diff --git a/include/swift/AST/DiagnosticsSema.def b/include/swift/AST/DiagnosticsSema.def
index 2368540dcc059..9fa095aa5852f 100644
--- a/include/swift/AST/DiagnosticsSema.def
+++ b/include/swift/AST/DiagnosticsSema.def
@@ -2795,7 +2795,7 @@ WARNING(differentiable_let_property_implicit_noderivative_fixit,none,
 NOTE(codable_extraneous_codingkey_case_here,none,
      "CodingKey case %0 does not match any stored properties", (Identifier))
 NOTE(codable_non_conforming_property_here,none,
-     "cannot automatically synthesize %0 because %1 does not conform to %0", (Type, Type))
+     "cannot automatically synthesize %0 because %1 does not conform to %0", (Type, TypeLoc))
 NOTE(codable_non_decoded_property_here,none,
      "cannot automatically synthesize %0 because %1 does not have a matching CodingKey and does not have a default value", (Type, Identifier))
 NOTE(codable_codingkeys_type_is_not_an_enum_here,none,
diff --git a/lib/Sema/DerivedConformanceCodable.cpp b/lib/Sema/DerivedConformanceCodable.cpp
index 843049dea1a4d..a76fd5e3be6fe 100644
--- a/lib/Sema/DerivedConformanceCodable.cpp
+++ b/lib/Sema/DerivedConformanceCodable.cpp
@@ -172,10 +172,20 @@ static bool validateCodingKeysEnum(DerivedConformance &derived,
         properties.erase(it);
         break;
 
-      case DoesNotConform:
+      case DoesNotConform: {
+        // We use a TypeLoc here so diagnostics can show the type
+        // as written by the user in source if possible. This is useful
+        // when the user has written an IUO type for example, since
+        // diagnostics would show the type as 'T?' instead of 'T!' if
+        // we use a Type instead.
+        TypeLoc typeLoc = {
+            it->second->getTypeReprOrParentPatternTypeRepr(),
+            it->second->getType(),
+        };
         it->second->diagnose(diag::codable_non_conforming_property_here,
-                             derived.getProtocolType(), it->second->getType());
+                             derived.getProtocolType(), typeLoc);
         LLVM_FALLTHROUGH;
+      }
 
       case TypeNotValidated:
         // We don't produce a diagnostic for a type which failed to validate.
@@ -352,10 +362,20 @@ static EnumDecl *synthesizeCodingKeysEnum(DerivedConformance &derived) {
         break;
       }
 
-      case DoesNotConform:
+      case DoesNotConform: {
+        // We use a TypeLoc here so diagnostics can show the type
+        // as written by the user in source if possible. This is useful
+        // when the user has written an IUO type for example, since
+        // diagnostics would show the type as 'T?' instead of 'T!' if
+        // we use a Type instead.
+        TypeLoc typeLoc = {
+            varDecl->getTypeReprOrParentPatternTypeRepr(),
+            varDecl->getType(),
+        };
         varDecl->diagnose(diag::codable_non_conforming_property_here,
-                          derived.getProtocolType(), varDecl->getType());
+                          derived.getProtocolType(), typeLoc);
         LLVM_FALLTHROUGH;
+      }
 
       case TypeNotValidated:
         // We don't produce a diagnostic for a type which failed to validate.
diff --git a/test/decl/protocol/special/coding/class_codable_excluded_optional_properties.swift b/test/decl/protocol/special/coding/class_codable_excluded_optional_properties.swift
index fe5c36bcc2acd..1b2d1ccb5f6a5 100644
--- a/test/decl/protocol/special/coding/class_codable_excluded_optional_properties.swift
+++ b/test/decl/protocol/special/coding/class_codable_excluded_optional_properties.swift
@@ -45,8 +45,8 @@ class ClassWithNonExcludedVarMembers : Codable { // expected-error {{type 'Class
     // AnyHashable does not conform to Codable.
     var p3: AnyHashable? // expected-note {{cannot automatically synthesize 'Decodable' because 'AnyHashable?' does not conform to 'Decodable'}}
     // expected-note@-1 {{cannot automatically synthesize 'Encodable' because 'AnyHashable?' does not conform to 'Encodable'}}
-    var p4: AnyHashable! // expected-note {{cannot automatically synthesize 'Decodable' because 'AnyHashable?' does not conform to 'Decodable'}}
-    // expected-note@-1 {{cannot automatically synthesize 'Encodable' because 'AnyHashable?' does not conform to 'Encodable'}}
+    var p4: AnyHashable! // expected-note {{cannot automatically synthesize 'Decodable' because 'AnyHashable!' does not conform to 'Decodable'}}
+    // expected-note@-1 {{cannot automatically synthesize 'Encodable' because 'AnyHashable!' does not conform to 'Encodable'}}
 }
 
 class ClassWithExcludedVarMembers : Codable {
diff --git a/test/decl/protocol/special/coding/class_codable_failure_diagnostics.swift b/test/decl/protocol/special/coding/class_codable_failure_diagnostics.swift
index ba35fb4e92e4a..e06724325099c 100644
--- a/test/decl/protocol/special/coding/class_codable_failure_diagnostics.swift
+++ b/test/decl/protocol/special/coding/class_codable_failure_diagnostics.swift
@@ -9,8 +9,8 @@ class C1 : Codable {
   var a: String = ""
   var b: Int = 0
   var c: Nested = Nested()
-  // expected-note@-1 {{cannot automatically synthesize 'Decodable' because 'C1.Nested' does not conform to 'Decodable'}}
-  // expected-note@-2 {{cannot automatically synthesize 'Encodable' because 'C1.Nested' does not conform to 'Encodable'}}
+  // expected-note@-1 {{cannot automatically synthesize 'Decodable' because 'Nested' does not conform to 'Decodable'}}
+  // expected-note@-2 {{cannot automatically synthesize 'Encodable' because 'Nested' does not conform to 'Encodable'}}
 }
 
