Describe the bug

The compiler implements both a tuple type conversion, and a tuple literal conversion from expression. The spec currently only covers that latter conversion. As a concrete example of a feature that explicitly designed for and allowed, but the spec says is illegal:

short s = 1;

(int, int) t1 = (s, s); // As said in the spec, this is a tuple expression conversion
(short, short) t2 = (s, s);
(int, int) t3 = t2; // By the spec, there shouldn't be a conversion here, but the compiler allows a tuple type conversion from (short, short) to (int, int)

Specifically, https://github.com/dotnet/csharpstandard/blob/draft-v8/standard/conversions.md#10213-implicit-tuple-conversions is the only conversions portion of the specification that touches tuples, and it covers tuple expressions. These are defined by https://github.com/dotnet/csharpstandard/blob/draft-v8/standard/expressions.md#1286-tuple-expressions. In t3 = t2, t2 is not a tuple expression, but is instead a simple name: https://github.com/dotnet/csharpstandard/blob/draft-v8/standard/expressions.md#1284-simple-names; since we're not dealing tuple expressions, implicit tuple conversions don't apply, and the spec says that no conversion should exist from t2 to t3.