diff --git a/src/Compilers/CSharp/Portable/Symbols/Source/SourceComplexParameterSymbol.cs b/src/Compilers/CSharp/Portable/Symbols/Source/SourceComplexParameterSymbol.cs
index 91bbb1b0e1f74..1f182b73abc9b 100644
--- a/src/Compilers/CSharp/Portable/Symbols/Source/SourceComplexParameterSymbol.cs
+++ b/src/Compilers/CSharp/Portable/Symbols/Source/SourceComplexParameterSymbol.cs
@@ -458,10 +458,15 @@ AttributeLocation IAttributeTargetSymbol.AllowedAttributeLocations
         /// <summary>
         /// Symbol to copy bound attributes from, or null if the attributes are not shared among multiple source parameter symbols.
         /// </summary>
+        /// <remarks>
+        /// This is inconsistent with analogous 'BoundAttributesSource' on other symbols.
+        /// Usually the definition part is the source, but for parameters the implementation part is the source.
+        /// This affects the location of diagnostics among other things.
+        /// </remarks>
         private SourceParameterSymbol? BoundAttributesSource
-            => PartialDefinitionPart;
+            => PartialImplementationPart;
 
-        private SourceParameterSymbol? PartialImplementationPart
+        protected SourceParameterSymbol? PartialImplementationPart
         {
             get
             {
@@ -481,7 +486,7 @@ private SourceParameterSymbol? PartialImplementationPart
             }
         }
 
-        private SourceParameterSymbol? PartialDefinitionPart
+        protected SourceParameterSymbol? PartialDefinitionPart
         {
             get
             {
@@ -501,7 +506,7 @@ private SourceParameterSymbol? PartialDefinitionPart
             }
         }
 
-        internal sealed override SyntaxList<AttributeListSyntax> AttributeDeclarationList
+        internal override SyntaxList<AttributeListSyntax> AttributeDeclarationList
         {
             get
             {
@@ -515,11 +520,13 @@ internal sealed override SyntaxList<AttributeListSyntax> AttributeDeclarationLis
         /// </summary>
         internal virtual OneOrMany<SyntaxList<AttributeListSyntax>> GetAttributeDeclarations()
         {
-            if (PartialImplementationPart is { } implementationPart)
+            // TODO2: should this be non-virtual and subtypes override 'AttributeDeclarationList' instead?
+            Debug.Assert(PartialImplementationPart is null);
+            if (PartialDefinitionPart is { } definitionPart)
             {
                 return OneOrMany.Create(
                     AttributeDeclarationList,
-                    implementationPart.AttributeDeclarationList);
+                    definitionPart.AttributeDeclarationList);
             }
             else
             {
diff --git a/src/Compilers/CSharp/Portable/Symbols/Source/SourceOrdinaryMethodSymbol.cs b/src/Compilers/CSharp/Portable/Symbols/Source/SourceOrdinaryMethodSymbol.cs
index db0e62af2e2c6..43a62f8f8efaf 100644
--- a/src/Compilers/CSharp/Portable/Symbols/Source/SourceOrdinaryMethodSymbol.cs
+++ b/src/Compilers/CSharp/Portable/Symbols/Source/SourceOrdinaryMethodSymbol.cs
@@ -401,6 +401,7 @@ protected sealed override SourceMemberMethodSymbol BoundAttributesSource
 
         internal sealed override OneOrMany<SyntaxList<AttributeListSyntax>> GetAttributeDeclarations()
         {
+            Debug.Assert(PartialDefinitionPart is null);
             if ((object)this.SourcePartialImplementation != null)
             {
                 return OneOrMany.Create(ImmutableArray.Create(AttributeDeclarationSyntaxList, this.SourcePartialImplementation.AttributeDeclarationSyntaxList));
diff --git a/src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertyAccessorSymbol.cs b/src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertyAccessorSymbol.cs
index 6c21244fb9c49..acf4d7ab312d3 100644
--- a/src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertyAccessorSymbol.cs
+++ b/src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertyAccessorSymbol.cs
@@ -631,6 +631,7 @@ public sealed override ImmutableArray<MethodSymbol> ExplicitInterfaceImplementat
 
         internal sealed override OneOrMany<SyntaxList<AttributeListSyntax>> GetAttributeDeclarations()
         {
+            Debug.Assert(PartialDefinitionPart is null);
             if (PartialImplementationPart is { } implementation)
             {
                 return OneOrMany.Create(AttributeDeclarationList, ((SourcePropertyAccessorSymbol)implementation).AttributeDeclarationList);
@@ -639,7 +640,7 @@ internal sealed override OneOrMany<SyntaxList<AttributeListSyntax>> GetAttribute
             return OneOrMany.Create(AttributeDeclarationList);
         }
 
