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spec: fix conversion rules (match implementation)
As written, the conversion P(x), where P and the type of x are type parameters with identical underlying types (i.e., identical constraints), is valid. However, unless the type of x and P are identical (which is covered with the assignability rule), such a conversion is not valid in general (consider the case where both type parameters are different type parameters with constraint "any"). This change adjusts the rules to prohibit type parameters in this case. The same reasoning applies and the analogue change is made for pointer types. The type checker already implements these updated rules. Change-Id: Id90187900cb2820f6a0a0cf582cf26cdf8addbce Reviewed-on: https://go-review.googlesource.com/c/go/+/371074 Trust: Robert Griesemer <gri@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org>
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@ -1,6 +1,6 @@
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<!--{
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<!--{
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"Title": "The Go Programming Language Specification - Go 1.18 Draft (incomplete)",
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"Title": "The Go Programming Language Specification - Go 1.18 Draft (incomplete)",
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"Subtitle": "Version of Nov 29, 2021",
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"Subtitle": "Version of Dec 13, 2021",
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"Path": "/ref/spec"
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"Path": "/ref/spec"
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}-->
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}-->
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@ -4782,14 +4782,16 @@ in any of these cases:
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</li>
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</li>
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<li>
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<li>
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ignoring struct tags (see below),
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ignoring struct tags (see below),
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<code>x</code>'s type and <code>T</code> have <a href="#Type_identity">identical</a>
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<code>x</code>'s type and <code>T</code> are not
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<a href="#Types">underlying types</a>.
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<a href="#Type_parameters">type parameters</a> but have
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<a href="#Type_identity">identical</a> <a href="#Types">underlying types</a>.
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</li>
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</li>
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<li>
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<li>
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ignoring struct tags (see below),
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ignoring struct tags (see below),
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<code>x</code>'s type and <code>T</code> are pointer types
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<code>x</code>'s type and <code>T</code> are pointer types
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that are not <a href="#Types">named types</a>,
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that are not <a href="#Types">named types</a>,
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and their pointer base types have identical underlying types.
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and their pointer base types are not type parameters but
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have identical underlying types.
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</li>
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</li>
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<li>
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<li>
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<code>x</code>'s type and <code>T</code> are both integer or floating
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<code>x</code>'s type and <code>T</code> are both integer or floating
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