[GSB] [Diag] Constraint to concrete type using ":" should offer a fix-it

include/swift/AST/DiagnosticsSema.def

@@ -1981,6 +1981,9 @@ ERROR(protocol_composition_one_class,none,
 ERROR(requires_conformance_nonprotocol,none,
       "type %0 constrained to non-protocol, non-class type %1",
       (Type, Type))
+NOTE(requires_conformance_nonprotocol_fixit,none,
+     "use '%0 == %1' to require '%0' to be '%1'",
+     (StringRef, StringRef))
 ERROR(requires_not_suitable_archetype,none,
       "type %0 in conformance requirement does not refer to a "
       "generic parameter or associated type",

include/swift/AST/GenericSignatureBuilder.h

@@ -314,6 +314,10 @@ class GenericSignatureBuilder {
     /// \c ConstraintResult::Unresolved so the caller knows what happened.
     GenerateUnresolved = 1,
   };
+
+  /// The set of constraints that are invalid because the constraint
+  /// type isn't constrained to a protocol or a class
+  std::vector<Constraint<Type>> invalidIsaConstraints;
 
 private:
   class InferRequirementsWalker;

lib/AST/GenericSignatureBuilder.cpp

@@ -4629,8 +4629,14 @@ ConstraintResult GenericSignatureBuilder::addTypeRequirement(
                         ->getDependentType(getGenericParams());
 
       Impl->HadAnyError = true;
-      Diags.diagnose(source.getLoc(), diag::requires_conformance_nonprotocol,
-                     subjectType, constraintType);
+
+      if (subjectType->is<DependentMemberType>()) {
+        subjectType = resolveDependentMemberTypes(*this, subjectType);
+      }
+
+      auto invalidConstraint = Constraint<Type>(
+          {subject, constraintType, source.getSource(*this, subjectType)});
+      invalidIsaConstraints.push_back(invalidConstraint);
     }
 
     return ConstraintResult::Conflicting;
@@ -5750,6 +5756,43 @@ GenericSignatureBuilder::finalize(SourceLoc loc,
       }
     }
   }
+
+  // Emit a diagnostic if we recorded any constraints where the constraint
+  // type was not constrained to a protocol or class. Provide a fix-it if
+  // allowConcreteGenericParams is true or the subject type is a member type.
+  if (!invalidIsaConstraints.empty()) {
+    for (auto constraint : invalidIsaConstraints) {
+      auto subjectType = constraint.getSubjectDependentType(getGenericParams());
+      auto constraintType = constraint.value;
+      auto source = constraint.source;
+      auto loc = source->getLoc();
+
+      Diags.diagnose(loc, diag::requires_conformance_nonprotocol,
+                     subjectType, constraintType);
+
+      auto getNameWithoutSelf = [&](std::string subjectTypeName) {
+        std::string selfSubstring = "Self.";
+
+        if (subjectTypeName.rfind(selfSubstring, 0) == 0) {
+          return subjectTypeName.erase(0, selfSubstring.length());
+        }
+
+        return subjectTypeName;
+      };
+
+      if (allowConcreteGenericParams ||
+          (subjectType->is<DependentMemberType>() &&
+           !source->isProtocolRequirement())) {
+        auto subjectTypeName = subjectType.getString();
+        auto subjectTypeNameWithoutSelf = getNameWithoutSelf(subjectTypeName);
+        Diags.diagnose(loc, diag::requires_conformance_nonprotocol_fixit,
+                       subjectTypeNameWithoutSelf, constraintType.getString())
+             .fixItReplace(loc, " == ");
+      }
+    }
+
+    invalidIsaConstraints.clear();
+  }
 }
 
 /// Turn a requirement right-hand side into an unresolved type.

test/Constraints/generics.swift

@@ -637,4 +637,43 @@ func compare<C: Collection, Key: Hashable, Value: Equatable>(c: C)
   -> Bool where C.Element == (key: Key, value: Value)
 {
   _ = Dictionary(uniqueKeysWithValues: Array(c))
-}
+}
+
+// https://bugs.swift.org/browse/SR-7984
+struct SR_7984<Bar> {
+  func doSomething() {}
+}
+
+extension SR_7984 where Bar: String {} // expected-error {{type 'Bar' constrained to non-protocol, non-class type 'String'}} expected-note {{use 'Bar == String' to require 'Bar' to be 'String'}} {{28-29= ==}}
+
+protocol SR_7984_Proto {
+  associatedtype Bar
+}
+
+extension SR_7984_Proto where Bar: String {} // expected-error {{type 'Self.Bar' constrained to non-protocol, non-class type 'String'}} expected-note {{use 'Bar == String' to require 'Bar' to be 'String'}} {{34-35= ==}}
+
+protocol SR_7984_HasFoo {
+  associatedtype Foo
+}
+protocol SR_7984_HasAssoc {
+  associatedtype Assoc: SR_7984_HasFoo
+}
+
+struct SR_7984_X<T: SR_7984_HasAssoc> {}
+extension SR_7984_X where T.Assoc.Foo: String {} // expected-error {{type 'T.Assoc.Foo' constrained to non-protocol, non-class type 'String'}} expected-note {{use 'T.Assoc.Foo == String' to require 'T.Assoc.Foo' to be 'String'}} {{38-39= ==}}
+
+struct SR_7984_S<T: Sequence> where T.Element: String {} // expected-error {{type 'T.Element' constrained to non-protocol, non-class type 'String'}} expected-note {{use 'T.Element == String' to require 'T.Element' to be 'String'}} {{46-47= ==}}
+func SR_7984_F<T: Sequence>(foo: T) where T.Element: String {} // expected-error {{type 'T.Element' constrained to non-protocol, non-class type 'String'}} expected-note {{use 'T.Element == String' to require 'T.Element' to be 'String'}} {{52-53= ==}}
+
+protocol SR_7984_P {
+  func S<T : Sequence>(bar: T) where T.Element: String // expected-error {{type 'T.Element' constrained to non-protocol, non-class type 'String'}} expected-note {{use 'T.Element == String' to require 'T.Element' to be 'String'}} {{47-48= ==}}
+}
+
+struct A<T: String> {} // expected-error {{type 'T' constrained to non-protocol, non-class type 'String'}}
+struct B<T> where T: String {} // expected-error {{type 'T' constrained to non-protocol, non-class type 'String'}}
+protocol C {
+  associatedtype Foo: String // expected-error {{inheritance from non-protocol, non-class type 'String'}} expected-error {{type 'Self.Foo' constrained to non-protocol, non-class type 'String'}}
+}
+protocol D {
+  associatedtype Foo where Foo: String // expected-error {{type 'Self.Foo' constrained to non-protocol, non-class type 'String'}}
+}