better errors for standalone explicit generic instantiations (#24276)

refs #8064, refs #24010

Error messages for standalone explicit generic instantiations are
revamped. Failing standalone explicit generic instantiations now only
error after overloading has finished and resolved to the default `[]`
magic (this means `[]` can now be overloaded for procs but this isn't
necessarily intentional, in #24010 it was documented that it isn't
possible). The error messages for failed instantiations are also no
longer a simple `cannot instantiate: foo` message, instead they now give
the same type mismatch error message as overloads with mismatching
explicit generic parameters.

This is now possible due to the changes in #24010 that delegate all
explicit generic proc instantiations to overload resolution. Old code
that worked around this is now removed. `maybeInstantiateGeneric` could
maybe also be removed in favor of just `explicitGenericSym`, the `result
== n` case is due to `explicitGenericInstError` which is only for niche
cases.

Also, to cause "ambiguous identifier" error messages when the explicit
instantiation is a symchoice and the expression context doesn't allow
symchoices, we semcheck the sym/symchoice created by
`explicitGenericSym` with the given expression flags.

#8064 isn't entirely fixed because the error message is still misleading
for the original case which does `values[1]`, as a consequence of
#12664.

(cherry picked from commit 0a058a6b8f)

tests/generics/tpointerprocs.nim

@@ -2,10 +2,17 @@ discard """
 cmd: "nim check $options --hints:off $file"
 action: "reject"
 nimout:'''
-tpointerprocs.nim(15, 11) Error: 'foo' doesn't have a concrete type, due to unspecified generic parameters.
-tpointerprocs.nim(27, 11) Error: cannot instantiate: 'foo[int]'; got 1 typeof(s) but expected 2
-tpointerprocs.nim(27, 14) Error: expression 'foo[int]' has no type (or is ambiguous)
-tpointerprocs.nim(28, 11) Error: expression 'bar' has no type (or is ambiguous)
+tpointerprocs.nim(22, 11) Error: 'foo' doesn't have a concrete type, due to unspecified generic parameters.
+tpointerprocs.nim(34, 14) Error: type mismatch: got <int>
+but expected one of:
+proc foo(x: int | float; y: int or string): float
+  first type mismatch at position: 2 in generic parameters
+  missing generic parameter: y:type
+
+expression: foo[int]
+tpointerprocs.nim(34, 14) Error: cannot instantiate: 'foo[int]'
+tpointerprocs.nim(34, 14) Error: expression 'foo[int]' has no type (or is ambiguous)
+tpointerprocs.nim(35, 11) Error: expression 'bar' has no type (or is ambiguous)
 '''
 """
 
@@ -25,4 +32,4 @@ block:
   proc foo(x: int | float, y: int or string): float = result = 1.0
   let
     bar = foo[int]
-    baz = bar
+    baz = bar

tests/overload/tambiguousexplicitgeneric.nim

@@ -0,0 +1,6 @@
+# related to issue #8064
+
+import tables
+
+let x = values[int] #[tt.Error
+        ^ ambiguous identifier: 'values' -- use one of the following:]#

tests/overload/texplicitgenericdiscard.nim

@@ -0,0 +1,7 @@
+# issue #8064
+
+import tables
+
+values[int] #[tt.Error
+^ ambiguous identifier: 'values' -- use one of the following:]#
+# this happens before discard check, so no discard error