-        private SyntaxList<AttributeListSyntax> AttributeDeclarationList
+        internal SyntaxList<AttributeListSyntax> AttributeDeclarationList
         {
             get
             {
diff --git a/src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertySymbol.cs b/src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertySymbol.cs
index ee17b1fbe541e..abafeb68ea878 100644
--- a/src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertySymbol.cs
+++ b/src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertySymbol.cs
@@ -175,6 +175,7 @@ public override OneOrMany<SyntaxList<AttributeListSyntax>> AttributeDeclarationL
         {
             get
             {
+                Debug.Assert(PartialDefinitionPart is null);
                 if (PartialImplementationPart is { } implementationPart)
                 {
                     return OneOrMany.Create(
diff --git a/src/Compilers/CSharp/Portable/Symbols/Synthesized/SynthesizedAccessorValueParameterSymbol.cs b/src/Compilers/CSharp/Portable/Symbols/Synthesized/SynthesizedAccessorValueParameterSymbol.cs
index 7252c0fee9068..bfe1cb85ababc 100644
--- a/src/Compilers/CSharp/Portable/Symbols/Synthesized/SynthesizedAccessorValueParameterSymbol.cs
+++ b/src/Compilers/CSharp/Portable/Symbols/Synthesized/SynthesizedAccessorValueParameterSymbol.cs
@@ -70,11 +70,13 @@ protected override IAttributeTargetSymbol AttributeOwner
             get { return (SourceMemberMethodSymbol)this.ContainingSymbol; }
         }
 
-        internal override OneOrMany<SyntaxList<AttributeListSyntax>> GetAttributeDeclarations()
+        internal sealed override SyntaxList<AttributeListSyntax> AttributeDeclarationList
         {
-            // Bind the attributes on the accessor's attribute syntax list with "param" target specifier.
-            var accessor = (SourceMemberMethodSymbol)this.ContainingSymbol;
-            return accessor.GetAttributeDeclarations();
+            get
+            {
+                var accessor = (SourcePropertyAccessorSymbol)this.ContainingSymbol;
+                return accessor.AttributeDeclarationList;
+            }
         }
 
         internal override void AddSynthesizedAttributes(PEModuleBuilder moduleBuilder, ref ArrayBuilder<SynthesizedAttributeData> attributes)
diff --git a/src/Compilers/CSharp/Test/Symbol/Symbols/PartialPropertiesTests.cs b/src/Compilers/CSharp/Test/Symbol/Symbols/PartialPropertiesTests.cs
index 9f498acdca7a5..7b4aa9c1fcea6 100644
--- a/src/Compilers/CSharp/Test/Symbol/Symbols/PartialPropertiesTests.cs
+++ b/src/Compilers/CSharp/Test/Symbol/Symbols/PartialPropertiesTests.cs
@@ -3197,6 +3197,8 @@ public partial int P
         [Fact]
         public void Attributes_SetValueParam()
         {
+            // Just as with parameter attributes on partial methods,
+            // the implementation part attributes are emitted before the definition part attributes.
             var source = """
                 #pragma warning disable CS9113 // Primary constructor parameter is unread
 
@@ -3231,8 +3233,8 @@ public partial int P
             var accessor = comp.GetMember<MethodSymbol>("C.set_P");
             AssertEx.Equal([], accessor.GetAttributes().SelectAsArray(a => a.ToString()));
             AssertEx.Equal([], accessor.PartialImplementationPart.GetAttributes().SelectAsArray(a => a.ToString()));
-            AssertEx.Equal(["A(1)", "B(1)", "A(2)", "B(2)"], accessor.Parameters.Single().GetAttributes().SelectAsArray(a => a.ToString()));
-            AssertEx.Equal(["A(1)", "B(1)", "A(2)", "B(2)"], accessor.PartialImplementationPart!.Parameters.Single().GetAttributes().SelectAsArray(a => a.ToString()));
+            AssertEx.Equal(["A(2)", "B(2)", "A(1)", "B(1)"], accessor.Parameters.Single().GetAttributes().SelectAsArray(a => a.ToString()));
+            AssertEx.Equal(["A(2)", "B(2)", "A(1)", "B(1)"], accessor.PartialImplementationPart!.Parameters.Single().GetAttributes().SelectAsArray(a => a.ToString()));
         }
 
