Championed issue: dotnet/csharplang#2460

• Trial VS insertion succeeds
• LangVersion [See TargetTypedConditionalOperatorTests.TestLanguageVersion and SemanticErrorTests.CS0172ERR_AmbigQM]
• target-typing to a nullable reference type [TestNullableReferenceType_*]
• target-typed 'ref' conditional (expect an error) [I don't know how to test this. Any attempt runs afoul of other language violations.]
• SemanticModel
  • shows an appropriate ConvertedType in GetTypeInfo [Most tests in TargetTypedConditionalOperatorTests]
  • ClassifyConversion(Expression, Type) [TestClassifyConversion_*]
• IOperation shows conversion appropriately [Tests show no conversion in the trees, as intended, like other conversion-from-expression cases. See, for example, ThrowFlow_32_Regular8]
• With user-defined conversions
  • C c = flag ? new A() : new B(); where A -> B, A -> C, B -> C, is consistent across language versions [TestUserDefinedConversionChain]
• Constant conversions
  • short x = true ? 1 : 2 continues to work in old language versions
  • short x = flag ? 1 : 2 requires new language version to work
  • short x = flag ? 1 << 16 : 1 always fails
  • short x = true ? 1 : 1 << 16 continues to work in old and new language versions
    • similar scenarios but with conditional expressions as constant patterns: case true ? 1 : int.MaxValue:
  • SomeEnum e = b ? 0 : 0; works in new language version (but works in any language version when 'b' is constant)
• Overload resolution
  • M(b ? 1 : 2) // calls M(long), not M(short)
  • M(true ? 1 : 2) // calls M(short), not M(long)
  • M(true ? 1 : 1 << 16) // calls M(short), not M(long)
• Lambdas Action a = b ? () => { } : () => { }
• Method group conversions Action a = b ? M1 : M2
• Iterators IEnumerable<short> M1() { yield return b ? 1 : 2; }
• Default literal Widget x = b ? default : default;
• Pointers void* ptr = b ? (int*)x : (long*)y;
• Function pointers, particularly variance in returns+parameters
• Expression trees
• dynamic conversions
• FormattableString formattable = flag ? $"a" : $"b"
  • also try with IFormattable?
• composition of tuples and conditional expressions
• throw expressions
  • object obj = b ? throw new Exception : throw new Exception();
  • object obj = b ? default : throw new Exception();