fixes #11515 (#11624)

* fixes #11515

* better fix for #11515; make tests green

compiler/semexprs.nim

@@ -2023,6 +2023,9 @@ proc tryExpr(c: PContext, n: PNode, flags: TExprFlags = {}): PNode =
   let oldErrorCount = c.config.errorCounter
   let oldErrorMax = c.config.errorMax
   let oldCompilesId = c.compilesContextId
+  # if this is a nested 'when compiles', do not increase the ID so that
+  # generic instantations can still be cached for this level.
+  if c.compilesContextId == 0:
     inc c.compilesContextIdGenerator
     c.compilesContextId = c.compilesContextIdGenerator
   # do not halt after first error:

compiler/seminst.nim

@@ -98,7 +98,7 @@ proc sameInstantiation(a, b: TInstantiation): bool =
 proc genericCacheGet(genericSym: PSym, entry: TInstantiation;
                      id: CompilesId): PSym =
   for inst in genericSym.procInstCache:
-    if inst.compilesId == id and sameInstantiation(entry, inst[]):
+    if (inst.compilesId == 0 or inst.compilesId == id) and sameInstantiation(entry, inst[]):
       return inst.sym
 
 when false:

tests/errmsgs/tnested_generic_instantiation.nim

@@ -1,6 +1,5 @@
 discard """
-errormsg: "generic instantiation too nested"
+action: compile
-file: "system.nim"
 """
 
 # bug #4766

tests/errmsgs/tnested_generic_instantiation2.nim

@@ -1,5 +1,5 @@
 discard """
-errormsg: "generic instantiation too nested"
+action: compile
 """
 
 #[