 // Codable class with non-enum CodingKeys.
diff --git a/test/decl/protocol/special/coding/struct_codable_excluded_optional_properties.swift b/test/decl/protocol/special/coding/struct_codable_excluded_optional_properties.swift
index 8d8d5939fee02..84e9b9c3022ec 100644
--- a/test/decl/protocol/special/coding/struct_codable_excluded_optional_properties.swift
+++ b/test/decl/protocol/special/coding/struct_codable_excluded_optional_properties.swift
@@ -12,8 +12,8 @@ struct StructWithNonExcludedLetMembers : Codable { // expected-error {{type 'Str
     // AnyHashable does not conform to Codable.
     let p3: AnyHashable? // expected-note {{cannot automatically synthesize 'Decodable' because 'AnyHashable?' does not conform to 'Decodable'}}
     // expected-note@-1 {{cannot automatically synthesize 'Encodable' because 'AnyHashable?' does not conform to 'Encodable'}}
-    let p4: AnyHashable! // expected-note {{cannot automatically synthesize 'Decodable' because 'AnyHashable?' does not conform to 'Decodable'}}
-    // expected-note@-1 {{cannot automatically synthesize 'Encodable' because 'AnyHashable?' does not conform to 'Encodable'}}
+    let p4: AnyHashable! // expected-note {{cannot automatically synthesize 'Decodable' because 'AnyHashable!' does not conform to 'Decodable'}}
+    // expected-note@-1 {{cannot automatically synthesize 'Encodable' because 'AnyHashable!' does not conform to 'Encodable'}}
 }
 
 struct StructWithExcludedLetMembers : Codable { // expected-error {{type 'StructWithExcludedLetMembers' does not conform to protocol 'Decodable'}}
@@ -46,8 +46,8 @@ struct StructWithNonExcludedVarMembers : Codable { // expected-error {{type 'Str
     // AnyHashable does not conform to Codable.
     var p3: AnyHashable? // expected-note {{cannot automatically synthesize 'Decodable' because 'AnyHashable?' does not conform to 'Decodable'}}
     // expected-note@-1 {{cannot automatically synthesize 'Encodable' because 'AnyHashable?' does not conform to 'Encodable'}}
-    var p4: AnyHashable! // expected-note {{cannot automatically synthesize 'Decodable' because 'AnyHashable?' does not conform to 'Decodable'}}
-    // expected-note@-1 {{cannot automatically synthesize 'Encodable' because 'AnyHashable?' does not conform to 'Encodable'}}
+    var p4: AnyHashable! // expected-note {{cannot automatically synthesize 'Decodable' because 'AnyHashable!' does not conform to 'Decodable'}}
+    // expected-note@-1 {{cannot automatically synthesize 'Encodable' because 'AnyHashable!' does not conform to 'Encodable'}}
 }
 
 struct StructWithExcludedVarMembers : Codable {
diff --git a/test/decl/protocol/special/coding/struct_codable_failure_diagnostics.swift b/test/decl/protocol/special/coding/struct_codable_failure_diagnostics.swift
index fa26222b73602..4928fb44ecc68 100644
--- a/test/decl/protocol/special/coding/struct_codable_failure_diagnostics.swift
+++ b/test/decl/protocol/special/coding/struct_codable_failure_diagnostics.swift
@@ -9,8 +9,8 @@ struct S1 : Codable {
   var a: String = ""
   var b: Int = 0
   var c: Nested = Nested()
-  // expected-note@-1 {{cannot automatically synthesize 'Decodable' because 'S1.Nested' does not conform to 'Decodable'}}
-  // expected-note@-2 {{cannot automatically synthesize 'Encodable' because 'S1.Nested' does not conform to 'Encodable'}}
+  // expected-note@-1 {{cannot automatically synthesize 'Decodable' because 'Nested' does not conform to 'Decodable'}}
+  // expected-note@-2 {{cannot automatically synthesize 'Encodable' because 'Nested' does not conform to 'Encodable'}}
 }
 
 // Codable struct with non-enum CodingKeys.
diff --git a/test/decl/protocol/special/coding/struct_codable_simple.swift b/test/decl/protocol/special/coding/struct_codable_simple.swift
index 05b04f37f8374..05b6423f98cd7 100644
--- a/test/decl/protocol/special/coding/struct_codable_simple.swift
+++ b/test/decl/protocol/special/coding/struct_codable_simple.swift
@@ -34,8 +34,8 @@ let _ = SimpleStruct.CodingKeys.self // expected-error {{'CodingKeys' is inacces
 struct SR_12248_1: Codable { // expected-error {{type 'SR_12248_1' does not conform to protocol 'Encodable'}}
   var x: Int // expected-note {{'x' previously declared here}}
   var x: Int // expected-error {{invalid redeclaration of 'x'}}
-  // expected-note@-1 {{cannot automatically synthesize 'Encodable' because '<<error type>>' does not conform to 'Encodable'}}
-  // expected-note@-2 {{cannot automatically synthesize 'Encodable' because '<<error type>>' does not conform to 'Encodable'}}
+  // expected-note@-1 {{cannot automatically synthesize 'Encodable' because 'Int' does not conform to 'Encodable'}}
+  // expected-note@-2 {{cannot automatically synthesize 'Encodable' because 'Int' does not conform to 'Encodable'}}
 }
 
 struct SR_12248_2: Decodable {