spec, cmd/compile: type inference for generic function assigned to single-type type set should work

[griesemer]

The 2nd assignment in this code should probably work, too (playground):

func g[P any](P) {}

func _[F func(int)]() {
	var _ func(int) = g
	var _ F = g // this should probably work
}

See also #77245 (related).

[gabyhelp]

Related Issues

• spec: infer type arguments from assignments of generic functions (reverse type inference) #59338 (closed)
• proposal: spec: function type inference should work in all assignment contexts #77245
• go/types, types2: assignment instead of inference error for invalid generic function assignment #60688 (closed)
• proposal: spec: infer generic function type even when it's called #61712
• go/types, types2: type inference fails when passing the same generic function twice #59956 (closed)
• spec: provide some way to refer to a generic function without instantiation #52654
• go/types, types2: cannot handle partially instantiated generic function argument #59958 (closed)
• go/types, types2: type inference fails if inferred type is generic function #59953 (closed)
• go/types, types2: inference fails where it should succeed when considering interface methods #60564 (closed)

Related Code Changes

• go/types, types2: improve type inference for assignment contexts

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[gopherbot]

Change https://go.dev/cl/744740 mentions this issue: go/types, types2: permit lone type set inference for generic functions