cmTarget: Compute link language of TARGET_OBJECTS with CMP0026 OLD (#15338)
Commit v3.1.0-rc1~297^2~5 (cmTarget: Drop 'head' argument from GetSourceFiles, 2014-07-10) exposed a dormant bug in source file computation, causing the test case to regress. After that commit, the source file computation and caching finds an existing container of source files. Prior to that patch, the GetSourceFiles method was called with either a null pointer for the head cmTarget, or it was called with the this pointer. The processSources method is eventually called, which normalizes the difference between the null pointer and the this pointer for the head target. However, the cache key depends on the actual pre-normalized pointer. The change in that commit caused the entry to be found in the cache where it was not before, which resulted in incorrect behavior. Prior to that commit, the test case also fails if the GetSourceFiles overload taking a vector<cmSourceFile*> is changed to normalize the head target at the beginning of the method: cmTarget const* head = head_ ? head_ : this; Such a construct was correctly used in other locations where similar caching was in place, before being removed in commit v3.1.0-rc1~310^2~25 (cmTarget: Remove 'head' argument from GetLinkInformation, 2014-06-12), but is not neccessary anymore. Commit v3.1.0-rc1~674^2~2 (cmTarget: Cache the cmSourceFiles in GetSourceFiles., 2014-04-05) introduced the caching, but fails the test case for an unrelated reason. That unrelated error was introduced in commit v3.1.0-rc1~688^2~5 (cmTarget: Allow any generator expression in SOURCES property., 2014-03-18) and fixed in commit v3.1.0-rc1~561^2~1 (cmTarget: Fix listing of source files at configure-time., 2014-04-13). All commits which fail the test case in the testable way do so when such a cached version of the source files is found and returned at generate time. In the test case, the cached content is populated at configure-time through the use of the deprecated LOCATION property with CMP0026 OLD. The cached content is an empty container for the bar target in the test case, because its source file 'foo.cpp.o' is not known until generate-time. That means that no source files are available to compute the link language and the reported error is issued. The actual problem is that the SourceFilesMap should be cleared after configure time by cmTarget::ClearLinkMaps. Clear it there now.