODO(diagnostics): This approach doesn't work for synthesized code
+    // because i.e. result builders would inject a new `buildBlock` or
+    // `buildExpression` for every kind of builder. We need to come up
+    // with the strategy to unify locators in such cases.
+    for (const auto &entry : fixes) {
+      auto *fix = entry.second;
+      if (!fix->getLocator()->isLastElement<LocatorPathElt::AnyRequirement>())
+        continue;
+
+      auto *calleeLoc = entry.first->getCalleeLocator(fix->getLocator());
+
+      if (isResultBuilderRef(calleeLoc->getAnchor())) {
+        auto *anchor = castToExpr<Expr>(calleeLoc->getAnchor());
+        builderFixes[anchor->getLoc()].push_back(entry);
+      } else {
+        requirementFixes[calleeLoc].push_back(entry);
+      }
+    }
+
+    SmallVector<FixInContext, 4> diagnosedFixes;
+
+    for (auto &entry : builderFixes) {
+      auto &aggregate = entry.second;
+
+      if (diagnoseAmbiguityWithGenericRequirements(*this, aggregate))
+        diagnosedFixes.append(aggregate.begin(), aggregate.end());
+    }
+
+    for (auto &entry : requirementFixes) {
+      auto &aggregate = entry.second;
+
+      // Ambiguity only if all of the solutions have a requirement
+      // fix at the given location.
+      if (aggregate.size() != solutions.size())
+        continue;
+
+      if (diagnoseAmbiguityWithGenericRequirements(*this, aggregate))
+        diagnosedFixes.append(aggregate.begin(), aggregate.end());
+    }
+
+    diagnosed |= !diagnosedFixes.empty();
+    // Remove any diagnosed fixes.
+    fixes.set_subtract(diagnosedFixes);
+  }
+
   llvm::MapVector<std::pair<FixKind, ConstraintLocator *>,
                   SmallVector<FixInContext, 4>>
       fixesByKind;
 
   for (const auto &entry : fixes) {
     const auto *fix = entry.second;
-    fixesByKind[{fix->getKind(), fix->getLocator()}].push_back(
-        {entry.first, fix});
+    fixesByKind[{fix->getKind(), fix->getLocator()}].push_back(entry);
   }
 
   // If leftover fix is contained in all of the solutions let's

test/Constraints/generics.swift

@@ -996,3 +996,38 @@ do {
   // expected-error@-3 {{local function 'foo' requires the types 'Set<Int>.Type.Element' and 'Array<String>.Type.Element' be equivalent}}
   // expected-note@-4 2 {{only concrete types such as structs, enums and classes can conform to protocols}}
 }
+
+// https://github.com/apple/swift/issues/56173
+protocol P_56173 {
+  associatedtype Element
+}
+protocol Q_56173 {
+  associatedtype Element
+}
+
+func test_requirement_failures_in_ambiguous_context() {
+  struct A : P_56173 {
+    typealias Element = String
+  }
+  struct B : Q_56173 {
+    typealias Element = Int
+  }
+
+  func f1<T: Equatable>(_: T, _: T) {} // expected-note {{where 'T' = 'A'}}
+
+  f1(A(), B()) // expected-error {{local function 'f1' requires that 'A' conform to 'Equatable'}}
+
+  func f2<T: P_56173, U: P_56173>(_: T, _: U) {}
+  // expected-note@-1 {{candidate requires that 'B' conform to 'P_56173' (requirement specified as 'U' : 'P_56173')}}
+  func f2<T: Q_56173, U: Q_56173>(_: T, _: U) {}
+  // expected-note@-1 {{candidate requires that 'A' conform to 'Q_56173' (requirement specified as 'T' : 'Q_56173')}}
+
+  f2(A(), B()) // expected-error {{no exact matches in call to local function 'f2'}}
+
+  func f3<T: P_56173>(_: T) where T.Element == Int {}
+  // expected-note@-1 {{candidate requires that the types 'A.Element' (aka 'String') and 'Int' be equivalent (requirement specified as 'T.Element' == 'Int')}}
+  func f3<U: Q_56173>(_: U) where U.Element == String {}
+  // expected-note@-1 {{candidate requires that 'A' conform to 'Q_56173' (requirement specified as 'U' : 'Q_56173')}}
+
+  f3(A()) // expected-error {{no exact matches in call to local function 'f3'}}
+}