Honor JsonNumberHandlingAttribute declarations on C# unions

[Copilot]

Honor a union type's JsonNumberHandling when building its configuration-time case map. This lets [JsonNumberHandling(JsonNumberHandling.Strict)] on IntOrString(int, string) opt out of quoted-number ambiguity under JsonSerializerDefaults.Web.

Adds regression coverage through reflection and source-generated metadata.

Fixes #133668.

Note

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[eiriktsarpalis]

@copilot I don't understand how the changes are relevant to the task at hand. Note that this should be a targeted fix.

[Copilot]

@copilot I don't understand how the changes are relevant to the task at hand. Note that this should be a targeted fix.

Narrowed in af64b38. The dispatch-map and hot-path case-resolution changes were removed; the production fix now only marks unions as number-handling-aware and propagates the selected case type’s explicit setting through the existing converter path.

[Copilot]

🟡 Changes recommended

One or more issues must be addressed before approval.

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Review details

Suppressed comments (2)

src/libraries/System.Text.Json/src/System/Text/Json/Schema/JsonSchemaExporter.cs:374

• This drops the caller's customNumberHandling whenever the union itself has no type-level override. MapJsonSchemaCore has already computed an effective value from the parent property/collection, union metadata, and options, but each case is given only typeInfo.NumberHandling; a union nested under a container/type-info override therefore falls back to the global options and can advertise a different numeric shape than the container uses. Preserve the parent override when typeInfo.NumberHandling is null.

                            JsonSchema caseSchema = MapJsonSchemaCore(ref state, caseTypeInfo, customNumberHandling: typeInfo.NumberHandling, cacheResult: false);

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/JsonTypeInfo.cs:1178

• The default map still ignores number-handling metadata on a nullable numeric case's element type. Nullable<T>.Converter.GetSupportedJsonValueTypes forwards the supplied handling to its element converter, and the structural classifier explicitly unwraps nullable cases to preserve elementTypeInfo.NumberHandling; this path only checks caseTypeInfo.NumberHandling. Consequently, an int type-info modifier of AllowReadingFromString on NullableNumericStringUnion(int?, string) under strict options produces a different dispatch map from the structural path and schema. Include the nullable element override when computing the effective handling.

                    target.NumberHandling ?? caseTypeInfo.NumberHandling ?? options.NumberHandling;

• Files reviewed: 14/14 changed files
• Comments generated: 1
• Review effort level: Lite

[Copilot]

🔵 Needs a closer look

The changes require final human review because they are too complex or risky for automated approval.

Review details

• Files reviewed: 15/15 changed files
• Comments generated: 0 new
• Review effort level: Lite

[tannergooding]

Rest of it looks good to me. Astra reports one more case where it can break though and gave a minimal repro

[Copilot]

🟡 Changes recommended

Three unresolved moderate findings remain around named floating-point literals and classifier invalidation.

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Review details

Suppressed comments (2)

src/libraries/System.Text.Json/gen/JsonSourceGenerator.Parser.cs:1801

• The compile-time approximation still treats numeric cases as string-shaped only for AllowReadingFromString. Once union-level attributes are honored here, [JsonNumberHandling(AllowNamedFloatingPointLiterals)] must also be reflected in ambiguity diagnostics; otherwise source generation silently omits a warning for a union whose floating-point case accepts named JSON strings, diverging from the runtime contract. Update the approximation and add a named-literal union diagnostic case.

                JsonNumberHandling? unionNumberHandling = GetNumberHandling(unionType);

                foreach (ITypeSymbol caseType in caseTypes)
                {
                    string caseTypeName = caseType.ToDisplayString(SymbolDisplayFormat.MinimallyQualifiedFormat);
                    JsonNumberHandling effectiveNumberHandling = unionNumberHandling ?? GetNumberHandling(caseType) ?? _contextNumberHandling;
                    JsonValueType valueTypes = GetSupportedJsonValueTypes(caseType, effectiveNumberHandling);

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/JsonTypeInfo.cs:1179

• AllowNamedFloatingPointLiterals is also a string-consuming number-handling mode for the built-in floating-point converters ("NaN", "Infinity", and "-Infinity"), but their GetSupportedJsonValueTypes currently reports String only for AllowReadingFromString. As a result, a union such as DoubleOrString(double, string) with this union-level attribute still builds a number-only dispatch map and either rejects named literals or routes them to the string case. Include named-literal handling in the value-shape calculation (and keep the structural/source-generator paths aligned) with a regression test.

                JsonNumberHandling effectiveNumberHandling =
                    target.NumberHandling ?? caseTypeInfo.NumberHandling ?? options.NumberHandling;
                JsonValueType valueTypes = converter.GetSupportedJsonValueTypes(effectiveNumberHandling);

• Files reviewed: 17/17 changed files
• Comments generated: 1
• Review effort level: Lite

[eiriktsarpalis]

/ba-g test failures are unrelated

[eiriktsarpalis]

/backport to release/11.0

[github-actions]

Started backporting to release/11.0 (link to workflow run)