         [Fact]
@@ -3269,6 +3271,8 @@ partial class C
         [Fact]
         public void Attributes_IndexerParameter()
         {
+            // Unlike other symbol kinds, for parameters, the implementation part attributes are emitted first, then the definition part attributes.
+            // This is consistent with partial methods.
             var source = """
                 #pragma warning disable CS9113 // Primary constructor parameter is unread
 
@@ -3302,7 +3306,7 @@ partial class C
 
             void verify(ParameterSymbol param)
             {
-                AssertEx.Equal(["A(1)", "B(1)", "A(2)", "B(2)"], param.GetAttributes().SelectAsArray(a => a.ToString()));
+                AssertEx.Equal(["A(2)", "B(2)", "A(1)", "B(1)"], param.GetAttributes().SelectAsArray(a => a.ToString()));
             }
         }
 
@@ -3317,27 +3321,27 @@ class Attr : Attribute { }
                 partial class C
                 {
                     [Attr]
-                    public partial int P { [Attr] get; [Attr] [param: Attr] set; }
+                    public partial int P { [Attr] get; [Attr] [param: Attr] set; } // 1 (param)
 
-                    [Attr]
-                    public partial int P { [Attr] get => 1; [Attr] [param: Attr] set { } }
+                    [Attr] // 2
+                    public partial int P { [Attr] get => 1; [Attr] [param: Attr] set { } } // 3, 4 (accessors)
                 }
                 """;
 
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
+                // (8,55): error CS0579: Duplicate 'Attr' attribute
+                //     public partial int P { [Attr] get; [Attr] [param: Attr] set; } // 1 (param)
+                Diagnostic(ErrorCode.ERR_DuplicateAttribute, "Attr").WithArguments("Attr").WithLocation(8, 55),
                 // (10,6): error CS0579: Duplicate 'Attr' attribute
-                //     [Attr]
+                //     [Attr] // 2
                 Diagnostic(ErrorCode.ERR_DuplicateAttribute, "Attr").WithArguments("Attr").WithLocation(10, 6),
                 // (11,29): error CS0579: Duplicate 'Attr' attribute
-                //     public partial int P { [Attr] get => 1; [Attr] [param: Attr] set { } }
+                //     public partial int P { [Attr] get => 1; [Attr] [param: Attr] set { } } // 3, 4 (accessors)
                 Diagnostic(ErrorCode.ERR_DuplicateAttribute, "Attr").WithArguments("Attr").WithLocation(11, 29),
                 // (11,46): error CS0579: Duplicate 'Attr' attribute
-                //     public partial int P { [Attr] get => 1; [Attr] [param: Attr] set { } }
-                Diagnostic(ErrorCode.ERR_DuplicateAttribute, "Attr").WithArguments("Attr").WithLocation(11, 46),
-                // (11,60): error CS0579: Duplicate 'Attr' attribute
-                //     public partial int P { [Attr] get => 1; [Attr] [param: Attr] set { } }
-                Diagnostic(ErrorCode.ERR_DuplicateAttribute, "Attr").WithArguments("Attr").WithLocation(11, 60));
+                //     public partial int P { [Attr] get => 1; [Attr] [param: Attr] set { } } // 3, 4 (accessors)
+                Diagnostic(ErrorCode.ERR_DuplicateAttribute, "Attr").WithArguments("Attr").WithLocation(11, 46));
 
             var property = comp.GetMember<SourcePropertySymbol>("C.P");
             AssertEx.Equal(["Attr", "Attr"], property.GetAttributes().SelectAsArray(a => a.ToString()));
@@ -3357,33 +3361,33 @@ class Attr : Attribute { }
                 partial class C
                 {
                     [Attr]
-                    public partial int this[[Attr] int x, [Attr] int y] { [Attr] get; [Attr] [param: Attr] set; }
+                    public partial int this[[Attr] int x, [Attr] int y] { [Attr] get; [Attr] [param: Attr] set; } // 1, 2, 3 (param)
 
