Test plan for nullable annotations on unconstrained type parameters

[jcouv]

See initial PR #45993

• draft spec on dotnet/csharplang
• language version errors for T? and where T : default
• nullability of default(T) and default(T?)
• nullability of var: var t = new T(); Local of unconstrained type parameter type is reported as NotAnnotated when using var #46236
• nullability of (T?)t
• nullability of (T)default(T)
• interop with T from outside nullable context: see UnconstrainedTypeParameter_45, _46
• T? outside nullable context
• T? with no constraints, with class? constraint, with notnull constraint
• T? as type argument
• T? as constraint
• T? substituted with U or U? where U : { T, T?, class, class?, struct, notnull }
• T? in variance
• T? in method type inference
• T? in lambda return type inference
• T? and T in best common type
• T? in ref types
• T? in out parameters: see UnconstrainedTypeParameter_47, No warning when overriding method with out T? rather than out T #49131
• T? in ref parameters
• T? in type argument constraint checks
• T? with [AllowNull], [MaybeNull]: see UnconstrainedTypeParameter_16
• T? with [DisallowNull], [NotNull]: see UnconstrainedTypeParameter_41, _42
• non-safety warnings for assignment or explicit cast of T? to T No warning reported for assignment or explicit cast of possibly null value of unconstrained type parameter type #46044
• conversions from U or U? to T or T? where U : { T, T? } No warning reported returning nullable version of unconstrained type parameter #46150
• default constraint on method overrides and explicit implementations
• default constraint on other methods
• default constraint combined with other constraints
• default constraint required to treat T? as unconstrained
• overrides that differ by default and struct constraints
• overrides that differ by default and other constraints { class, class?, notnull, I, I? }
• overrides that differ by T and T?: see UnconstrainedTypeParameter_09, _10
• T? consumed in C#8
• T? consumed in C#8 with [AllowNull], [MaybeNull]
• overrides and implementations with T? and [MaybeNull]T, etc.
• symbol display of T? and IEnumerable<T?>
• symbol display of method with where T : default constraint
• unassigned fields of type T and T?
• annotations from GetDeclaredSymbol()
• flow state from GetSymbolInfo() on a method re-inferred with T?
• flow state from IOperation on a method re-inferred with T?

Productivity:

• check publicAPI analyzer on T?
• recommend default keyword
• UseExplicitType