Document breaking change for Type.MakeGenericSignatureType argument validation in .NET 10 Preview 3 (#49124)

* Add breaking change documentation for Type.MakeGenericSignatureType argument validation

Co-authored-by: gewarren <[email protected]>

* Address review feedback: rename folder to 10, update issue number, and remove Preview 3 from New behavior section

Co-authored-by: gewarren <[email protected]>

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: gewarren <[email protected]>

Author: Copilot

docs/core/compatibility/10.0.md

@@ -53,6 +53,7 @@ If you're migrating an app to .NET 10, the breaking changes listed here might af
 | [MacCatalyst version normalization](core-libraries/10.0/maccatalyst-version-normalization.md) | Behavioral change | Preview 1 |
 | [.NET runtime no longer provides default termination signal handlers](core-libraries/10.0/sigterm-signal-handler.md) | Behavioral change | Preview 5 |
 | [System.Linq.AsyncEnumerable included in core libraries](core-libraries/10.0/asyncenumerable.md) | Source incompatible | Preview 1 |
+| [Type.MakeGenericSignatureType argument validation](reflection/10/makegeneric-signaturetype-validation.md) | Behavioral change | Preview 3 |
 | [YMM embedded rounding removed from AVX10.2](core-libraries/10.0/ymm-embedded-rounding.md) | Behavioral change | Preview 5 |
 
 ## Cryptography

docs/core/compatibility/reflection/10/makegeneric-signaturetype-validation.md

@@ -0,0 +1,46 @@
+---
+title: "Breaking change: Type.MakeGenericSignatureType argument validation"
+description: Learn about the .NET 10 breaking change in core .NET libraries where Type.MakeGenericSignatureType validates that the genericTypeDefinition argument is a generic type definition.
+ms.date: 10/13/2025
+ai-usage: ai-assisted
+ms.custom: https://github.com/dotnet/docs/issues/48902
+---
+# Type.MakeGenericSignatureType argument validation
+
+Starting in .NET 10, the <xref:System.Type.MakeGenericSignatureType(System.Type,System.Type[])?displayProperty=nameWithType> API validates that the `genericTypeDefinition` argument is a generic type definition.
+
+## Version introduced
+
+.NET 10 Preview 3
+
+## Previous behavior
+
+Previously, <xref:System.Type.MakeGenericSignatureType(System.Type,System.Type[])?displayProperty=nameWithType> accepted any type for the `genericTypeDefinition` argument, including non-generic types.
+
+## New behavior
+
+Starting in .NET 10, <xref:System.Type.MakeGenericSignatureType(System.Type,System.Type[])?displayProperty=nameWithType> requires the `genericTypeDefinition` argument to be a generic type definition. If the argument is not a generic type definition, the method throws an <xref:System.ArgumentException>.
+
+## Type of breaking change
+
+This change is a [behavioral change](../../categories.md#behavioral-change).
+
+## Reason for change
+
+The type created by <xref:System.Type.MakeGenericSignatureType(System.Type,System.Type[])?displayProperty=nameWithType> had non-sensical behavior when the `genericTypeDefinition` argument was not a generic type definition.
+
+## Recommended action
+
+Avoid calling <xref:System.Type.MakeGenericSignatureType(System.Type,System.Type[])?displayProperty=nameWithType> for types that are not generic type definitions. For example:
+
+```csharp
+// Before
+Type instantiatedType = Type.MakeGenericSignatureType(originalType, instantiation);
+
+// After
+Type instantiatedType = originalType.IsGenericTypeDefinition ? Type.MakeGenericSignatureType(originalType, instantiation) : originalType;
+```
+
+## Affected APIs
+
+- <xref:System.Type.MakeGenericSignatureType(System.Type,System.Type[])?displayProperty=fullName>

docs/core/compatibility/toc.yml

@@ -110,6 +110,10 @@ items:
                 href: networking/10.0/default-http-streaming.md
               - name: "'Uri' length limits removed"
                 href: networking/10.0/uri-length-limits-removed.md
+          - name: Reflection
+            items:
+              - name: Type.MakeGenericSignatureType argument validation
+                href: reflection/10/makegeneric-signaturetype-validation.md
           - name: SDK and MSBuild
             items:
               - name: .NET CLI `--interactive` defaults to `true` in user scenarios