-                    [Attr]
-                    public partial int this[[Attr] int x, [Attr] int y] { [Attr] get => 1; [Attr] [param: Attr] set { } }
+                    [Attr] // 4
+                    public partial int this[[Attr] int x, [Attr] int y] { [Attr] get => 1; [Attr] [param: Attr] set { } } // 5, 6 (accessor)
                 }
                 """;
 
             var comp = CreateCompilation(source);
             comp.VerifyEmitDiagnostics(
+                // (8,30): error CS0579: Duplicate 'Attr' attribute
+                //     public partial int this[[Attr] int x, [Attr] int y] { [Attr] get; [Attr] [param: Attr] set; } // 1, 2, 3 (param)
+                Diagnostic(ErrorCode.ERR_DuplicateAttribute, "Attr").WithArguments("Attr").WithLocation(8, 30),
+                // (8,44): error CS0579: Duplicate 'Attr' attribute
+                //     public partial int this[[Attr] int x, [Attr] int y] { [Attr] get; [Attr] [param: Attr] set; } // 1, 2, 3 (param)
+                Diagnostic(ErrorCode.ERR_DuplicateAttribute, "Attr").WithArguments("Attr").WithLocation(8, 44),
+                // (8,86): error CS0579: Duplicate 'Attr' attribute
+                //     public partial int this[[Attr] int x, [Attr] int y] { [Attr] get; [Attr] [param: Attr] set; } // 1, 2, 3 (param)
+                Diagnostic(ErrorCode.ERR_DuplicateAttribute, "Attr").WithArguments("Attr").WithLocation(8, 86),
                 // (10,6): error CS0579: Duplicate 'Attr' attribute
-                //     [Attr]
+                //     [Attr] // 4
                 Diagnostic(ErrorCode.ERR_DuplicateAttribute, "Attr").WithArguments("Attr").WithLocation(10, 6),
-                // (11,30): error CS0579: Duplicate 'Attr' attribute
-                //     public partial int this[[Attr] int x, [Attr] int y] { [Attr] get => 1; [Attr] [param: Attr] set { } }
-                Diagnostic(ErrorCode.ERR_DuplicateAttribute, "Attr").WithArguments("Attr").WithLocation(11, 30),
-                // (11,44): error CS0579: Duplicate 'Attr' attribute
-                //     public partial int this[[Attr] int x, [Attr] int y] { [Attr] get => 1; [Attr] [param: Attr] set { } }
-                Diagnostic(ErrorCode.ERR_DuplicateAttribute, "Attr").WithArguments("Attr").WithLocation(11, 44),
                 // (11,60): error CS0579: Duplicate 'Attr' attribute
-                //     public partial int this[[Attr] int x, [Attr] int y] { [Attr] get => 1; [Attr] [param: Attr] set { } }
+                //     public partial int this[[Attr] int x, [Attr] int y] { [Attr] get => 1; [Attr] [param: Attr] set { } } // 5, 6 (accessor)
                 Diagnostic(ErrorCode.ERR_DuplicateAttribute, "Attr").WithArguments("Attr").WithLocation(11, 60),
                 // (11,77): error CS0579: Duplicate 'Attr' attribute
-                //     public partial int this[[Attr] int x, [Attr] int y] { [Attr] get => 1; [Attr] [param: Attr] set { } }
-                Diagnostic(ErrorCode.ERR_DuplicateAttribute, "Attr").WithArguments("Attr").WithLocation(11, 77),
-                // (11,91): error CS0579: Duplicate 'Attr' attribute
-                //     public partial int this[[Attr] int x, [Attr] int y] { [Attr] get => 1; [Attr] [param: Attr] set { } }
-                Diagnostic(ErrorCode.ERR_DuplicateAttribute, "Attr").WithArguments("Attr").WithLocation(11, 91));
+                //     public partial int this[[Attr] int x, [Attr] int y] { [Attr] get => 1; [Attr] [param: Attr] set { } } // 5, 6 (accessor)
+                Diagnostic(ErrorCode.ERR_DuplicateAttribute, "Attr").WithArguments("Attr").WithLocation(11, 77));
 
             var property = (SourcePropertySymbol)comp.GetMember<NamedTypeSymbol>("C").Indexers.Single();
             AssertEx.Equal(["Attr", "Attr"], property.GetAttributes().SelectAsArray(a => a.ToString()));