Fix NullReferenceException for mistyped default values on non-string columns in migrations

[Copilot]

Migrations authored in older EF Core versions can carry a default value whose CLR type doesn't match the column's store type — e.g. table.Column<int>(nullable: false, defaultValue: ""). This worked through EF Core 7 but throws an NullReferenceException when applied under EF Core 8+.

new AddColumnOperation { ClrType = typeof(int), ColumnType = "int", DefaultValue = "", IsNullable = false }
// EF8: NullReferenceException in RelationalTypeMappingSource.FindCollectionMapping

Root cause

MigrationsSqlGenerator.DefaultValue calls FindMapping(typeof(string), "int"). The scalar lookup misses, so resolution falls into the collection-mapping path, where TypeMappingSourceBase.TryFindJsonCollectionMapping treats string as a primitive collection of char (string : IEnumerable<char>). The follow-up mapping lookup returns null, and null!.WithComposedConverter(...) dereferences it.

Changes

• TypeMappingSourceBase.TryFindJsonCollectionMapping: guard against modelClrType == typeof(string). string always has dedicated scalar mappings and is never a JSON/primitive collection, so it should not enter this path. The mistyped default now resolves to null and DefaultValue falls back to GetMappingForValue, yielding valid SQL (... DEFAULT N''), matching legacy behavior.
• SqlServerMigrationsSqlGeneratorTest: added AddColumnOperation_mistyped_default_legacy covering the int-column / string-default case.

 if ((providerClrType == null || providerClrType == typeof(string))
+    && modelClrType != typeof(string)
     && modelClrType.TryGetElementType(typeof(IEnumerable<>)) is { } elementType
     && elementType != modelClrType
     && !modelClrType.GetGenericTypeImplementations(typeof(IDictionary<,>)).Any())

[Copilot]

Pull request overview

This PR fixes a regression where applying older migrations with a mistyped DefaultValue (e.g. int column with defaultValue: "") can throw a NullReferenceException in EF Core 8+ during type mapping resolution. The change prevents string from being misclassified as a primitive collection (IEnumerable<char>) in the JSON collection-mapping path, restoring the prior “fallback” behavior and producing valid SQL.

Changes:

• Update TypeMappingSourceBase.TryFindJsonCollectionMapping to explicitly exclude modelClrType == typeof(string) from the JSON/primitive-collection mapping flow.
• Add a SQL Server migrations SQL generator test covering AddColumnOperation with an int column and string default value to ensure no exception and stable SQL generation.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/EFCore/Storage/TypeMappingSourceBase.cs	Prevents string from entering JSON collection mapping, avoiding the NRE and allowing downstream fallback mapping for mistyped defaults.
test/EFCore.SqlServer.FunctionalTests/Migrations/SqlServerMigrationsSqlGeneratorTest.cs	Adds a regression test asserting the generated SQL for the mistyped-default scenario.